Introduction to Microbiol - Complete

8/10/2019 Introduction to Microbiol - Complete

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From earliest times, people had believed in spontaneous generation that living organisms could

develop from nonliving matter. Even Aristotle (384322 B.C.)thought some of the simpler invertebrates

could arise by spontaneous generation.

This view finally was challenged by the Italian physician Francesco Redi (16261697), who carried out a

series of experiments on decaying meat and its ability to produce maggots spontaneously.

In 1748, the English priest John Needham (17131781) reported the results of his experiments on

spontaneous generation. Needham boiled mutton broth and then tightly stoppered the flasks.

Eventually many of the flasks became cloudy and contained microorganisms. He thought organic matter

contained a vital force that could confer the properties of life on nonliving matter.

A few years later, the Italian priest and naturalist Lazzaro Spallanzani (17291799) improved on

Needhams experimental design by first sealing glass flasks that contained water and seeds. If the

sealed flasks were placed in boiling water for 3/4 of an hour, no growth took place as long as the flasks

remained sealed. He proposed that air carried germs to the culture medium, but also commented that

the external air might be required for growth of animals already in the medium. The supporters of

spontaneous generation maintained that heating the air in sealed flasks destroyed its ability to support

life.

Several investigators attempted to counter such arguments. Theodore Schwann (18101882)allowed

air to enter a flask containing a sterile nutrient solution after the air had passed through a red-hot tube.

The flask remained sterile.

Subsequently Georg Friedrich Schroder and Theodor von Dusch allowed air to enter a flask of heat-

sterilized medium after it had passed through sterile cotton wool. No growth occurred in the medium

even though the air had not been heated.

Despite these experiments the French naturalist Felix Pouchet claimed in 1859 to have carried out

experiments conclusively proving that microbial growth could occur without air contamination. This

claim provoked Louis Pasteur (18221895)to settle the matter once and for all.

Pasteur first filtered air through cotton and found that objects resembling plant spores had been

trapped. If a piece of the cotton was placed in sterile medium after air had been filtered through it,microbial growth occurred. Next he placed nutrient solutions in flasks, heated their necks in a flame, and

drew them out into a variety of curves, while keeping the ends of the necks opento the atmosphere.

Pasteur then boiled the solutions for a few minutes and allowed them to cool. No growth took place

even though the contents of the flasks were exposed to the air. Pasteur pointed out that no growth

occurred because dust and germs had been trapped on the walls of the curved necks. If the necks were

broken, growth commenced immediately. Pasteur had not only resolved the controversy by 1861 but

also had shown how to keep solutions sterile.

The English physicist John Tyndall (18201893)dealt a final blow to spontaneous generation in 1877 by

demonstrating that dust did indeed carry germs and that if dust was absent, broth remained sterile


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even if directly exposed to air. During the course of his studies, Tyndall provided evidence for the

existence of exceptionally heat-resistant forms of bacteria.

Germ theory of disease

Agostino Bassi (17731856)first showed a microorganism could cause disease when he demonstrated

in 1835 that a silkworm disease was due to a fungal infection. He also suggested that many diseases

were due to microbial infections.

In 1845, M. J. Berkeley proved that the great Potato Blight of Ireland was caused by a water mold, and

in 1853, Heinrich de Baryshowed that smut and rust fungi caused cereal crop diseases.

Following his successes with the study of fermentation, Pasteur was asked by the French government to

investigate the pbrine disease of silkwormsthat was disrupting the silk industry. After several years of

work, he showed that the disease was due to a protozoan parasite. The disease was controlled by

raising caterpillars from eggs produced by healthy moths.

Indirect evidence for the germ theory of disease came from the work of the English surgeon Joseph

Lister (18271912)on the prevention of wound infections. Lister, impressed with Pasteursstudies on

the involvement of microorganisms in fermentation and putrefaction, developed a system of antiseptic

surgery designed to prevent microorganisms from entering wounds. Instruments were heat sterilized,

and phenol was used on surgical dressings and at times sprayed over the surgical area. The approach

was remarkably successful and transformed surgery after Lister published his findings in 1867.

