HISTORY OF MICROBIOLOGY

Microbiology and Origin of Life:

Many explanations have been offered for the origin of Life on the planet earth. One

of the most acceptable ones suggests that the life originated in the sea. Following

millions of years of a chemical evolutionary process. This hypothesis proposes that

the inorganic compounds of the atmosphere subject to the influence of UV light,

electrical discharges and /or high temperatures interacted, resulting in the formation

of organic compounds which precipitated in the sea where they accumulated. These

organic compounds subject to additional physical effects of the environment

combined and formed peptides, polypeptides and other more complex organic

substances which served as the precursors of the first form of life. One of the many

unanswered questions is 'that of the process of duplication (multiplication) in the first

forms of life.

History of microbiology:

History is the story of achievements of men and women but it records relatively few

outstanding names and events. Similarly, it has been said that, in science, the credit

of-one finding goes to one who convinces the world and hot to the one who first had

the idea. Microbiology began when people learned to grind lenses from pieces of glass

and combine them to produce magnifications great enough to see the microbes. The

history of microbiology can be divided into following topics.

• Discovery of microscope and developments in microscopy

• Theory of spontaneous generation and its disproval (Biogenesis vs. abiogenesis)

• Establishment of germs theory of fermentation

• Establishment of germs theory of disease '

• Developments in medical microbiology

• Developments in Agricultural microbiology . .

A) Discovery of microscope and developments in microscopy;

Roger Bacon (1220-1292): He postulated that the disease is caused by invisible living

creatures.

Fracastpro (1483-1553) and Pienciz (1762): They also made again a similar

suggestion, but these people had no proof.

Kircher(1658): He referred to "worms" invisible to the naked eye in decaying

bodies, meat milk and diarrheal secretions. Although his descriptions lacked

accuracy, he was first to recognize the significance of bacteria and other microbes in

disease. ,

Robert Hooke (1665): Described cells in a piece of cork. Discovered compound

microscope. He was a member of secretaries of the Royal Philosophical society of

London. He published a book called “Micrographia ". In which he described many

molds and bacteria.

Antony Van Leeuwenhoek (1632-1723): He was the first to report his

observations of bacteria and protozoa with accurate descriptions and drawings. He

observed living creatures in a drop of rain water and called them as little

"animalcules". He was a lens grinder and made more than 250 microscopes

consisting of home ground lenses mounted in brass and silver having magnification up

to 200-300 times. He made drawings of bacteria in rainwater, saliva, vinegar and

other substances and described them with pictures; He related his exciting

discoveries in a series of more than 300 letters to his friends In the "Royal Society of

London and French Academy of Sciences. The significance of his discoveries,

however, went unrecognized as there was little awareness that microbes cause

diseases. .

Joblet (France) in 1754 wrote extensively on microscopic-objects. In Muller (Danish

Scientist) 1773, examined many forms and made attempts to describe and classify

them into different groups. He introduced terms like Bacillus, Vibrio and Spirillum.

During the next 5O years better microscopes where developed and in 1808, a

German scientist Ehrenberg made surveys of microscopic forms and published it in

two volumes. In 1844, Dolland demonstrated the usefulness of oil immersion lens

it observe small objects more clearly. In 1870, Abbe developed sub stage condense

for better illumination of objects. By using better microscopes, German botanist Chon

and his students published series of papers on bacteria during 1872 - 76.

The different techniques in microscopy such as bright field, phase contrast

fluorescent and ultraviolet microscopy made observation of minute objects more easy

The development of Electron Microscope by Zwophin in 1963 has made it possible

to magnify objects up to 2,00,000 times and some of the minute objects like

"viruses" which are not easily visible under tight microscope can be seen and

studied.

B) Theory of spontaneous generation (abiogenesis) vs Biogenesis:

Spontaneous generation (abiogenesis): Origin of living things spontaneously from

the non-living ones i.e. non-living origin or inanimate origin.

Biogenesis: Origin of living things from the living things only i.e. life from life.

The discovery of microbes-spurred interest in the origin of living things. As far as the

human beings were concerned, the Greek explanation that the Goddess Gaea was

able to create people from stones and other inanimate objects had been largely

discarded. The idea of spontaneous generation. dates back at least to the ancient

Greeks who believed that decaying meat produced maggots and that the flies and

frogs arose from the mud under appropriate climatic conditions.

Aristotle (384-322 BC): He taught that animals might originate spontaneously

from the soil, plants or others unlike animals.

Francesco Redii (1626): He doubted the spontaneous generation of maggots from

the meat. He performed an experiment by placing meat in a jar covered with wire

gauze. Attracted by the odor of meat, the flies laid eggs on the covering, and from

these eggs, the maggots developed. Thus, he concluded that the origin of maggots

was from the flies and not from the meat.

Thus, the matter was settled for the forms of life such as maggots, mice and

scorpions, but the origin of microbes was yet doubtful. There appeared champions

for and challengers of spontaneous generation, each with a new, and sometimes

fantastic explanation or bit of experimental evidence.

Louis Joblet (1710): He observed that hay, when infused in water and allowed to

stand for some days gave rise to countless microbes. He boiled this infusion and

placed one portion in a closed vessel and the other in an open vessel. The infusion in

the open vessel was full of microbes after incubation whereas no life (microbes)

appeared in the closed vessel. Thus, he proved that infusion alone was incapable of

generating a new life spontaneously. .

John Needham (1713-1781): He conducted a similar experiment as that of Joblet

and got conflicting results. The microbes developed in the heated closed vessel as

well as in the unheated ones. He, therefore,-believed in the spontaneous generation.

The conflicting results were due to insufficient heating, which failed to kill the heat

resistant, "spores", and nothing was known about spores at that time.

Lazaro Spailanzani (1729-1799): He boiled the beef broth for an hour and then

sealed the flasks and incubated. No microbes appeared after incubation. He

confirmed the results by repeated experiments. However, he failed to convince

Needham, who insisted that air was essential for the spontaneous generation of

microbes and that it had been excluded from the flasks by sealing.

Two workers answered this argument some 60-70 years later independently.

Franz Schulze (18TB-1873): He passed the air in to the boiled infusions through

strong acid solutions. The microbes did riot appear even after a long period of

Incubation.

Theodor Schwann (1810-1882): He passed the air into his flasks containing

boiled beef broth through red hot tubes. In this case also, the microbes did not

appear.

The die heard advocates of spontaneous generation were still not convinced. They

said, "acid and heat altered the air so that it would not support the growth of

microbes".

Schroder and Dusch (1850): They performed a more convincing experiment by

passing air through cotton into the flasks containing heated broth. Thus, the

microbes were filtered out of the air by the cotton fibers so that microbial growth did

not occur. Thus a basic technique of plugging bacterial culture tubes with cotton a

stopper was initiated, (cotton plugging technique),

The concept of spontaneous generation was revived for the last time, by Pouchet

(1859) who published an extensive report proving the occurrence of spontaneous

generation. ,

Louis Pasteur (1822-1895): He performed experiments that ended the argument of

spontaneous generation forever. He prepared a special flask with a long, narrow,

goose neck opening. The nutrient solutions were heated in the flasks and the air-

untreated and unfiltered could pass in and out of the flask. The germs (microbes)

settled in the goose neck area; and no microbes appeared in the solution even after a

long period of incubation.

John Tyndall (1820-1893): Finally, Tyndall conducted experiments in a specially

designed box (dust free box) to prove that the dust particles carried the germs. He

demonstrated that, if no dust was present, the sterile broth remained free of microbial

growth for indefinite periods."

C) Establishment of Germs Theory of Fermentations

Louis Pasteur was a Professor of Chemistry at France. He was studying the methods

and processes of making consistently good wines and beer. He found that the

fermentation of fruits and grains resulting in the production of alcohol was

brought .about by the microbes. By examining many batches of "ferment” he f6und

microbes of different kinds. One type of microbes predominated in good lots whereas

the poor quality wines and beer contained the other type of microbes. He suggested

that these undesirable microbes might be removed by heating, not enough to hurt

the original flavor of fruit juice, but enough to destroy a very high percentage of

microbial population. He found that, holding the fruit juices at 62.8°C or 145bF

temperature for 30 minutes did the" job Today, this process known as

"pasteurization" is widely used in fermentation and the dairy industries.

The methods of pasteurization used in the dairy industry are as follows.

a) Low Temperature Holding (LTH) Method: Heating every particle of milk or

milk product to at least 145°F (62.8°C) for 30 minutes.

b) High Temperature Short Time (HTST) Method: Heating every particle of

milk or milk product to at least 161°F (71.7°C) for 15 seconds. ' -

D) Establishment of Germs Theory of Disease:

Before Pasteur had proved by experiment that bacteria are the cause of some

diseases, many students had expressed strong arguments for the germs theory of

disease.

Fracastoro (1483-1553) : He suggested that the diseases might be due

invisible organisms transmitted from one person to the other.

Plenciz (1762): He proposed that the living;-agents are the cause of diseases

and different germs may be responsible for different diseases,

Holmes (1809-1884) Stated that Puerperal fever, a disease of childbirth was

contagious, and was probably caused by germs carried from one mother to

another by midwives and physicians.

Joseph Lister (1827-1912) : He used a dilute solution of phenol (carbolic acid)

to" soak the surgical dressings and spray the operating room. The wounds

became rarely infected and healed rapidly. This technique of "antiseptic surgery"

was quickly accepted by the surgeons.

Louis Pasteur (1822-1895) :After a great success in solving the problem of

undesirable microbes in the French wine industry, the French Government

requested Pasteur to investigate the pebrine disease of silkworm which was

ruining the French silk industry. Pasteur isolated the parasite (protozoan) causing

the pebrine disease. After investigation he showed that the farmers could

eliminate the disease –by using only healthy, disease-free caterpillars for the

breeding stock. Pasteur also studied the problem of anthrax, a disease of cattle,

sheep and sometimes human beings. H e isolated the organism causing anthrax

disease (Bacillus anthracis) and grows it in the laboratory flasks. He isolated

these bacteria from the blood of animate that had died of anthrax. .

Robert Koch (1843-1910): Koch was busy with the anthrax problem in Germany;

He isolated the typical bacilli with squarish ends from the blood of the cattle died

of anthrax. He grows these bacteria in laboratory pure, cultures and then injected

them into other healthy, susceptible animals, where the disease was produced.

From these experimentally infected animals he isolated the bacterium similarly to

'the previously inoculated one.. This was the first time a bacterium had seen proved

to be the cause of an animal disease (pebrine is caused by a protozoan). This led to

the establishment of following Koch's Postulates, which provided guidelines to

identify the-causative agent of an infectious disease.