Kochs Postulates

The first direct demonstration of the role of bacteria in causing disease came from the study of anthrax

by the German physician Robert Koch (18431910). Koch used the criteria proposed by his former

teacher, Jacob Henle (18091885),to establish the relationship between Bacillus anthracisand anthrax,

and published his findings in 1876. Koch injected healthy mice with material from diseased animals, and

the mice became ill. After transferring anthrax by inoculation through a series of 20 mice, he incubated apiece of spleen containing the anthrax bacillus in beef serum. The bacilli grew, reproduced, and

produced endospores. When the isolated bacilli or their spores were injected into mice, anthrax

developed. His criteria for proving the causal relationship between a microorganism and a specific

disease are known as Kochs postulates.

Kochs proof that B. anthracis caused anthrax was independently confirmed by Pasteur and his

coworkers. They discovered that after burial of dead animals, anthrax spores survived and were brought

to the surface by earthworms. Healthy animals then ingested the spores and became ill.

Although Koch used the general approach described in the postulates during his anthrax studies, he did

not outline them fully until his work on the cause of tuberculosis. In 1884, he reported that this diseasewas caused by a rod-shaped bacterium, Mycobacterium tuberculosis; he was awarded the Nobel Prize in

Physiology or Medicine in 1905 for his work.


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The Development of Techniques for Studying Microbial Pathogens

During Kochs studies on bacterial diseases, it became necessary to isolate suspected bacterial

pathogens in pure culturea culture containing only one type of microorganism. At first Koch cultured

bacteria on the sterile surfaces of cut, boiled potatoes, but this was unsatisfactory because the bacteria

would not always grow well. Eventually he developed culture media using meat extracts and protein

digests because of their similarity to body fluids. He first tried to solidify the media by adding gelatin.

Separate bacterial colonies developed after the surface of the solidified medium had been streaked with

a bacterial sample. The sample could also be mixed with liquefied gelatin medium. When the gelatin

medium hardened, individual bacteria produced separate colonies. Despite its advantages, gelatin was

not an ideal solidifying agent because it can be digested by many bacteria and melts at temperatures

above 28C.A better alternative was provided by Fannie Eilshemius Hesse,the wife of Walther Hesse,

one of Kochs assistants. She suggested the use of agar as a solidifying agentshe had been using it

successfully to make jellies for some time. Agar was not attacked by most bacteria and did not melt

until reaching a temperature of 100C. Furthermore, once melted, it did not solidify until it reached atemperature of 50C, eliminating the need to handle boiling liquid and providing time for manipulation

of the medium. Some of the media developed by Koch and his associates, such as nutrient broth and

nutrient agar, are still widely used. Another important tool developed in Kochs laboratory was a

container for holding solidified mediathe petri dish (plate),named after Richard Petri, who devised it.

These developments directly stimulated progress in all areas of bacteriology.

Viral pathogens were also studied during this time. The discovery of viruses and their role in disease was

made possible when Charles Chamberland (18511908), one of Pasteurs associates, constructed a

porcelain bacterial filter in 1884. Dimitri Ivanowski and Martinus Beijerinck (pronounced by-a-rink)


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used the filter to study tobacco mosaic disease. They found that plant extracts and sap from diseased

plants were infectious, even after being filtered with Chamberlands filter. Because the infectious agent

passed through a filter that was designed to trap bacterial cells, the agent must be something smaller

than a bacterium. Beijerinck proposed that the agent was a filterable virus. Eventually viruses were

shown to be tiny, acellular infectious agents.

Protection against micro-organisms (Immunological Studies)

During studies on chicken cholera, Pasteur and Rouxdiscovered that incubating their cultures

for long intervals between transfers would attenuate the bacteria, which meant they had lost their

ability to cause the disease. If the chickens were injected with these attenuated cultures, they remained

healthy but developed the ability to resist the disease. He called the attenuated culture a vaccine [Latin

vacca, cow] in honor of Edward Jenner because, many years earlier, Jenner had used material from

cowpox lesions to protect people against smallpox. Shortly after this, Pasteur and Chamberland

developed an attenuated anthrax vaccine in two ways: by treating cultures with potassium bichromate

and by incubating the bacteria at 42 to 43C.