Kochs Postulates:

1) Association: A specific organism can always be found in association with a

given disease.

2) Isolation : the organism can be isolated and grown in pure culture in

laboratory.

3) Inoculation: the pure culture will produce the same disease when inoculated

into a healthy susceptible animals

4) Re isolation: It is possible to recover the same organism in pure culture from

the experimentally infected animal.

ROLE OF MICROBES IN FERMENTATION:

The souring of milk and the production of alcoholic beverages has been

throughout recorded history, but the fermentation process concerned has

understood for only a century. Berzelius. Liebig, Wohler, and other distinguished

influential organic chemists of the last century interpreted the transformation of

sugar in lactic acid or in to ethyl alcohol and carbon dioxide as purely chemical

phenomena. When was pointed out that yeast or other microbes are always present,

they devised explanation other than one which eventually proved top be true. Liebig

for example,, noting how yeast is destroyed and decomposes, was communicated in

some way to sugar and substances to contact with it. Berzelius interpreted

fermentation as a contact phenomenon and Mitscherlich saw in yeast globules a

catalyst similar In behavior to spongy platinum contact with hydrogen peroxide,

None of these investigators accepted the thesis, first proposed by Cagniard

de Latour, Schwann, and Kutzing between -1835 and 1638, that yeasts actively

transform sugar in to alcohol and carbon dioxide. They postulated that in other

fermentations, various microbes formed characteristic end products during their

growth.

Pasteur in 1857: Observed the formation of lactic acid from sugar by

several kinds of bacteria. He noted that a gray deposit in fermentation vessels

consisted of microscopic, very short globules, occurring either singly or in small,

irregular masseses. These globules were much smaller than those of beer yeast.

When they were transferred to a fresh nutrient solution containing sugar, yeast

extract and chalk, lactic acid was produced and the globules increases greatly in

number. Pasteur demonstrated that the presence of globules was a necessary

prerequisite to lactic acid formation. He later showed that in alcoholic, acetic,

butyric and other, fermentations, the typical end product appeared only when a

specific microorganism was present.

A further discovery-in connection with butyric fermentation was that, the

organism responsible grows only in the absence of air. It was when found that

alcoholic fermentation also occurs only in the absence of air, but yeast can grow in

presence of air and, in fact, grows more rapidly arid abundantly with than without

air. However, oxygen is toxic to the butyric bacterium. This was apparently the first

indication that organisms could exist in the complete absence of oxygen - a

revolutionary concept.

The germ theory of fermentation, stating that the microorganisms bring

about specific changes in their substrates laid the foundation of important

industrial developments. The research necessary to prove the germ theory of

fermentation also demonstrated the necessity for strict control of the various

factors associated with the fermentation process: the composition of the

fermenting solution, the identity-and purity of the microbial population, and

incubation conditions such as temperature and aeration.

Fermentation:

Fermentation is defined as the incomplete, oxidation produced by

microorganisms acting on compounds, which for most art- are carbohydrates

or carbohydrate like in nature.

It is a anaerobic .processes, final products being H2O and CO2. Bacteria,

fungi and yeasts can carry out the fermentation.

Bacteria: 1) Lactobacillus 2)

Streptococcus

3) Pseudomonas linineri 4) Sarcina

yehtriculi

Fungi: 1) Aspergillus , 2) Rhiropus 3} Mucor

4) Yeasts: Saccharomyces cervislae

Saccharomyces ellipsoideus

METABOLISM IN BACTERIA

Term metabolism refers to the sum total of all biochemical transformations that occur

in the cells. It is in fact the chemistry of life. This chemistry is generally divided

into two sections:

Anabolism and Catabolism.

1. Anabolism or Biosynthesis: It includes all such transformations that are

involved in the synthesis of organic macromolecules. Making the major portion or

cellular mass. From the simpler compounds, present in the extra cellular

environment. Anabolism is normally energy utilizing process. Energy is used in the

form of ATP.

2. Catabolism: The mechanism of generating ATP is diverse among organisms. The

ATP required for anabolism, may be produced by the process of photosynthesis.

However, chemical energy in the form of inorganic or organic compound (that are

degraded) can also be used to drive biosynthesis. Such process involving the

direct use of chemical energy (obtained from breakdown of chemical compounds-

inorganic or organic) is termed catabolism or degradative metabolism .Thus

catabolism is characterized by the release of energy.

The three important groups of organic compounds involved in metabolism are

carbohydrates, lipids and proteins. The fourth group, the nucleic acids remains

unchanged for long periods of time -in the cell.

Chemotrophs are those organisms that obtain energy by the oxidation of

electron donors in their environments.

These molecules can be organic (chemo organo trophs) or inorganic

(chemolithotrophs). The chemotroph designation is in contrast to

phototrophs, which utilize solar energy.

Chemotrophs can be either autotrophic or heterotrophic.

Chemoautotrophs (Gr: Chemo = chemical, auto = self, troph =

nourishment).

They deriving energy from chemical reactions, synthesize all necessary

organic compounds from carbon dioxide.

Chemoautotrophs use inorganic energy sources, such as hydrogen sulfide,

elemental sulfur, ferrous iron, molecular hydrogen, and ammonia.

Most are bacteria or archaea that live in environments such as deep sea

vents and are the primary producers in such ecosystems.

Chemoautotrophs generally fall into several groups: methanogens,

halophiles, sulfur oxidizers and reducers, nitrifiers, and thermoacidophiles.

Chemoheterotrophs (or chemotrophic heterotrophs) (Gr: Chemo =

chemical, hetero = (an)other, troph = nourishment) are unable to fix

carbon and form their own organic compounds.

Chemoheterotrophs can be chemolithoheterotrophs, utilizing inorganic

energy sources such as sulfur or chemoorganoheterotrophs, utilizing

organic energy sources such as carbohydrates, lipids, and proteins

Phototrophs (Gr: = light, = nourishment) are the organisms that carry

out photon capture to acquire energy.

They use the energy from light to carry out various cellular

metabolic processes.

It is a common misconception that phototrophs are obligatorily

photosynthetic.

Many, but not all, phototrophs often photosynthesize: they

anabolically convert carbon dioxide into organic material to be

utilized structurally, functionally, or as a source for later catabolic

processes (e.g. in the form of starches, sugars and fats).

All phototrophs either use electron transport chains or direct proton

pumping to establish an electro-chemical gradient which is utilized

by ATP synthase, to provide the molecular energy currency for the

cell.

Most of the well-recognized phototrophs are autotrophs, also known

as photoautotrophs, and can fix carbon.

They can be contrasted with chemotrophs that obtain their energy

by the oxidation of electron donors in their environments.

Photoheterotrophs produce ATP through photophosphorylation but

use environmentally obtained organic compounds to build structures

and other bio-molecules.

Photoautotrophic organisms are sometimes referred to as holophytic

Fermentation versus Respiration:

Heterotrophs exhibit two basic strategies, the fermentation and respiration for

oxidizing organic compounds to synthesis ATP from ADP. In fermentation the

organic substrate acts as an. electron acceptor {oxidizing agent). Therefore

both, the electron donor and the acceptor are internal to the organic

substrate. There is no net change in the oxidation state of the products

relative to the starting substrate molecule. The oxidized products are exactly

counterbalanced by the reduced products, and thus the required oxidation-

reduction balance is achieved. The coenzymes,, that are reduced are

reoxidized by its end, so that they are in fact not consumed in the process.

There is no. requirement of oxygen or other electron acceptor.

In contrast to fermentation, respiration requires an external electron

acceptor; that is a molecule other than the one derived from" the electron

donor must act as electron acceptor "(oxidizing agent) to achieve a balance of

oxidation-reduction reactions. These balance of Oxidation Reduction-is thus

also achieved without consumption of co-enzymes. The most common external

electron acceptor in respiration is molecular oxygen-thus called aerobic

respiration. When another molecule as nitrate or sulphate serves as the

terminal electron acceptor, the pathway is called anaerobic respiration.

Fermentation yields far less ATP per substrate molecule than respiration since

same substrate serves both donor and acceptor of electrons. There is no

complete oxidation! The AG° for complete oxidation of glucose to carbon

dioxide and water is 686 kcal/mole, compared to only 58 kcal/mole when

glucose is partially oxidized to two molecules of lactic acid in fermentation.

Depending upon the conditions of growth, an organism used other respiration

or-fermentation catabolic pathways (i:e, respiration-anaerobic or aerobic, OR

fermentation). Therefore three general methods exist by which a carbon arid

energy source can be broken down to provide energy.

1. Aerobic respiration. Respiration can be defined as an ATP generating

metabolic process in which either organic or inorganic compounds serve as

electron donors (become oxidized) and inorganic compounds serve as 'the

ultimate acceptors (become reduced). Usually the ultimate electron acceptor- is

molecular oxygen and this is called aerobic respiration. Thus carbon and energy

source is broken-down by a series of reactions, the oxidation stages occurring at

the expense of oxygen as the terminal electron acceptor. Aerobic respiration is

also simply termed respiration.

In aerobic respiration, sugars are first converted to the key metabolic

intermediate, pyruvic acid. Certain catabolic reaction pathways are common to

both respiration and fermentation. Among these are the three pathways of

conversion of sugars, to pyruvic acid. They are: i) the-EMP (also called the

glycolytic pathway), (ii) the..Pentose phosphate. Pathway, also called .the hexose.

Mono phosphate shunt and (iii) the Entner-Doudoroff pathway. The first two occur

in "many organisms (pro-as well as eukaryotes),.whereas the third is restricted to

some prokaryotes. We shall consider the glycolytic pathway, which is most

common not only in microorganisms but also in plants and animals. Glucose is first

converted to pyruvic acid through glycolysis and this acid is then oxidized to CO2

through the TCA or Krebs' cycle...

BACTERIAL VIRUSES

Viruses are infections agents so small that they can only be seen at

magnifications-provided by the electron microscope.

They are 10 to 100 times smaller than most bacteria. With an approximate

size range 20 to 300 nm.

Thus they pass through, the pores of filters which do not permit the passage of

most bacteria.

Viruses are incapable of independent growth in artificial media.

They can grow only in animal or plant cells or in microorganisms.

They reproduce in these cells by replication (a process In which many copies

or replicas are made of each-viral component and are then assemble to

produce progeny virus),Thus viruses are referred to as obligate intracellular

parasites. (If the least requirement for life is that an organism duplicates

itself, then-viruses may be viewed as microorganisms.

Viruses largely lack metabolic machinery of their own to generate energy or

to synthesize, proteins. They depend on the host cells to carry out these viral

functions. However, like the host cells, viruses have the genetic information for

replication and viruses have information in their genes for usurping the host cell's

energy-generating and protein-synthesizing systems.