Pasteur then prepared rabies vaccine by a different approach. The pathogen was attenuated by

growing it in an abnormal host, the rabbit. After infected rabbits had died, their brains and spinal cords

were removed and dried. During the course of these studies, Joseph Meister, a nine-year-old boy who

had been bitten by a rabid dog, was brought to Pasteur. Since the boys death was certain in the

absence of treatment, Pasteur agreed to try vaccination. Joseph was injected 13 times over the next 10

days with increasingly virulent preparations of the attenuated virus. He survived.

After the discovery that thediphtheria bacillusproduced a toxin, Emil von Behring (18541917)

and Shibasaburo Kitasato (18521931)injected inactivated toxin into rabbits, inducing them to produce

an antitoxin, a substance in the blood that would inactivate the toxin and protect against the disease. A

tetanus antitoxinwas then prepared and both antitoxins were used in the treatment of people.

The antitoxin work provided evidence that immunity could result from soluble substances in the

blood, now known to be antibodies (humoral immunity). It became clear that blood cells were also

important in immunity (cellular immunity)when Elie Metchnikoff (18451916)discovered that some

blood leukocytes could engulf disease-causing bacteria. He called these cells phagocytes and the process

phagocytosis [Greekphagein,eating].

Innate Immunity


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Microorganisms or toxins that successfully enter an organism encounter the cells and

mechanisms of the innate immune system. The innate response is usually triggered when microbes are

identified by pattern recognition receptors, which recognize components that are conserved among

broad groups of microorganisms, or when damaged, injured or stressed cells send out alarm signals,

many of which (but not all) are recognized by the same receptors as those that recognize pathogens.

Innate immune defenses are non-specific, meaning these systems respond to pathogens in a generic

way. This system does not confer long-lasting immunity against a pathogen. The innate immune system

is the dominant system of host defense in most organisms.

1. Surface barriers

Several barriers protect organisms from infection, including mechanical, chemical, and biological

barriers. The waxy cuticle of many leaves; the exoskeleton of insects, the shells and membranes of

externally deposited eggs, and skin are examples of mechanical barriers that are the first line of defense

against infection. However, as organisms cannot be completely sealed from their environments, other

systems act to protect body openings such as the lungs, intestines, and the genitourinary tract. In the

lungs, coughing and sneezing mechanically eject pathogens and other irritants from the respiratory

tract. The flushing action of tears and urine also mechanically expels pathogens, while mucus secreted

by the respiratory and gastrointestinal tract serves to trap and entangle microorganisms.

Chemical barriers also protect against infection. The skin and respiratory tract secrete

antimicrobial peptides such as the -defensins. Enzymes such as lysozyme and phospholipase A2 in

saliva, tears, and breast milk are also antibacterials. Vaginal secretions serve as a chemical barrier

following menarche, when they become slightly acidic, while semen contains defensins and zinc to kill

pathogens. In the stomach, gastric acid and proteases serve as powerful chemical defenses against

ingested pathogens.

Within the genitourinary and gastrointestinal tracts, commensal flora serve as biological barriers by

competing with pathogenic bacteria for food and space and, in some cases, by changing the conditions

in their environment, such as pH or available iron. This reduces the probability that pathogens will reach

sufficient numbers to cause illness. However, since most antibiotics non-specifically target bacteria and

do not affect fungi, oral antibiotics can lead to an "overgrowth" of fungi and cause conditions such as a

vaginal candidiasis (a yeast infection). There is good evidence that re-introduction of probiotic flora,

such as pure cultures of the lactobacilli normally found in unpasteurized yogurt, helps restore a healthy

balance of microbial populations in intestinal infections in children and encouraging preliminary data in

studies on bacterial gastroenteritis, inflammatory bowel diseases, urinary tract infection and post-

surgical infections.

2. Inflammation

Inflammation is one of the first responses of the immune system to infection. The symptoms of

inflammation are redness, swelling, heat, and pain, which are caused by increased blood flow into

tissue. Inflammation is produced by eicosanoids and cytokines, which are released by injured or infected

cells. Eicosanoids include prostaglandins that produce fever and the dilation of blood vessels associated

with inflammation, and leukotrienes that attract certain white blood cells(leukocytes). Common

cytokines include interleukins that are responsible for communication between white blood cells;

chemokines that promote chemotaxis; and interferons that have anti-viral effects, such as shutting

down protein synthesis in the host cell. Growth factors and cytotoxic factors may also be released.


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These cytokines and other chemicals recruit immune cells to the site of infection and promote healing of

any damaged tissue following the removal of pathogens.

3. Complement system

The complement system is a biochemical cascade that attacks the surfaces of foreign cells. It

contains over 20 different proteins and is named for its ability to "complement" the killing of pathogens

by antibodies. Complement is the major humoral component of the innate immune response. Many

species have complement systems, including non-mammals like plants, fish, and some invertebrates.

In humans, this response is activated by complement binding to antibodies that have attached

to these microbes or the binding of complement proteins to carbohydrates on the surfaces of microbes.

This recognition signal triggers a rapid killing response. The speed of the response is a result of signal

amplification that occurs following sequential proteolytic activation of complement molecules, which

are also proteases. After complement proteins initially bind to the microbe, they activate their protease

activity, which in turn activates other complement proteases, and so on. This produces a catalytic

cascade that amplifies the initial signal by controlled positive feedback. The cascade results in the

production of peptides that attract immune cells, increase vascular permeability, and opsonize (coat)

the surface of a pathogen, marking it for destruction. This deposition of complement can also kill cells

directly by disrupting their plasma membrane.

4. Cellular barriers

Leukocytes (white blood cells) act like independent, single-celled organisms and are the second

arm of the innate immune system. The innate leukocytes include the phagocytes (macrophages,

neutrophils, and dendritic cells), mast cells, eosinophils, basophils, and natural killer cells. These cells

identify and eliminate pathogens, either by attacking larger pathogens through contact or by engulfing

and then killing microorganisms. Innate cells are also important mediators in the activation of the

adaptive immune system.

Phagocytosis is an important feature of cellular innate immunity performed by cells called

'phagocytes' that engulf, or eat, pathogens or particles. Phagocytes generally patrol the body searching

for pathogens, but can be called to specific locations by cytokines. Once a pathogen has been engulfed

by a phagocyte, it becomes trapped in an intracellular vesicle called a phagosome, which subsequently

fuses with another vesicle called a lysosome to form a phagolysosome. The pathogen is killed by the

activity of digestive enzymes or following a respiratory burst that releases free radicals into the

phagolysosome. Phagocytosis evolved as a means of acquiring nutrients, but this role was extended in

phagocytes to include engulfment of pathogens as a defense mechanism. Phagocytosis probably

represents the oldest form of host defense, as phagocytes have been identified in both vertebrate and

invertebrate animals.

Neutrophils and macrophages are phagocytes that travel throughout the body in pursuit of

invading pathogens. Neutrophils are normally found in the bloodstream and are the most abundant

type of phagocyte, normally representing 50% to 60% of the total circulating leukocytes. During the

acute phase of inflammation, particularly as a result of bacterial infection, neutrophils migrate toward

the site of inflammation in a process called chemotaxis, and are usually the first cells to arrive at the

scene of infection. Macrophages are versatile cells that reside within tissues and produce a wide array of

chemicals including enzymes, complement proteins, and regulatory factors such


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as interleukin. Macrophages also act as scavengers, ridding the body of worn-out cells and other debris,

and as antigen-presenting cells that activate the adaptive immune system.

Dendritic cells (DC)are phagocytes in tissues that are in contact with the external environment;

therefore, they are located mainly in the skin, nose, lungs, stomach, and intestines. They are named for

their resemblance to neuronal dendrites, as both have many spine-like projections, but dendritic cells

are in no way connected to the nervous system. Dendritic cells serve as a link between the bodily tissues

and the innate and adaptive immune systems, as they present antigen to T cells, one of the key cell

types of the adaptive immune system.

Mast cellsreside in connective tissues and mucous membranes, and regulate the inflammatory

response. They are most often associated with allergy andanaphylaxis. Basophils and eosinophils are

related to neutrophils. They secrete chemical mediators that are involved in defending against parasites

and play a role in allergic reactions, such as asthma. Natural killer (NK cells) cells are leukocytes that

attack and destroy tumor cells, or cells that have been infected by viruses.