Actually, viruses in transit from one host cell to another are small packers of

genes.

The viral genetic material is either DNA or RNA but the virus does not have

both. (Host cells have both DNA and RNA.).

The nucleic acid is enclosed in a highly specialize protein coat of varying

design.

The coat protects the genetic material when the virus is outside of any host

cell and serves as a vehicle for entry into another specific host cell. The

structurally complete mature and infectious virus is called the Virion.

During reproduction in the host cells viruses may cause disease.

In fact, viruses incite the most common acute infectious diseases of humans

{like the "cold" of flu").

And there is growing evidence that they any cause many chronic diseases

as well.

Significantly, all viruses' are generally insensitive to the broad range of available

antibiotics such as penicillin, streptomycin, and others.

From the above discussion of what does or does not constitute a virus,- we

may now attempt a definition for this group of infectious agents. We can define

Defination

Viruses as noncellular infectious entities whose genomes are a nucleic acid either

DNA or RNA; which reproduce, only in living cells: and which use the cells biosynthetic

machinery to direct the/synthesis of specialized particles (virions), which contain

the viral genomes and transfer them efficiently to other cells.

Bacterial viruses, or bacteriophage (or simply phages) have provided the

microbiologist with a model for virology (the study of viruses) and molecular

biology (a discipline which examines the structure, function and organization of

the macromolecules in which biological specificity is encoded);

BACTERIPHAGES: DISCOVERY AND SIGNIFICANCE:

Bacteriophages, viruses that infect bacteria, were discovered independently

by Frederick W. Twort in England in 1915 and by Felix d’Herelle at the Pasteur

Institute in Paris in 1917.

Twort, Observed that bacterial: colonies sometimes underwent lysis (dissolved

and disappeared) and that this lytic effect could be transmitted from colony to

colony. Even high dilutions of material from a lysed colony that had been passed

through a bacterial filter could transmit the lytic effect. However, heating the

filtrate destroyed its lytic property. From these observations Twort cautiously

suggested that the lytic agent might be a virus.

D'Herelle rediscovered this phenomenon in 1947 (hence the term Towrt-

d'Herellle phenomenon) and coined the word bacteriophage, which means

"bacteria eater." He considered the filterable agent to be an invisible microbe for

example a virus that was parasitic for bacteria.

Since the bacterial hosts of phages are easily cultivated under controlled

conditions, demanding relatively little in terms of time, labor and space compared

with the maintenance of plant and animal hosts, Bacteriophages have received

considerable attention in viral research, Furthermore, since Bacteriophages are

the smallest and simplest biological entities known which are capable of self-

replication (making copies of themselves), they have been used widely in genetic

research. Of importance too have. been studies on the bacterium bacteriophage

interaction. Much has been learned about host-parasite ; relationships from these

studies, which have provided a better understanding of plant and animal

infections with viral pathogens. Thus the bacterium-bacteriophage interaction has

become the model system for the study of viral pathogenicity.

GENERAL CHARACTERISTICS

Bacterial viruses are widely distributed in nature.

Phages exist for most, if not all, bacteria.

With the proper techniques, these phages can be isolated quite easily in the

laboratory.

Bacteriophages, like all viruses, are composed of a nucleic acid core

surrounded by a protein coat.

Bacterial viruses occur in different shapes, although many have a tail through

which they inoculate the host cell with viral nucleic acid.

There are two main types of bacterial viruses: lytic, or virulent, and lysogenic.

When lytic phages infect cell, the cells respond, by producing large numbers of

lyses, releasing new phages infect other host cells. This is called a lytic cycle.

In the lysogenic type of infection, the result is not so readily apparent.

The viral nucleic acid is carried and replicated in the host bacterial cells from

one generation to another without any cell lysis. However, lysogenic phages

may spontaneously become virulent "at some subsequent generation and lyse

the host cells.

In addition, there are some filamentous phages which simply "leak" out of cells

without killing them.

MORPHOLOGY AND STRUCTURE:

1. The electron microscope had made it possible to determine the structural

characteristics of bacterial viruses.

2. All phages have a nucleic acid core covered by a protein coat, or capsid.

3. The capsid is made up of morphological subunits (as seen under the

electron microscope) called capsomeres.

4. The capsomeres consist of a number of protein subunits or molecules

called protomers.

Bacterial viruses may be grouped into six morphological types .

A. This most complex type has hexagonal head, a rigid fail with a

contractile

sheath, and tail fibers.

B. Similar to :A, this type has hexagonal, head. However, it lacks a

contractile

sheath, its tail Is flexible, and it may or may not have tail fibers.

C. This type is characterized by a hexagonal head and a tail shorter than

the

head. The tail has no contractile sheath and may or may not have tail

fibers.

D. This type has a head made up of large capsomeres, but has no tail.

E. This type has a head made up of small capsomeres, but has no tail.

F. This type is filamentous.

a. Types A,B, and C show a morphology unique to bacteriophages — The

morphological types in groups D and E are found in plant and animal

(including insect) viruses as well.

b. The filamentous form of group F is found in some plant viruses.

c. Pleomorphic viruses recently discovered to have a lipid-containing

envelope, have no detectable capsid, and possess double - stranded

DNA (ds-DNA). The representative phage is MV-L2.

d. Phage Structure: Most phages occur in one of two structural forms,

having either cubic or helical symmetry. In overall appearance, cubic-

phages are regular solids or, more specifically, polyhedral, (singular,

polyhedron); helical phages are rod-shaped.

2. Polyhedral phages are icosahedral in shape. (The-icosahedron is a regular

polyhedron with 20 triangular facets and 12 vertices.) This means that, the

capsid has 20 facets, each of which is an equilateral triangle; these facets

come together to the form the 12 corners. In the simplest capsid, there is

capsomere at each of the 12 vertices; this capsomere, which is surrounded by

five other capsomeres, is termed a penton.

For example, the phage X174 exhibits the simplest capsid.

In larger and more complex capsid, the triangular facets and subdivided into

a progressively larger number of equilateral triangles.

Thus a capsid may be composed of hundreds of capsomeres but it is still

based on the simple icosahedron model.

The elongated heads of some tailed phages are derivatives of the

icosahedron. For example the head of the T2 and T4 phages is an icosahedron

elongated by one or two extra bands of hexons.

Rod-shaped viruses have their capsomeres arranged helically and not in

stacked rings. An example is the bacteriophage M l3. . . . . .

Some bacteriophages, such as the T elven coliphages (T2.T4, and T6), have

very complex structures, including a head and a tail. They are said to have binal

symmetry be caused each virion has both an icosahedral head and a hollow helical

tail.

PHAGE REPLICATION :

The Bacteriophage can exist in three phases:

(i) as a free particle virion

(ii) in a lysogenic state as a prophage , and

(iii) in the vegetative state (in lytic cycle )

"As a virion, it is inert and cannot reproduce. In the lysogenic state, the

DNA of the phase is integrated within the bacterial DNA and exists Jrif a tion-

infectious form ( prophage ) and replicates in synchrony with the bacterial'

DNA. In the lytic cycle, the phase particle infects the susceptible host,

multiplies and causes the lysis of bacterial cell with concomittent release of

progeny viral particles. Ateo

when the integrated phase is induced to become the vegetative phage the

lytic

cycle follows. Phase that cause lysis are called virulent phages as opposed to

these which can exist in a lysogenic state which are called as temperate

"phases".

Bacteria which carry temperate phages are called lysogenic bacteria and

such

bacteria are immune to super infection by the same phase .

Reproduction (Life cycle):

LIFE CYCLE OF BACTERIOPHAGE

Bacteriophage exhibits two different types of life cycle-

1. Lytic or Virulent cycle

2. Temparate or Avirulent or Lysogenic cycle:

Lytic or Virulent cycle:

In virulent cycle there is intracellular multiplication of phage followed by

the lysis and release of progeny virion. This is called lytic cycle

Temparate or Avirulent or Lysogenic cycle:

In Lysogenic cycle the phage DNA become integrated with the bacterial

genome, replicating without any cell lysis.

LIFE CYCLE: Multiplication of Bacteriophages

The replication of virulent phage was initially using T even numbered

(T2,T4,T6) phage of E.coli.

The multiplication cycle of phage occur in five steps-

1. Attachment or Adsorption

2. Penetration

3. Biosynthesis of phage component

4. Maturation

5. Release of progeny phage particle

Attachment or Adsorption:

The first step in infection of host bacterial cell by phage is adsorption.

Phage particle come into contact with bacterial cell by random collision.

A phage attaches to the surface of the bacterium by the tail.

Adsorption depends on the presence of chemical group called as receptor

on the surface of bacterial cell.

The receptor of bacterial cell is a lipopolysaccharide.

Host specificity of phage its affinity at the adsorption.

Infection of bacterium by naked phage genetic material is known as

transfection.

Penetration:

Attachment is followed by injection of genetic material (nucleic acid /DNA)

in to the bacterial cell .

The phage DNA is injected into the bacterial cell through the hollow core.

Penetration may be enhanced by the presence of phage tail lysozyme with

break small portion of the cell wall for the entry of phage DNA.

After penetration of DNA the empty head and tail of phage remain outside

the bacterial cell is called shell.

If many phages are attached to the bacterial cell multiple holes are

produced on the bacterial cell with the consequent leakages of cell

component.

Bacterial lysis occurs without viral multiplication.

Phage such as T1 and T5 that do not have contractile sheath also inject

their nucleic acid through the cell envelop by adhesion site between the

inner and outer membrane.

Biosynthesis of phage component:

After the infection and penetration of DNA transcription of part of viral

genome produce early mRNA molecules which is translated into a set of

early protein.

These cause the switch off host cell macromolecule synthesis, degrade

the host DNA/ chromosome and start the synthesis of viral components.

Viral DNA replicate and also produce the late mRNA molecule transcribe

from gene which specify the protein of phage coat.

The late messages are translated into subunit of capsid. Rest of the

structure gets condensed to form phage head, tail and tail fibre.

Maturation:

The phage DNA, head and tail protein are synthesized separately in the

bacterial cell.

DNA condense into compact polyhydron and packaged into head and

finally the tail structure are added.

The process of assembly of the phage from its component is called

maturation.

Release of progeny phage particle

By the sudden explosion or breakaging the bacterial cell wall.

o Lysozyme synthesized with in the cell caused the bacterial cell wall to

breakdown and newly produced Bacteriophage are release from the host

cell

VIROIDS:

Viroids constitute a novel class of micro-organisms and are the smallest known agents of infectious diseases So far, viroid are definitely known to exist only in higher plant.

The first viroid was discovered in attempt to purify and characterize the causative agent of potato spindle tuber, a disease that, for many years, had been assumed to be of viral etiology.