Natural killer cells,or NK cells, are a component of the innate immune system which does not directly

attack invading microbes. Rather, NK cells destroy compromised host cells, such as tumor cells or virus-

infected cells, recognizing such cells by a condition known as "missing self." This term describes cells

with low levels of a cell-surface marker called MHC I (major histocompatibility complex) a situation

that can arise in viral infections of host cells. They were named "natural killer" because of the initial

notion that they do not require activation in order to kill cells that are "missing self." For many years it

was unclear how NK cells recognize tumor cells and infected cells. It is now known that the MHC makeup

on the surface of those cells is altered and the NK cells become activated through recognition of

"missing self". Normal body cells are not recognized and attacked by NK cells because they express

intact self MHC antigens. Those MHC antigens are recognized by killer cell immunoglobulin receptors

(KIR) which essentially put the brakes on NK cells.

Adaptive immune system

The adaptive immune system evolved in early vertebrates and allows for a stronger immune

response as well as immunological memory, where each pathogen is "remembered" by a signature

antigen. The adaptive immune response is antigen-specific and requires the recognition of specific

"non-self" antigens during a process called antigen presentation. Antigen specificity allows for the

generation of responses that are tailored to specific pathogens or pathogen-infected cells. The ability to

mount these tailored responses is maintained in the body by "memory cells". Should a pathogen infect

the body more than once, these specific memory cells are used to quickly eliminate it.

Lymphocytes: The cells of the adaptive immune system are special types of leukocytes, called

lymphocytes. B cells and T cells are the major types of lymphocytes and are derived from hematopoieticstem cells in the bone marrow. B cells are involved in the humoral immune response, whereas T cells are

involved in cell-mediated immune response.


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Both B cells and T cells carry receptor molecules that recognize specific targets. T cells recognize

a "non-self" target, such as a pathogen, only after antigens (small fragments of the pathogen) have been

processed and presented in combination with a "self" receptor called a major histocompatibility

complex (MHC) molecule. There are two major subtypes of T cells: the killer T cell and the helper T cell.

Killer T cells only recognize antigens coupled to Class I MHC molecules, while helper T cells onlyrecognize antigens coupled to Class II MHC molecules. These two mechanisms of antigen presentation

reflect the different roles of the two types of T cell. A third, minor subtype is the T cells that recognize

intact antigens that are not bound to MHC receptors.

In contrast, the B cell antigen-specific receptor is an antibody molecule on the B cell surface, and

recognizes whole pathogens without any need for antigen processing. Each lineage of B cell expresses a

different antibody, so the complete set of B cell antigen receptors represent all the antibodies that the

body can manufacture.


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1.

Killer T cells

Killer T cells are a sub-group of T cells that kill cells that are infected with viruses (and other

pathogens), or are otherwise damaged or dysfunctional. As with B cells, each type of T cell recognizes a

different antigen. Killer T cells are activated when their T cell receptor (TCR) binds to this specific antigen

in a complex with the MHC Class I receptor of another cell. Recognition of this MHC:antigen complex is

aided by a co-receptor on the T cell, called CD8. The T cell then travels throughout the body in search of

cells where the MHC I receptors bear this antigen. When an activated T cell contacts such cells, it

releases cytotoxins, such as perforin, which form pores in the target cell's plasma membrane,

allowing ions, water and toxins to enter. The entry of another toxin called granulysin (a protease)

induces the target cell to undergo apoptosis. T cell killing of host cells is particularly important in

preventing the replication of viruses. T cell activation is tightly controlled and generally requires a very

strong MHC/antigen activation signal, or additional activation signals provided by "helper" T cells (see

below).

2. Helper T Cells

Helper T cells regulate both the innate and adaptive immune responses and help determine

which immune responses the body makes to a particular pathogen. These cells have no cytotoxic activity

and do not kill infected cells or clear pathogens directly. They instead control the immune response by

directing other cells to perform these tasks.

Helper T cells express T cell receptors (TCR) that recognize antigen bound to Class II MHC

molecules. The MHC:antigen complex is also recognized by the helper cell'sCD4 co-receptor, which

recruits molecules inside the T cell (e.g., Lck) that are responsible for the T cell's activation. Helper T cells

have a weaker association with the MHC:antigen complex than observed for killer T cells, meaning many

receptors (around 200300) on the helper T cell must be bound by an MHC:antigen in order to activate

the helper cell, while killer T cells can be activated by engagement of a single MHC:antigen molecule.

Helper T cell activation also requires longer duration of engagement with an antigen-presenting cell. The

activation of a resting helper T cell causes it to release cytokines that influence the activity of many cell

types. Cytokine signals produced by helper T cells enhance the microbicidal function of macrophages

and the activity of killer T cells. In addition, helper T cell activation causes an upregulation of molecules

expressed on the T cell's surface, such as CD40 ligand (also called CD154), which provide extra

stimulatory signals typically required to activate antibody-producing B cells.

3. T cells

T cells possess an alternative T cell receptor (TCR) as opposed to CD4+ and CD8+ () T cells and

share the characteristics of helper T cells, cytotoxic T cells and NK cells. The conditions that produce

responses from T cells are not fully understood. On one hand, T cells are a component ofadaptiveimmunity as they rearrange TCR genes to produce receptor diversity and can also develop a memory

phenotype. On the other hand, the various subsets are also part of the innate immune system, as

restricted TCR or NK receptors may be used as pattern recognition receptors.

4. B lymphocytes and antibodies

A B cell identifies pathogens when antibodies on its surface bind to a specific foreign antigen.

This antigen/antibody complex is taken up by the B cell and processed by proteolysis into peptides. The

B cell then displays these antigenic peptides on its surface MHC class II molecules. This combination of


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MHC and antigen attracts a matching helper T cell, which releases lymphokines and activates the B cell.

As the activated B cell then begins to divide, its offspring (plasma cells)secrete millions of copies of the

antibody that recognizes this antigen. These antibodies circulate in blood plasma and lymph, bind to

pathogens expressing the antigen and mark them for destruction by complement activation or for

uptake and destruction by phagocytes. Antibodies can also neutralize challenges directly, by binding to

bacterial toxins or by interfering with the receptors that virus and bacteria use to infect cells.

Immunization

Passive Immunization:

Passive immunization is achieved by injecting a recipient with preformed immunoglobulins (Igs)

obtained from human (or, occasionally, equine) serum. Passive immunization provides immediate

protection to individuals who have been exposed to an infectious organism and who lack active

immunity to that pathogen. Because passive immunization does not activate the immune system, it

generates no memory response. Passive immunity dissipates after a few weeks to months as the Igs are

cleared from the recipients serum.

Active Immunization:

Active immunization is achieved by injection of viable or nonviable pathogens, or purified pathogen

product, prompting the immune system to respond as if the body were being attacked by an intact

infectious microorganism. Whereas passive immunization provides immediate protection, active

immunization may require several days to months to become effective. Active immunization leads to

prolonged immunity and is generally preferred over the short-term immunity provided by passive

immunization with preformed Igs. Simultaneous administration of active and passive immunizations

may be required after exposure to certain infections such as hepatitis B.

A. Formulations for active immunization

Vaccines are 1) live, attenuated microorganisms; 2) killed micro -organisms; 3) microbial extracts; 4)

vaccine conjugates; or 5) inactivated toxins (toxoids). Both bacterial and viral pathogens are targeted by

these diverse means.

1. Live pathogens: When live pathogens are used, they are attenuated (weakened) to preclude clinical

consequences of infection. Attenuated microbes reproduce in the recipient, typically leading to a more

robust and long-lasting immune response than can be obtained through vaccination with killed

organisms. However, with live, attenuated vaccines, there is a possibility that the attenuated vaccine

strain will revert to an active pathogen after administration to the patient. For example, vaccine-associated poliomyelitis occurs following administration of approximately 1 of every 2.4 million doses of

live polio vaccine. All recent cases of polio in the United States are vaccine associated. Also, live,

attenuated vaccines should not be given to immunocompromised individuals because there is the

potential for a disseminated infection.

2. Killed microorganisms: Killed vaccines have the advantage over attenuated microorganisms in that

they pose no risk of vaccine associated infection. As noted above, killed organisms often provide a weak

or short-lived immune response. Some vaccines, such as polio and typhoid vaccines, are available both

in live and killed versions.