In 1967, Diener and Raymer reported that the transmissible agent of this disease was a free RNA and that no viral micro protein particles (virions) were detectable in infected tissue.

Diener, by using sedimentation and gel electrophoresis, had shown conclusively that the infectious RNA was far smaller than the smaller genomes of viruses. No evidence of the involvement of helper viruses in the replication of RNA could be obtained. Despite in small size, the RNA appeared to be replicated autonomously insusceptible cells. Because of the basic difference between the potato spindle tuber disease agent and the conventional viruses, the term viroid wasintroduced by T. O. Diener

Viroids are low molecular weight RNA and represent minimal genetic and

biological system. Viroids, unlike viruses, lack the protective coat protein and are

composed entirely of single stranded covalently close circuler forms of 1ow mw

RNA.

They are not encapsulated like the viral nucleic acids and are present in

certain species of higher plants infected with specific disease.

They are: not detectable in healthy individuals of the same species but when

introduce in to such individuals, they replicate autonomously despite their small

size and produce the characteristic syndrome. All known viroids infect their hosts

in a persistent manner.

In symptom, viroid disease do not differ, significantly from virus disease,

although stunting of plants is a predominant symptom of most of the viriod

diseases. However, stunting is a symptom of many conventional plant viruses also.

Other important symptoms of viriods include- stunting, veinal discolouration, leaf

distoration, vein clearing, localized chlorotic and necrosis spots, mottling leaves ,

necrosis of leaves and death of whole plant.

Viroid infection is known to be responsible for plant diseases such as potato

spindle tuber, citrus exocortis., avocado sun blotch coconut cadang cadahg,

cucumber pale fruit, tomato bunchy top , chrysanthemum stunt, chrysanthemum

chlorotic mottle and hop stunt, etc. Viroid etiology has not yet been demonstrated

definitely in coconut cadang cadang and avocado sun biotch disease,.but viriod like

RNA appears to be associated with these diseases

Viriods can be as small as 246 nucleotides or as large as 400, but their RNA is not

organized in to plant viruses where translation of viral genetic information is

essential for virus replication.

Prions -

Prions are infectious proteins that can reproduce within living systems. They

appear to be proteinacious due to due to their degradation by proteases and

appear to lack nucleic acids due to their resistance to digestion by nucleases. This

new type of sub viral entities were discoursed by Alper, Haig and Clarke (1966), as

causative agent of "Scarpie' disease. The term prion was coined by S. P. Prusiner

who was awarded Nobel Prize in 1997 for the work on the structure and function of

prions.

The prions are considered to be devoid of their own genetic material (DNA or

RNA) and consist of just single or two or three protein molecules. As stated above,

prions were discovered during the search for the cause of scarpie which is an

infectious and fatal disease of sheep and goat. It is characterized by a wild, facial

expression, nervousness, twitching of the neck and head, grinding of the teeth, and

scrapping of portion of the skin against rocks with subsequent loss of wool in

sheep.

The prions, at present, are thought to be the causative agents of following

diseases.

Sr. Disease Natural host1 Scarpie Sheep & goat2 Bovine spongiform encephalopathy Cattle (Mad cow Disease)3 Parkirison's diseases Human4 Multiple Sclerosis Human

Structure:

Prions are hundred times smaller than smallest viruses, contain only protein

and can reproduce in the living cells maintaining their own identity They are

heterogeneous in size and density and can exist in many molecular forms gel

electrophoresis investigations have revealed that prion possess an apparent mw of

between 27,000 to 30,000. Electron microscopic studies have shown that a large

number of "prion molecules (approx. 1000) aggregate together to form a composite

structure called, prion rods, ' '

Chemical Nature:

The chemical nature of prions, as stated above, is considered to be

proteinacious and they have no nucleic acids of their own. It has been observed that

nucleases have no effect on prion infectivity whereas proteases can drastically

reduce prion infectivity.

Replication:

If prion in fact does not contain nucleic acids, their ability to replicate would

seem to pose a challenge to the dogma of molecular biology. However, a recent

hypothesis states that the existence of small piece of 'DNA genes' (also called Prp

genes) is necessary to encode the amino acids sequence of prion protein at the

time of its replication. Therefore; appears has 'Prp gene' is component of the host

genetic material (host DNA) that remains closely associated with scarpie agent

infectivity during extensive purification,

Lysogenic' cycle: Lysogeny is a process in which the viral nucleic acid does not

usurp the functions of the host bacterium's synthetic processes but becomes an

integral part of the bacterial chromosome. As the bacterium reproduces, viral

nucleic acid is transmitted to the daughter cells at each cell division. In the

lysogenic state the virus Is simple one of the bacterial genes. Under certain natural

conditions or artificial stimuli (such as exposure to ultraviolet light), the synthesis of

virus may. take over, and lyses occurs.

Bacterial chromosomee

Fig :Mechanism of lysogeny

Lysogenic cycle

Infective .vir

us

SOIL MICROBIOLOGY

Introduction:

The development of Science Soil Microbiology is very recent It

relates to study of different microorganirns in soil and their different

activities and their influence on the crop growth, the, large variety of

micro-organisms inhabit the soil of which the bacteria covers about

50 % of the, total population, followed by actinomycetes, and the fungi

ranked 3rd as the population of soil micro-organisms is concerned.

Soil :It is the most dynamic site of biological interactions which

contains a vast population of bacteria, actinomycetes, fungi, algae,

protozoa and other microorganisms.

OR

It is the outer covering of the earth which consists of loosely

arranged layers of materials composed of inorganic, and organic

constituents in different- stages of organization.

Soil Microbiology is the branch of science which deals with the study of

different soil microorganisms like bacteria, actinomycetes,fungi others

which are beneficial for plant growth and nutrition.

Physical characteristics of soil :

.. 1) Mineral matter 2) Organic matter

3) Soil water 4) Soil air

5) Microbial flora

Microbial flora of soil :

A) Bacteria: Bacteria are the most numerous and form about 50 % of total

microbial flora or microbial biomass. The DMCs as high as one to several billions

per gram have been recorded. The plate counts, however, yield a very less total

count because of certain limitations of the method, in general, the population of

bacteria decreases with depth of the soil. The bacteria of -all shapes like cocci,

bacilli and spirilli live in the soil. Winogradsky classified soil bacteria into 2

groups

viz., autochthonous and zymogenous. . :

Autochthonus bacteria: These' are the indigenous or native bacteria in the soil

which are, present in more or less constant numbers. They thrive on the native

organic matter and nutrients in soil. Ex. Nocardia, Arthrobacter.

Zymogenous bacteria: These are the fermentative bacteria present in soil.

Their normal population in soil is low. They require a source of energy; their

number increases when the substrate is added to soil and again decreases when

the substrate is exhausted Eg Pseudomonas, Bacillus, Clostridium.

There is a great variety of nutritional and physiological types of bacteria in

the soil. It includes aerobes, anaerobes, facultative and microaerophilic bacteria

the psychrophilic, mesophilic, and thermophilic bacteria, autotrops and

heterotrophs cellulose decomposers. S oxidizer, N fixers, P solubilzers, protein

digester and other.

Actinomycetes:

These are next numerous to bacteria in dry, warm soil. The

predominent

NITROGEN FIXATION

A number of microorganism are able to use molecular nitrogen from the

atmosphere as their source of Nitrogen. The conversion of molecular

nitrogen into Ammonia is known as Nitrogen fixation.

There are two types of biological Nitrogen fixation involving different group

of Microorganisms.

SYMBIOTIC NITROGEN FIXATION

It involve the bacteria which lives in the root of plant having symbiotic

association with the later.

Nitrogen Fixing Bacteria Leguminous plants

Rhizobium melitoli Alfa Alfalfa

Rhizobium leguminosarum Peas

Rhizobium japonicum Soyabean

NON- SYMBIOTIC NITROGEN FIXATION

It involve the microorganism which are live freely and independently in soil.

Aerobic

Autotrophic Heterotrophic

The sequence of changes from atmospheric nitrogen to fixed inorganic

nitrogen to simple organic compound to complex in the tissue and release of

this nitrogen back to atmospheric nitrogen is known as Nitrogen cycle.

SOIL MICROBIOLOGY

Definition:

It is branch of science/microbiology which deals with study of soil microorganisms and

their activities in the soil.

Soil:

It is the outer, loose material of earth’s surface which is distinctly different from the

underlying bedrock and the region which support plant life. Agriculturally, soil is the

region which supports the plant life by providing mechanical support and nutrients

required for growth. From the microbiologist view point, soil is one of the most dynamic

sites of biological interactions in the nature.

It is the region where most of the physical, biological and biochemical reactions related

to decomposition of organic weathering of parent rock take place.

Components of Soil:

Soil is an admixture of five major components viz. organic matter, mineral matter, soil-

air, soil water and soil microorganisms/living organisms. The amount/ proposition of

these components varies with locality and climate.

1. Mineral / Inorganic Matter: It is derived from parent rocks/bed rocks through

decomposition, disintegration and weathering process. Different types of inorganic

compounds containing various minerals are present in soil. Amongst them the dominant

minerals are Silicon, Aluminum and iron and others like Carbon, Calcium Potassium,

Manganese, Sodium, Sulphur, Phosphorus etc. are in trace amount. The proportion of

mineral matter in soil is slightly less than half of the total volume of the soil.

2. Organic matter/components: Derived from organic residues of plants and animals

added in the soil. Organic matter serves not only as a source of food for microorganisms

but also supplies energy for the vital processes of metabolism which are characteristics

of all living organisms. Organic matter in the soil is the potential source of N, P and S for

plant growth. Microbial decomposition of organic matter releases the unavailable

nutrients in available from. The proportion of organic matter in the soil ranges from 3-

6% of the total volume of soil.

3. Soil Water: The amount of water present in soil varies considerably. Soil water

comes from rain, snow, dew or irrigation. Soil water serves as a solvent and carrier of

nutrients for the plant growth. The microorganisms inhabiting in the soil also require

water for their metabolic activities. Soil water thus, indirectly affects plant growth

through its effects on soil and microorganisms. Percentage of soil-water is 25% total

volume of soil.

4. Soil air (Soil gases): A part of the soil volume which is not occupied by soil particles

i.e. pore spaces are filled partly with soil water and partly with soil air. These two

components (water & air) together only accounts for approximately half the soil's

volume. Compared with atmospheric air, soil is lower in oxygen and higher in carbon

dioxide, because CO2 is continuous recycled by the microorganisms during the process

of decomposition of

organic matter. Soil air comes from external atmosphere and contains nitrogen, oxygen

Co2 and water vapour (CO2 >oxygen). Co2 in soil air (0.3-1.0%) is more than

atmospheric

air (0.03%). Soil aeration plays important role in plant growth, microbial population, and

microbial activities in the soil.

5. Soil microorganisms: Soil is an excellent culture media for the growth and

development of various microorganisms. Soil is not an inert static material but a

medium pulsating with life. Soil is now believed to be dynamic or living system.

Soil contains several distinct groups of microorganisms and amongst them bacteria,

fungi, actinomycetes, algae, protozoa and viruses are the most important. But bacteria

are more numerous than any other kinds of microorganisms.

Microorganisms form a very small fraction of the soil mass and occupy a volume of less

than one percent. In the upper layer of soil (top soil up to 10-30 cm depth i.e. Horizon

A), the microbial population is very high which decreases with depth of soil. Each

organisms or a group of organisms are responsible for a specific change /

transformation in the soil. The final effect of various activities of microorganisms in the

soil is to make the soil fit for

the growth & development of higher plants.

Living organisms present in the soil are grouped into two categories as

follows.

1. Soil flora (micro flora) e.g. Bacteria, fungi, Actinomycetes, Algae and

2. Soil fauna (micro fauna) animal like eg. Protozoa, Nematodes, earthworms, moles,

ants, rodents.

Relative proportion / percentage of various soil microorganisms are:

Bacteria-aerobic (70%), anaerobic (13 %), Actinomycetes (13%), Fungi /molds (03 %)

and others (Algae Protozoa viruses) 0.2-0.8 %.

Soil organisms play key role in the nutrient transformations.

Scope and Importance of Soil Microbiology

Living organisms both plant and animal types constitute an important component of

soil. Though these organisms form only a fraction (less than one percent) of the total

soil mass, but they play important role in supporting plant communities on the earth

surface. While studying the scope and importance of soil microbiology, soil-plant-animal

ecosystem as such must be taken into account. Therefore, the scope and importance of

soil

microbiology, can be understood in better way by studying aspects like

1. Soil as a living system

2. Soil microbes and plant growth

3. Soil microorganisms and soil structure

4. Organic matter decomposition

5. Humus formation

6. Biogeochemical cycling of elements

7. Soil microorganisms as bio-control agents

8. Soil microbes and seed germination

9. Biological N2 fixation

10. Degradation of pesticides in soil.

1. Soil as a living system: Soil inhabit diverse group of living organisms, both micro

flora (fungi, bacteria, algae and actinomycetes) and micro-fauna (protozoa, nematodes,

earthworms, moles, ants). The density of living organisms in soil is very high i.e. as

much as billions / gm of soil, usually density of organisms is less in cultivated soil than

uncultivated / virgin land and population decreases with soil acidity. Top soil, the

surface

layer contains greater number of microorganisms because it is well supplied with

Oxygen and nutrients. Lower layer / subsoil is depleted with Oxygen and nutrients

hence it contains fewer organisms. Soil ecosystem comprises of organisms which are

both, autotrophs (Algae, BOA) and heterotrophs (fungi, bacteria).

Autotrophs use inorganic carbon from CO2 and are "primary producers" of organic

matter, whereas heterotrophs use organic carbon and are decomposers/consumers.

2. Soil microbes and plant growth: Microorganisms being minute and microscopic,

they are universally present in soil, water and air. Besides supporting the growth of

various biological systems, soil and soil microbes serve as a best medium for plant

growth. Soil fauna & flora convert complex organic nutrients into simpler inorganic

forms which are readily absorbed by the plant for growth. Further, they produce variety

of substances like IAA, gibberellins, antibiotics etc. which directly or indirectly promote

the plant growth

3. Soil microbes and soil structure: Soil structure is dependent on stable aggregates

of soil particles-Soil organisms play important role in soil aggregation. Constituents of

soil are viz. organic matter, polysaccharides, lignins and gums, synthesized by soil

microbes plays important role in cementing / binding of soil particles. Further, cells and

mycelial strands of fungi and actinomycetes, Vormicasts from earthworm is also found

to play important role in soil aggregation. Different soil microorganisms, having soil

aggregation / soil binding properties are graded in the order as fungi > actinomycetes >

gum

producing bacteria > yeasts.

Examples are: Fungi like Rhizopus, Mucor, Chaetomium, Fusarium, Cladasporium,

Rhizoctonia, Aspergillus, Trichoderma and Bacteria like Azotobacler, Rhizobium Bacillus

and

Xanlhomonas.

4. Soil microbes and organic matter decomposition: The organic matter serves

not only as a source of food for microorganisms but also supplies energy for the vital

processes of metabolism that are characteristics of living beings.

Microorganisms such as fungi, actinomycetes, bacteria, protozoa etc. and macro

organisms such as earthworms, termites, insects etc. plays important role in the

process of decomposition of organic matter and release of plant nutrients in soil. Thus,

organic

matter added to the soil is converted by oxidative decomposition to simpler nutrients /

substances for plant growth and the residue is transformed into humus. Organic

matter / substances include cellulose, lignins and proteins (in cell wall of plants),

glycogen

(animal tissues), proteins and fats (plants, animals).

Cellulose is degraded by bacteria, especially those of genus Cytophaga and other

genera (Bacillus, Pseudomonas, Cellulomonas, and Vibrio Achromobacter) and fungal

genera (Aspergillus, Penicilliun, Trichoderma, Chactomium, Curvularia).

Lignins and proteins are partially digested by fungi, protozoa and nematodes.

Proteins are degraded to individual amino acids mainly by fungi, actinomycetes and

Clostridium. Under unaerobic conditions of waterlogged soils, methane are main carbon

containing product which is produced by the bacterial genera (strict anaerobes)

Methanococcus, Methanobacterium and Methanosardna.

5. Soil microbes and humus formation: Humus is the organic residue in the soil

resulting from decomposition of plant and animal residues in soil, or it is the highly

complex organic residual matter in soil which is not readily degraded by microorganism,

or

it is the soft brown/dark coloured amorphous substance composed of residual organic

matter along with dead microorganisms.

6. Soil microbes and cycling of elements: Life on earth is dependent on cycling of

elements from their organic / elemental state to inorganic compounds, then to organic

compounds and back to their elemental states. The biogeochemical process through

which organic compounds are broken down to inorganic compounds or their constituent

elements is known “Mineralization”, or microbial conversion of complex organic

compounds into simple inorganic compounds & their constituent elements is known as

mineralization.

Soil microbes plays important role in the biochemical cycling of elements in the

biosphere where the essential elements (C, P, S, N & Iron etc.) undergo chemical

transformations. Through the process of mineralization organic carbon, nitrogen,

phosphorus, Sulphur, Iron etc. are made available for reuse by plants.

7. Soil microbes and biological N2 fixation: Conversion of atmospheric nitrogen in

to ammonia and nitrate by microorganisms is known as biological nitrogen fixation.

Fixation of atmospheric nitrogen is essential because of the reasons:

1. Fixed nitrogen is lost through the process of nitrogen cycle through denitrification.

2. Demand for fixed nitrogen by the biosphere always exceeds its availability.

3. The amount of nitrogen fixed chemically and lightning process is very less (i.e. 0.5%)

as compared to biologically fixed nitrogen 4. Nitrogenous fertilizers contribute only 25%

of the total world requirement while biological nitrogen fixation contributes about 60%

of the earth's fixed nitrogen

5. Manufacture of nitrogenous fertilizers by "Haber" process is costly and time

consuming.

The numbers of soil microorganisms carry out the process of biological nitrogen fixation

at normal atmospheric pressure (1atmosphere) and temp (around 20 °C).

Two groups of microorganisms are involved in the process of BNF.

A. Non-symbiotic (free living) and B. Symbiotic (Associative)

Non-symbiotic (free living): Depending upon the presence or absence of oxygen,

non symbiotic N2 fixation prokaryotic organisms may be aerobic heterotrophs

(Azotobacter,

Pseudomonas, Achromobacter) or aerobic autotrophs (Nostoc, Anabena, Calothrix, BGA)

and anaerobic heterotrophs (Clostridium, Kelbsiella. Desulfovibrio) or anaerobic

Autotrophs

(Chlorobium, Chromnatium, Rhodospirillum, Meihanobacterium etc)

Symbiotic (Associative): The organisms involved are Rhizobium, Bratfyrhizobium in

legumes (aerobic): Azospirillum (grasses), Actinonycetes frantic(with Casuarinas, Alder).

8. Soil microbes as biocontrol agents: Several ecofriendly bioformulations of

microbial origin are used in agriculture for the effective management of plant diseases,

insect pests, weeds etc.

eg: Trichoderma sp and Gleocladium sp are used for biological control of seed and soil

borne diseases.

Fungal genera

Entomophthora, Beauveria, Metarrhizium and protozoa Maltesia grandis. Malameba

locustiae etc are used in the management of insect pests. Nuclear polyhydrosis virus

(NPV) is used for the control of Heliothis / American boll worm.

Bacteria like Bacillus thuringiensis, Pseudomonas are used in cotton against Angular

leaf spot and boll worms.

8. Degradation of pesticides in soil by microorganisms: Soil receives different

toxic chemicals in various forms and causes adverse effects on beneficial soil micro

flora / micro fauna, plants, animals and human beings. Various microbes present in soil

act as the scavengers of these harmful chemicals in soil. The pesticides/chemicals

reaching the soil are acted upon by several physical, chemical and biological forces

exerted by microbes in

the soil and they are degraded into non-toxic substances and thereby minimize the

damage caused by the pesticides to the ecosystem.

For example, bacterial genera like Pseudomonas, Clostridium, Bacillus, Thiobacillus,

Achromobacter etc. and

Fungal genera like Trichoderma, Penicillium, Aspergillus, Rhizopus, and Fusarium are

playing important role in the degradation of the toxic chemicals / pesticides in soil.

9. Biodegradation of hydrocarbons: Natural hydrocarbons in soil like waxes,

paraffin’s, oils etc are degraded by fungi, bacteria and actinomycetes. E.g. ethane (C2

H6) a paraffin hydrocarbon is metabolized and degraded by Mycobacteria, Nocardia,

Streptomyces Pseudomonas, Flavobacterium and several fungi.

Soil Microorganism: Bacteria

Amongst the different microorganisms inhabiting in the soil, bacteria are the

most abundant and predominant organisms.

These are primitive, prokaryotic, microscopic and unicellular microorganisms

without chlorophyll.

Morphologically, soil bacteria are divided into three groups viz Cocci

(round/spherical), (rodshaped) and Spirilla I Spirllum (cells with long wavy

chains).

Bacilli are most numerous followed by Cocci and Spirilla in soil.