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3. Microbial extracts: Instead of using whole organisms, vaccines can be composed of antigen molecules

(often those located on the surface of the microorganism) extracted from the pathogen or prepared by

recombinant DNA techniques. The efficacy of these vaccines varies. In some instances, the vaccine

antigen is present on all strains of the organism, and the vaccine, thus, protects against infection by all

strains. With other pathogens, such as pneumococcus, protective antibody is produced against only a

specific capsular polysaccharide, one among more than 80 distinct types. Immunity to one

polysaccharide type does not confer immunity to any other type. For this reason, the pneumococcal

vaccine is composed of 23 different polysaccharides, comprising the antigens produced by the most

common types of disease causing pneumococci. Some pathogens, such as influenza virus, frequently

change their antigenic determinants. Therefore, influenza virus vaccines must also change regularly to

counter the different antigens of influenza A and B virus strains in circulation. In the case of rhinovirus

infections (the leading cause of the common cold) at least 100 types of the virus are known. It is not

feasible to develop a vaccine that confers protection to this large number of antigenic types.

4. Vaccine conjugates: Vaccines can produce humoral immunity through B cell proliferation leading to

antibody production, which may or may not involve helper T cells. For example, pneumococcal

polysaccharide and the polysaccharide of Hib induce B-cell type specific protective antibodywithout

involvement of helper T cells. These T cellindependent responses are characterized by low antibody

titers, particularly in children younger than age 18 months. Thus, conventional H. influenzae

polysaccharide vaccine does not provide protection for children ages 3 to 18 months. Consequently, this

organism has, in the past, produced severe infections in this age group. However, by covalently

conjugating the Haemophilus polysaccharide to a protein antigen, such as diphtheria toxoid, H.

influenzae vaccines produce a robust T celldependent antibody response even in 3-month-old infants.

Conjugate vaccines are also currently available for Streptococcus pneumoniaand Neisseria meningitidis.

5. Toxoids: These are derivatives of bacterial exotoxins produced by chemically altering the natural toxin

or by engineering bacteria to produce harmless variants of the toxin. Vaccines containing toxoid are

used when the pathogenicity of the organism is a result of the secreted toxin. Depending on the specific

vaccine, administration is generally via intramuscular or subcutaneous routes. Details of the various

vaccines are presented in the chapters in which the target micro - organisms are discussed.

C. Effect of age on efficacy of immunization

1. Passive immunity from mother: Newborns receive serum IgG antibodies from their mothers, which

gives them temporary protection against those diseases to which the mother was immune. In addition,

maternal milk also contains secretory antibodies that provide some protection against gastrointestinal

(GI) and respiratory tract infections.


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2. Active immunization: The infants antibody-producing capacity develops slowly during the first year

of life. Although the immune system is not fully developed, it is desirable to begin immunization at age 2

months because diseases are common in this age group and can be particularly severe (for example,

pertussis, H. influenzae meningitis). As with infants, older adults have a reduced antibody response to

vaccines.

Types of immune response to vaccines

Vaccines containing killed pathogens (such as hepatitis A or the Salk polio vaccine) or antigenic

components of pathogens (such as hepatitis B subunit vaccine) do not enter host cells, thereby eliciting

a primary B cellmediated humoral response. These antibodies are ineffective in attacking intracellular

organisms. By contrast, attenuated live vaccines (usually viruses) do penetrate cells. This results in the

production of intracellular antigens that are displayed on the surface of the infected cell, prompting a

cytotoxic T-cell response, which is effective in eliminating intracellular pathogens.

DNA VACCINESDNA vaccines represent a new approach to vaccination. The proposedmechanis m for these vaccines is

that the gene for the antigen of interest is cloned into a bacterial plasmid, which is engineered to

increase the expression of the inserted gene in mammalian cells. After being injected, the plasmid

enters a host cell where it remains in the nucleus as an episome (that is, it is not integrated into the

cells DNA). Using the host cells protein synthesis machinery, the plasmid DNA in the episome directs

the synthesis of the protein it encodes. This antigenic microbial protein may leave the cells and interact

with T helper and B cells, or it may be cleaved into fragments and presented as major histocompatiblity

complex I antigen complex on the cell surface, resulting in activation of killer T cells. To date, the

potency of DNA vaccines in humans has been disappointing.


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OVERVIEW OF ANTIMICROBIAL AGENTS

Antimicrobial drugs are effective in the treatment of infections because of their selective toxicity (that is,

they have the ability to kill or inhibit the growth of an invading microorganism without harming the cells

of the host). In most instances, the selective toxicity is relative, rather than absolute,requiring that the

concentration of the drug be carefully controlled to attack the microorganism while still being tolerated

by the host. Selective antimicrobial therapy takes advantage of the biochemical differences that exist

between microorganisms and human beings. The clinically useful antibacterial drugs are mainly

organized into six families: penicillins, cephalosporins, tetracyclines, aminoglycosides, macrolides, and

fluoroquinolones.