The most common method used for isolation of soil bacteria is the "dilution plate

count" method which allows the enumeration of only viable/living cells in the soil.

The size of soil bacteria varies from 0.5 to 1.0 micron in diameter and 1.0 to 10.0

microns in length. They are motile with locomotory organs flagella.

Bacterial population is one-half of the total microbial biomass in the soil ranging

from 1,00000 to several hundred millions per gram of soil, depending upon the

physical, chemical and biological conditions of the soil.

Winogradsky (1925), on the basis of ecological characteristics classified soil

microorganisms in general and bacteria in particular into two broad categories i.e.

Autochnotus (Indigenous species) and the Zymogenous (fermentative).

Autochnotus bacterial population is uniform and constant in soil, since their nutrition is

derived from native soil organic matter (eg. Arthrobacter and Nocardia whereas

Zymogenous bacterial population in soil is low, as they require an external source of

energy, eg. Pseudomonas & Bacillus.

The population of Zymogenous bacteria increases gradually when a specific substrate is

added to the soil. To this category belong the cellulose decomposers, nitrogen utilizing

bacteria and ammonifiers.

As per the system proposed in the Bergey's Manual of Systematic Bacteriology, most of

the bacteria which are predominantly encountered in soil are taxonomically included in

the three orders, Pseudomonadales, Eubacteriales and Actinomycetales of the class

Schizomycetes.

The most common soil bacteria belong to the genera Pseudomonas, Arthrobacter,

Clostridium Achromobacter, Sarcina, Enterobacter etc.

The another group of bacteria common in soils is the Myxobacteria belonging to the

genera Micrococcus, Chondrococcus, Archangium, Polyangium, Cyptophaga.

Bacteria are also classified on the basis of physiological activity or mode of nutrition,

especially the manner in which they obtain their carbon, nitrogen, energy and other

nutrient requirements.

They are broadly divided into two groups i.e. a) Autotrophs and b) Heterotrophs

1. Autotrophic bacteria are capable synthesizing their food from simple inorganic

nutrients, while heterotrophic bacteria depend on pre-formed food for nutrition. All

autotrophic bacteria utilize Co2 (from atmosphere) as carbon source and derive energy

either from sunlight (photoautotrophs, eg. Chromatrum. Chlorobium.

Rhadopseudomonas or from the oxidation of simple inorganic substances present in soil

(chemoautotrophs eg. Nitrobacter, Nitrosomonas, Thiaobacillus).

2. Majority of soil bacteria are heterotrophic in nature and derive their carbon and

energy from complex organic substances/organic matter, decaying roots and plant

residues. They obtain their nitrogen from nitrates and ammonia compounds (proteins)

present in soil and other nutrients from soil or from the decomposing organic matter.

Certain bacteria also require amino acids, B- Vitamins, and other growth promoting

substances also.

Functions / Role of Bacteria:

Bacteria bring about a number of changes and biochemical

transformations in the soil and thereby directly or indirectly help

in the nutrition of higher plants growing in the soil. The important

transformations and processes in which soil bacteria play vital

role are: decomposition of cellulose and other carbohydrates,

ammonification (proteins ammonia), nitrification (ammonianitrites-

nitrates), denitrification (release of free elemental

nitrogen), biological fixation of atmospheric nitrogen (symbiotic

and non-symbiotic) oxidation and reduction of sulphur and iron

compounds. All these processes play a significant role in plant

nutrition,

Process/reaction Bacterial genera

Cellulose decomposition (celluloytic bacteria ) most cellulose decomposers are

mesophilic

a. Aerobic : Angiococcus, Cytophaga, Polyangium, Sporocytophyga, Bacillus,

Achromobacter, Cellulomonas

b. anaerobic: Clostridium Methanosarcina, Methanococcus Ammonification Bacillus,

Pseudomonas

(Ammonifiers)

Nitrification (Nitrifying bacteria)

Nitrosomonas, Nilrobacter Nitrosococcus

Denitrification (Denitrifies) Achromobacter, Pseudomonas, Bacillus, Micrococcus

Nitrogen fixing bacteria a Symbiotic- Rhizobium, Bradyrrhizobium

b Non-symbiotic: aerobic –

Azotobacter Beijerinckia (acidic soils), anaerobic-Clostridium

Bacteria capable of degrading various plant residues in soil are :

Cellulose

Hemicelluloses

Lignin Pectin Proteins

Pseudomon as

Bacillus Pseudomonas

Erwinia Clostridium

Cytophaya Vibrio Micrococcus Proteus

Spirillum Pseudomonas

Flavobacteriumm

Pseudomonas

Actinomycetes

Erwinia Xanthomonas

Bacillus Cellulomona s

Streptomyc es

Soil Microorganism – Actinomycetes

These are the organisms with characteristics common to both bacteria and fungi but yet

possessing distinctive features to delimit them into a distinct category. In the strict

taxonomic sense, actinomycetes are clubbed with bacteria the same class of

Schizomycetes and confined to the order Actinomycetales.

They are unicellular like bacteria, but produce a mycelium which is non-septate

(coenocytic) and more slender, tike true bacteria they do not have distinct cell-wall and

their cell wall is without chitin and cellulose (commonly found in the cell wall of fungi).

On

culture media unlike slimy distinct colonies of true bacteria which grow quickly,

actinomycetes colonies grow slowly, show powdery consistency and stick firmly to agar

surface. They produce hyphae and conidia / sporangia like fungi. Certain actinomycetes

whose hyphae undergo segmentation resemble bacteria, both morphologically and

physiologically.

Actinomycetes are numerous and widely distributed in soil and are next to bacteria in

abundance. They are widely distributed in the soil, compost etc. Plate count estimates

give values ranging from 10^4 to 10^8 per gram of soil. They are sensitive to

acidity / low PH (optimum PH range 6.5 to 8.0) and waterlogged

soil conditions. The population of actinomycetes increases with

depth of soil even up to horizon ‘C’ of a soil profiler They are

heterotrophic, aerobic and mesophilic (25-30 ^c) organisms and

some species are commonly present in compost and manures are

thermophilic growing at 55-65° c temperature (eg.

Thermoatinomycetes, Streptomyces).

Actinomycetes belonging to the order of Actinomycetales are

grouped under four families viz Mycobacteriaceae,

Actinomycetaceae, Streptomycetaceae and Actinoplanaceae.

Actinomycetous genera which are agriculturally and industrially

important are present in only two families of Actinomycetaceae

and Strepotmycetaceae.

In the order of abundance in soils, the common genera of

actinomycetes are Streptomyces (nearly 70%), Nocardia and

Micromonospora although Actinomycetes, Actinoplanes,

Micromonospora and Streptosporangium are also generally

encountered.

Functions / Role of actinomycetes:

1. Degrade/decompose all sorts of organic substances like

cellulose, polysaccharides, protein fats, organic-acids etc.

2. Organic residues / substances added soil are first attacked by

bacteria and fungi and later by actinomycetes, because they are

slow in activity and growth than bacteria and fungi.

3. They decompose / degrade the more resistant and

indecomposable organic substance/matter and produce a number

of dark black to brown pigments which contribute to the dark

colour of soil humus.

4. They are also responsible for subsequent further decomposition

of humus (resistant material) in soil.

5. They are responsible for earthy / musty odor / smell of freshly

ploughed soils.

6. Many genera species and strains (eg. Streptomyces if

actinomycetes produce/synthesize number of antibiotics like

Streptomycin, Terramycin, Aureomycin etc.

7. One of the species of actinomycetes Streptomyces scabies

causes disease "Potato scab" in potato.

Soil Microorganism – Fungi

Fungi in soil are present as mycelial bits, rhizomorph or as

different spores. Their number varies from a few thousand to a

few -million per gram of soil. Soil fungi possess filamentous

mycelium composed of individual hyphae. The fungal hyphae may

be aseptate /coenocytic (Mastigomycotina and Zygomycotina) or

septate (Ascomycotina, Basidiomycotina & Deuteromycotina).

As observed by C.K. Jackson (1975), most commonly encountered

genera of fungi in soil are; Alternaria, Aspergillus, Cladosporium,

Cephalosporium Botrytis, Chaetomium, Fusarium, Mucor,

Penicillium, Verticillium, Trichoderma, Rhizopus, Gliocladium,

Monilia, Pythium, etc. Most of these fungal genera belong to the

subdivision Deuteromycotina / Fungi imperfeacta which lacks

sexual mode of reproduction.

As these soil fungi are aerobic and heterotrophic, they require

abundant supply of oxygen and organic matter in soil. Fungi are

dominant in acid soils, because acidic environment is not

conducive / suitable for the existence of either bacteria or

actinomycetes. The optimum PH range for fungi lies-between 4.5

to 6.5. They are also present in neutral and alkaline soils and

some can even tolerate PH beyond 9.0

Functions / Role of Fungi

1. Fungi plays significant role in soils and plant nutrition.

2. They plays important role in the degradation / decomposition of

cellulose, hemi cellulose, starch, pectin, lignin in the organic

matter added to the soil.

3. Lignin which is resistant to decomposition by bacteria is mainly

decomposed by fungi.

4. They also serve as food for bacteria.

5. Certain fungi belonging to sub-division Zygomycotina and

Deuteromycotina are predaceous in nature and attack on

protozoa & nematodes in soil and thus, maintain biological

equilibrium in soil.

6. They also plays important role in soil aggregation and in the

formation of humus.

7. Some soil fungi are parasitic and causes number of plant

diseases such as wilts, root rots, damping-off and seedling blights

eg. Pythium, Phyiophlhora, Fusarium, Verticillium etc.

8. Number of soil fungi forms mycorrhizal association with the

roots of higher plants (symbiotic association of a fungus with the

roots of a higher plant) and helps in mobilization of soil

phosphorus and nitrogen eg. Glomus, Gigaspora, Aculospora,

(Endomycorrhiza) and Amanita, Boletus, Entoloma, Lactarius

(Ectomycorrhiza).

Soil Microorganism – Algae

Algae are present in most of the soils where moisture and

sunlight are available. Their number in soil usually ranges from

100 to 10,000 per gram of soil. They are photoautotrophic,

aerobic organisms and obtain CO2 from atmosphere and energy

from sunlight and synthesize their own food. They are unicellular,

filamentous or colonial. Soil algae are divided in to four main

classes or phyla as follows:

1. Cyanophyta (Blue-green algae)

2. Chlorophyta (Grass-green algae)

3. Xanthophyta (Yellow-green algae)

4. Bacillariophyta (diatoms or golden-brown algae)

Out of these four classes / phyla, blue-green algae and grassgreen

algae are more abundant in soil. The green-grass algae and

diatoms are dominant in the soils of temperate region while bluegreen

algae predominate in tropical soils. Green-algae prefer acid

soils while blue green algae are commonly found in neutral and

alkaline soils. The most common genera of green algae found in

soil are: Chlorella, Chlamydomonas, Chlorococcum, Protosiphon

etc. and that of diatoms are Navicula, Pinnularia. Synedra,

Frangilaria.