Applied Areas of Microbiology

Applied Microbiology refer to the use of microbes in their natural environment to perform processes

useful to humankind. Such processes include wastewater treatment and bioremediation. Like applied

microbiology, industrial microbiology involves the use of microorganisms to achieve specific goals.

Industrial microbiology, however, generally focuses on products such as pharmaceutical and medical

compounds (e.g., antibiotics, hormones, transformed steroids), solvents, organic acids, chemical

feedstocks, amino acids, and enzymes that have economic value.

One of the most active and important fields in microbiology is medical microbiology, which

deals with diseases of humans and animals. Medical microbiologists identify the agents causing

infectious diseases and plan measures for their control and elimination. Frequently they are involved in

tracking down new, unidentified pathogens such as the agent that causes variant Creutzfeldt-Jakob

disease, (the human version of mad cow disease) the hantavirus, the West Nile virus, and the virus

responsible for SARS. These microbiologists also study the ways in which microorganisms cause disease.

Public health microbiology is closely related to medical microbiology. Public health

microbiologists try to identify and control the spread of communicable diseases. They often monitor

community food establishments and water supplies in an attempt to keep them safe and free from

infectious disease agents.

Immunology is concerned with how the immune system protects the body from pathogens and

the response of infectious agents. It is one of the fastest growing areas in science; for example,

techniques for the production and use of monoclonal antibodies have developed extremely rapidly.

Immunology also deals with practical health problems such as the nature and treatment of allergies and

autoimmune diseases like rheumatoid arthritis.

Agricultural microbiology is concerned with the impact of microorganisms on agriculture.

Agricultural microbiologists try to combat plant diseases that attack important food crops, work on

methods to increase soil fertility and crop yields, and study the role of microorganisms living in thedigestive tracts of ruminants such as cattle. Currently there is great interest in using bacterial and viral

insect pathogens as substitutes for chemical pesticides.

Microbial ecology is concerned with the relationships between microorganisms and the

components of their living and nonliving habitats. Microbial ecologists study the global and local

contributions of microorganisms to the carbon, nitrogen, and sulfur cycles. The study of pollution effects

on microorganisms also is important because of the impact these organisms have on the environment.

Microbial ecologists are employing microorganisms in bioremediation (The use of biologically mediated

processes to remove or degrade pollutants from specific environments) to reduce pollution.


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Scientists working in food and dairy microbiologytry to prevent microbial spoilage of food and

the transmission of foodborne diseases such as botulism and salmonellosis. They also use

microorganisms to make foods such as cheeses, yogurts, pickles, and beer. In the future,

microorganisms themselves may become a more important nutrient source for livestock and humans.

Major products of Industrial Biology:


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In 1929, Alexander Fleming discovered that the fungus Penicillium produced what he called

penicillin, the first antibiotic that could successfully control bacterial infections. Although it took World

War II for scientists to learn how to mass produce it, scientists soon found other microorganisms

capable of producing additional antibiotics as well as compounds such as citric acid, vitamin B12, and

monosodium glutamate. Today, industrial microbiologists use microorganisms to make products such

as antibiotics, vaccines, steroids, alcohols and other solvents, vitamins, amino acids, and enzymes.

Industrial microbiologists identify microbes of use to industry. They also engineer microbes with

desirable traits and devise systems for culturing them and isolating the products they make.

Microbiologists working in microbial physiology and biochemistry study many aspects of the

biology of microorganisms. They may study the synthesis of antibiotics and toxins, microbial energy

production, the ways in which microorganisms survive harsh environmental conditions, microbial

nitrogen fixation, and the effects of chemical and physical agents on microbial growth and survival.

Microbial genetics andmolecular biology focus on the nature of genetic information and how it

regulates the development and function of cells and organisms. The use of microorganisms has been

very helpful in understanding gene structure and function. Microbial geneticists play an important role

in applied microbiology because they develop techniques that are useful in agricultural microbiology,

industrial microbiology, food and dairy microbiology, and medicine.

Documents

Introduction to Microbiol - Complete