Blue green algae are unicellular, photoautotrophic prokaryotes

containing Phycocyanin pigment in addition to chlorophyll. They

do not posses flagella and do not reproduce sexually. They are

common in neutral to alkaline soils. The dominant genera of BGA

in soil are: Chrococcus, Phormidium, Anabaena, Aphanocapra,

Oscillatoria etc. Some BGA posses specialized cells know as

"Heterocyst" which is the sites of nitrogen fixation. BGA fixes

nitrogen (non-symbiotically) in puddle paddy/water logged paddy

fields (20-30 kg/ha/season). There are certain BGA which possess

the character of symbiotic nitrogen fixation in association with

other organisms like fungi, mosses, liverworts and aquatic ferns

Azolla, eg Anabaena-Azolla association fix nitrogen symbiotically

in rice fields.

Functions / role of algae or BGA:

1. Plays important role in the maintenance of soil fertility

especially in tropical soils.

2. Add organic matter to soil when die and thus increase the

amount of organic carbon in soil.

3. Most of soil algae (especially BGA) act as cementing agent in

binding soil particles and thereby reduce/prevent soil erosion.

4. Mucilage secreted by the BGA is hygroscopic in nature and thus

helps in increasing water retention capacity of soil for longer

time/period.

5. Soil algae through the process of photosynthesis liberate large

quantity of oxygen in the soil environment and thus facilitate the

aeration in submerged soils or oxygenate the soil environment.

6. They help in checking the loss of nitrates through leaching and

drainage especially in un-cropped soils.

7. They help in weathering of rocks and building up of soil

structure.

Soil Microorganism – Protozoa

These are unicellular, eukaryotic, colourless, and animal like

organisms (Animal kingdom). They are larger than bacteria and

size varying from few microns to a few centimeters. Their

population in arable soil ranges from l0,000 to 1,00,000 per gram

of soil and are abundant in surface soil. They can withstand

adverse soil conditions as they are characterized by "cyst stage"

in their life cycle. Except few genera which reproduce sexually by

fusion of cells, rest of them reproduces asexually by fission /

binary fission. Most of the soil protozoa are motile by flagella or

cilia or pseudopodia as locomotors organs. Depending upon the

type of appendages provided for locomotion, protozoa are

1. Rhizopoda (Sarcondia)

2. Mastigophora

3. Ciliophora (Ciliata)

4. Sporophora (not common Inhabitants of soil)

Class-Rhizopoda: Consists protozoa without appendages usually

have naked protoplasm without cell-wall, pseudopodia as

temporary locomotory organs are present some times. Important

genera are Amoeba, Biomyxa, Euglypha, etc.

Class Mastigophora: Belongs flagellated protozoa, which are

predominant in soil. Important genera are: Allention, Bodo,

Cercobodo, Cercomonas, Entosiphon Spiromonas, Spongomions

and Testramitus. Many members are saprophytic and some

posses chlorophyll and are autotrophic in nature. In this respect,

they resemble unicellular algae and hence are known as

"Phytoflagellates".

The soil protozoa belonging to the class ciliate / ciliophora are

characterized by the presence of cilia (short hair-like appendages)

around their body, which helps in locomotion. The important soil

inhabitants of this class are Colpidium, Colpoda, Balantiophorus,

Gastrostyla, Halteria, Uroleptus, Vortiicella, Pleurotricha etc.

Protozoa are abundant in the upper layer (15 cm) of soil. Organic

manures protozoa. Soil moisture, aeration, temperature and PH

are the important factors affecting soil protozoa.

Function / Role of Protozoa

1. Most of protozoans derive their nutrition by feeding or

ingesting soil bacteria belonging to the genera Enterobacter,

Agrobacterium, Bacillus, Escherichia, Micrococcus, and

Pseudomonas and thus, they play important role in maintaining

microbial / bacterial equilibrium in the soil.

2. Some protozoa have been recently used as biological control

agents against phytopathogens.

3. Species of the bacterial genera viz. Enterobacter and

Aerobacter are commonly used as the food base for isolation and

enumeration of soil protozoans.

4. Several soil protozoa cause diseases in human beings which

are carried through water and other vectors, eg. Amoebic

dysentery caused by Entomobea histolytica.

STEP INVOLVED IN NITROGEN CYCLE ARE

1. NITROGEN FIXATION: Nitrogen to Ammonia

a) Symbiotic Nitrogen Fixation

b) Non- Symbiotic Nitrogen Fixation

2. PROTEOLYSIS: Breakdown of complex organic nitrogen compound into

amino acid.

3. AMMONIFICATION: Breakdown of amino acid to ammonia

4. NITRIFICATION: Conversion of ammonia to nitrate

5. DENITRIFICATION: Nitrate to atmospheric nitrogen

NITROGEN FIXATION

There are large amount or number of microorganism which are able to use

molecular nitrogen present in the atmosphere as their source of nitrogen.

This conversion of molecular nitrogen to ammonia is known as nitrogen

fixation.

The atmospheric nitrogen is thus first fixed to ammonia which is inorganic

nitrogen compound. This ammonia is now converted to amino acid which is

simple organic nitrogen and later get converted into protein, which is

complex organic nitrogen compound, which are eaten up by animals which

increases the protein. After the death of plants and animals also the excreta

and waste in the soil get decomposed by soil microorganism.

PROTEOLYSIS

The proteolysis is defined as an enzymatic hydrolysis of protein by

proteinase to peptide and to amino acid by peptidase.

The nitrogen in the protein is locked and is not available as a nutrient to

plant. The first process that must take place in the enzymatic hydrolysis of

protein.

This is achieved by the microorganism which is capable of seceraating

extracellular proteinase that converts protein to peptide to amino acid by

peptidase.

Protein-------------------peptides---------------------------amino acid

AMMONIFICATION

The end product of proteolysis are amino acid. The amino acid are further

attached by microorganism leading to liberation of ammonia by a process

known as deamination.

Alanine + O2 --------------------------------------- Pyruvic acid + NH3

This reaction is classified as an oxidative deamination and the production

of ammonia is referred to as ammonification.

The ammonia thus formed being volatile may be lost in the air, however if

solubilised in water its form NH4+.

The ammonium ion may be accumulated in the soil and becomes available

to plant and under favorable condition may oxidized to nitrate.

NITRIFICATION:

The oxidation of ammonia to nitrite (NO2) and further oxidation of nitrite

to nitrate (NO3) is called nitrification. Winogradsky studied the nitrification

process and stated that rt occurs in 2 steps by the Gram -ve

chemoautotroph, (chemolithotrophic) bacteria.

,a) Oxidation of ammonia (NH3) to nitrite (NO2) : Some bacteria

like

Nitrosornonas eurppea, Nitrosovibrio tenuiq, Nitrosococcus nitrosus, Nitrosdf?

occus oceanus, etc; oxidize ammonia to nitrites.

2NH3 + 3O2 ----------------------- 2HNO2 +2H20

b) Oxidation of nitrite (NO2) to nitrate(NO3): Some bacteria like

Nitrobacter winogradskyi, Nitrospina gracilis, etc. further oxidize the nitrites

to nitrates.

HNO2 + 1/a O2 ---------------------- HNO3

D) Nitrate Reduction

Several heterotrophic bacteria are capable of converting nitrates into

nitrites or amrnonia normally under anaerobic conditions

i.e; in waterlogged conditions. The O2 of the nitrateserves as an

acceptor for electrons and hydrogen.

HNO3 + 4H2 --------------------- N3 +3H2O .

This reaction is not of major significance in well aerated, cultivated

agricultural soils,

DETIFTRIFICATION

The transformation of nitrates to gaseous nitrogen by microorganisms in a

series of biochemical changes is called denitrification. It is an undesirable

process for agriculture as the available form of nitrogen (nitrate) is lost to the

atmosphere as N2gas. -

2NO3 -----------2NO2---------- -2Nb-------------------N2Q--------------N2-

(Nitrate) (Nitrite) (Nitric oxide)' (Nitrous oxide)'-

(Nitrogen) _ .^

Organisms : Pseudomonas, Thiobacillus. Bacillus-. Achromobacter. AfkaligerKS. -

Agrobacterium, icrococcus denitrificahs, Tahiobacillus dehifrif'tcans.'

The process of denitrification is enhanced by, ,

i) Abunciance of organic matter ii) Elevated temperatures (25-60°C)

iii) Neutral or alkaline pH; |v) Limited O2 supply (anaerobidsis)

nitrogen cycle diagram

SULPHUR CYCLE

The sulphur cycle can be studied under the following---

1. ASSIMILATION OF ELEMENTAL SULPHUR

2. ASSIMILATION OF SULPHATE BY PLANTS

3. REDUCTION OF SULPHATE TO H2S

4. DEGRADATION OF ORGANIC SULPHUR COMPOUND TO HYDROGEN

SULPHIDE

5. OXIDATION OF H2S TO ELEMENTAL SULPHUR

Assimilation of elemental sulphur

The sulphur in its elemental form cannot be utilized by plants or

animals. It is the function of bacteria specially Thiobacillus thiooxidans

which can oxidize elemental sulphur to sulphate.

2S + 2H2O + 3O2---------------------------------- 2H2SO4

The energy obtained by the oxidation of elemental sulphur to sulphate

is utilized by Thiobacillus thiooxidans.

Assimilation of sulphate by plants

The sulphate that became available in the soil is now assimilated by the

plant and is converted into organic sulphur compound ie. Protein having

sulphur containing amino acid eg. Cysteine.

The plants are eaten up by the animals an dit build up the animal

protein which also contain sulphur containing amino acid.

Reduction of sulphate to H2S

The sulphate can also be reduced to hydrogen sulphide by soil

microorganism

eg: bacteria involved in the process of reduction of sulphate of H2S by

Desulfotomaculum

4H2 + CaSO4 ----------------------- H2S + Ca(OH)2 + 2H2O

Degradation of organic sulphur compound to hydrogen sulphide

The death of plant and animals increases the protein content in the soil.

The protein are degraded to liberate large number of amino acid, some of

them are sulphur containing amino acid. eg. Cysteine.

The sulphur is released from the amino acid by enzymatic activity of

many heterotrophic bacteria.

Oxidation of H2S to elemental sulphur

The H2S resulting from sulphate reduction and amino acid

decomposition is oxidized by a group of photosynthetic bacteria to elemental

sulphur and cycle continue.

CO2 + 2H2S ------------ (CH2O)x + H2O + 2S

Significance of sulphur cycle

It help in generation of sulphate which is most suitable source of

sulphur for plant

Sulphate is the anion of strong mineral acid and prevent excessive

alkalinity due to ammonia formation by microorganism.

Acculmulation of H2SO$ solubilises inorganic salts that contain

phosphate and metals.

Sulphate also help the growth of sulphate reducing bacteria.

Generation of H2S help the phosynthic bacteria in which H2s act as an

electron donor.

Suphur cycle.

RHIZOSPHERE

The zone of soil surrounding the plant root where the nutrient released

from the root increase the microbial population and its activities is termed the

rhizosphere

The term rhizosphere was coined by the German scientist Lorenz Hiltner

in 1904.

The plant roots surface usually including the adhering soil particles is

called rhizoplane.

The rhizosphere can be defined as the region extending a few millimeter

from the surface of each root, where the microbial population of soil is

influenced by the chemical activities of the plant..

The rhizosphere provides certain characteristics condition for the

increased occurrence of microflora in it, which is attributed to the rich

food material provided by the added sloughed off position of root tissue

and root exudates.

In rhizosphere it is not only the quantitative increase in the microbial

population, their composition i.e. qualitative change also occur.

These microbes serve as liable source of nutrients for other microbes

thus creating a microbial loop and in addition to playing critical role in

the organic matter synthesis and degradation.

Two common bacteria found in the rhizosphere

Pseudomonas Achromobacter have been referred to

a plant growth promoting rhizobacteria.

Several bacteria such as Azotobacter, azosprillium and Acetobacter, the

nitrogen fixing are found in the rhizoplane and rhizosphere and fix

nitrogen, the process termed as the associative nitrogen fixation.

Applied AREAS OF MICROBIOLOGY

Applied Areas of Microbiology:

Sr.

No.

.Applied area Deals with ,

1 Medical microbiology Etiology and diagnosis of infectious diseases.

2. Public health microbiology Measures to control 'occurrence and spread of

diseases. 3. Industrial microbiology Fermentation products such as alcohols, acids and

pharmaceutical products such as antibiotics and

vitamins.4.

«

Milk and food

microbiology

Quality; "control of milk and foods and production

of milk products like butter, cheese, yoghurt,

pickles and others.5 Agricultural microbiology Soil fertility, animal and plant .diseases.6. Aerobiology Occurrence of MICROBES in air and spread of

microbes through air.7. Microbiology Of domestic

water and waste, water

Quality control of domestic water and treatment -

processes for waste water

8. Marina microbiology 'Rote of microorganisms in marine environments.9. Exobiology Possible occurrence of microorganisms1 in the

outer space and on planets through contamination

by astronauts or space, vehicles. 10. , Insect microbiology Control of insect vectors of diseases ' like malaria

and yellow fever.

Beneficial and harmful activities of microbes in agriculture and allied branches .of

Microbiology.

Microorganisms are ubiquitous in nature and therefore occur everywhere -in

soil water and air. Microbes during their growth and reproduction cause beneficial

and harmful effects which are of economic importance in Agriculture and Industries

viz- dairy, biochemical, pharmaceutical, cdaliarid petroleum industry. Microbes

also play important rote in sanitation and human, animal and plant health as well as

in space technology. They are vital also in biological warfare .

Relation of microbiology with other science :

Agricultural science

A) Agronomy : Agronomy is primarily related with crop production. Two

beneficial effects are noteworthy in Agriculture.

1) Nitrogen fixation : Microbes, especially are Bacteria are fixing

atmospheric nitrogen symbolically and non symbiotically . Rhizobium spp. fixing

the nitrogen symbolically in leguminous crop, however, the Azotobacter and

Azospirilium in cereals and non legumes . The genera, Blue, green algae, especially like

Anabaena, Aulosiia, Nostoc, etc, in paddy fields , Actinomycets (frankla in non-

legumes) fix the atmospheric nitrogen (N2) to the form (NH3) available to the

plants.Thus the nitrogen nutrition of plants is. drastically improved.

2) Organic matter decomposition : The microbes because of their organic acid

and enzymes producing ability (cellulytic nature), decompose the complex organic

matter and convert it to simpler and soluble substances which become available to

the plants as nutrients on which the higher animals depend.

3) Phosphorus solubilization :The bacteria like Pseudomonas striate, Bacillus

megatherium var, phosphaticum and the fungi like Aspergillus awamori and Penicillium

digttaturri soiublize the insoluble phosphates from the soil and make phosphorus

available to the plants.

4) Sulfur oxidation and transformation of other elements: The elemental

sulfur (S) can not be used by plants. Certain autotrophic bacteria like Thiobacillus

thiooxidans oxidize elemental sulfur to sulfates (SO*), which are taken up by the

plants. The nutrient substances like Fe, Mn, Cu, etc. are also made available to the

plants by degrading the complex substances into simpler ones.

5) Animal husbandry & Dairy science :

I- Microbes play an important role in digestion of cellulose in the rumen of

ruminants. They synthesis proteins & vitamins .

II - Microbes are employed in the production of dairy products, like cheese , butter &

shrikhand. - •' .

Preparation of fermented milk products ; A number of fermented milk products like

cultured buttermilk, cultured sour cream, butter, cheese, Bulgarian milk, acidophilus milk,

kumis, kefir, yogurt are prepared by using different microbes like species of

Lactobacillus, Streptococcus, Luconostoc, etc,

C) Other agricultural uses: The microbes are used in other industries tike retting

of jute, fermentation of tobacco and preparation of silage.

Mushroom production: Some soil fungi like Agahcus bisporus, Pleurotus sajor-

caju, Pleurotus flodda, Votvariella spp. produce protein-rich edible fruiting structures

which are used for consumption by the human beings thus leading to improvement

in protein nutrition.

2. Industrial science

1. Beverage: The microbes are commercially important in beverage industry. Many

types of yeast are employed in the fermentations of fruits and grains for production of

alcohol e.g. Saccharomyces cerevisiae.

2. Bakery: Microbes are also used in preparation of food supplements and brads in bakery

industry, e.g: confectionaty

3.Pharmacy : Microbes are of vital importance in pharmaceutical industry for the

production of antibiotics and vaccines .The "wonder drugs" (antibiotics) are also

produced by the microorganisms which are used to treat a number of infectious

diseases, eg. Penicillin is produced from a fungus, Penicillium chrysogenum.

Streptomycin is produced from an actinomycets, Streptomyces griseus, of other

antibiotics are prepared from species ; actinomycets The microbes are "also"

used for preparation of vaccines for immunization against some infectious

diseases. Some genera are also known to produce vitamins.

4. Biochemistry : The microbes are also exploited In biochemical industry for the

production of organic acids like lactic acid (Rhizopus oryzae, Lactobacillus

delbrueckii, L bulgaricus], glutamic acid (Brevibacterium spp.), citric acid (Aspergillus

niger A. wentit), fumaric acid (Rhizopus nigricans), gibberellic acid (Fusarium

moniliforme) and acetic acid i.e. vinegar (Acetobacter aeeti), enzymes like amylase

and protease (Bacillus subtilis), acetone-butanol (Clostridium acetobutylicum), amino

acid like lysine (Micrococcus glutamicus), insulin, interfered and somatostatin

(recombinant DNA varieties of Escherichia coll.

3. Basic sciences:

Microorganisms. are ideal experimental, tools in the hands of scientists to

studying the basic laws of nature. They are used to investigate different biological

phenomenon due to following reasons :

1,Being cultivable in test tube, microbes requires less space for growth,

reproduction and maintenance as compared to higher plants and animals.

2. Due to unusual high rate of growth are(d reproduction, microbes are very

convenient for generation studies, eg. bacteria can complete 100 generations In

just 24 hours.

3. Microbes have many similarities with higher plants and animals in respect of

enzyme systems, metabolic process and synthetic abilities. Therefore conclusion

based on the studies of microbes can be applied to higher plants.and animal as

well.

4. Microbes, especially bacteria and bacteriophages are very simple models for

understanding host-pathogen interaction. Virus- host cell interaction as observed in

case of bacteriophage-bacteteria combination is also applicable to virus-higher

plant or virus -higher animal combination.

5.Due to above reasons microbes are ideal experimental toois in the hancis of

biochemists, geneticists, cytolog;fsts,: molectrt'ar biologists and advocates of

Darwinism.

4. Energy sciences

Goal and petroleum technology:

1. Some microorganisms are also reported to be involved in the oxidation of organic

matter to compounds similar to petroleum.

?. Microorganisms are also responsible for biogas ( methane ) production from

#trganiCfWast$ products in biogas plants .The methanogenic bacteria like species of

-•^thanobreyibaQterium, Methanosarcina, Methanocpccus, etc. are used for the

biogas (methane) production from the domestic and dairy wastes such as human

and animal urine and excreta. The methane gas produced can be used as a ft>el

for combustion as well as for generation of electricity.

3. Photosynthetic- bacteria and algae can convert solar energy into chemical

energy, and therefore play an important role in maintenance of oxygen level

and energy balance of ecosystem.

5. Sanitation and health sciences :

1. Microorganisms can impair the quality of drinking water, and makes it polluted.

2. Microorganisms can digest as well as oxidize organic matter from sewage and

purify it for irrigation.

3. Microbial parasites are employed for the control of mosquitoes and other

vectors of human and plant pathogens.

4. Microbes can also bemused for biological control of weed, plant, animal and human

pathogens.

Biological control of plant diseases.; The fungi like Trichoderma ha&ianum,

T. viride and Gliocladium virens arid the bacteria like Pseudomonas, Bacillus, etc.

because of their antibiotic producing ability are used for biological management of soil

borne plant diseases like root rots, wilts, etc.

Biological control of insect pests : The bacteria like Bacillus thuringiensis are used

for the biological control of some lepidopterous insect pests.

Diseases in humans, plants and animals : Some microorganisms cause

diseases in human beings (cholera, tuberculosis, pneumonia, dysentery,

influenza,

common cold, malaria, anthrax, hepatitis, diphtheria, etc.), plants {leaf

spots',

blights, wilts,* root rots, etc.) and animals (Q fever, foot and mouth,

tuberculosis,

etc.). . :

6. Space technology:

1 .Microbes can serve as food and source of energy to astronauts

eg. Mushrooms.

2 They are also employed for the maintenance of suitable oxygen-carbon

dioxide balance in space vehicles.

3. Some microbes are also employed in biological warfare. Some microbes

causing infectious diseases may be used in the biological wars. Eg. Use of

Bacillus anthracis causing anthrax disease in human beings.