Microbial Genetics 6

7/31/2019 Microbial Genetics 6

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Microbial Genetics:Prokaryotic Genome:

-Most prokaryotic genes are carried on the bacterial chromosome,

a single circle containing about 4000 kbp of DNA.

-Many bacteria contain additional genes onplasmids.

-Plasmid is an extra-circular supercoiled DNA that carry some

important genes.

-Bothbacteria chromosome

and plasmid arecalled replicons.

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-Genes essential for bacterial growth are carried on chromosome,

and plasmid carry genes of specialized functions.

Examples on plasmid genes:

1- E. coliplasmids carry genes responsible for sucrose and

citrate uptake.

2- Pseudomonas species plasmids carry genes responsible for

degradation of octane, and salicylic acid.

Transposonsare genetic elements that contain several kbp of

DNA, including the information necessary for their migration

from one genetic locus to another inside the bacterial cell.


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Mutation:

Mutations are changes in the DNA sequence that greatly enhanced

by exposure to mutagens.

Types of Mutations:

1- Base Substitution.

2- Deletion.

3- Insertions

4- Inversion.


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Genetic Transfer:

-Transfer of DNA among prokaryotes is widespread between

different strains of same species.

-DNA transfer makes a major contribution to the remarkable genetic

diversity of bacteria.

Mechanisms of Gene Transfer:1- Conjugation. 2- Transduction.

3- Transformation.

Conjugation: plasmids are the genetic elements most frequently

transferred by conjugation.

Conjugation is a mechanism by which genes ( plasmids) will be

transferred from one bacterial cell to another by a mean of Sex pili.


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Conjugation:n


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Transduction:

Transduction is a phage-mediated genetic transfer.

In this process, a fragment of bacterial DNA present in bacteriophage

will be transferred to another bacterial cell by means ofinfection.

The phage viruses will infect the second bacterium forming plaques.

Under certain conditions, lyses of the second bacterium will not be

induced by the virus; this will produce a new bacterial strain that

carry a recombinant DNA.


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Transduction:n


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Transformation:

Transformation is a change of cellular phenotype due to direct

uptake of specific gene or incorporation of that gene inside host cell

Chromosome by electrical shock (electroporation).


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The Microbial Virulence factors:

Virulence factors are external cellular structures, enzymes, and

toxins that enhance microbial pathogenicity.

In general, the most important virulence factors are:

1-Microbial capsule:

Microbe resist host acidic environment (stomach gastric acid).

Microbe resist host proteolytic enzyme(Present in Saliva, and stomach).Microbe resist phagocytosis.

2-Fimbriae or Pili:

Microbial adhesion to the host cell surface.

Adhesion could be also enhanced by receptor-antigen interaction.3-Microbial Enzymes:

Collagenase enzyme enhances microbial invasion; due to degradation

of extracellular matrix components.

Urease: Neutralization of acidic pH ( urine, stomach).


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4-Bacteria Toxins:

A-Exotoxins:

- Well known poisonous substances.

- Chemical nature: Proteins (two polypeptide components).

- Almost all are Heat-labile at 60 C.

- Intracellular toxin fraction could:

1-inhibit cellular protein biosynthesis.

2-Cause ionic imbalance and loss of water.

3-inhibit the release of neurotransmitters.

B-Endotoxins:-Chemical nature: Lipopolysaccharide, the component of Grams

negative bacterial outer membrane.

-Heat-Stable at 100 C.


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5-The microbial Hemolysin:

-Degradation of RBCs, Hemoglobin and NADH will be released.

6-The microbial Haemagglutinine and Coagulase enzyme:

-Agglutination of RBCs; the microbe escapes Humoral

immunity.

7-The microbial Beta-Lactamases:

-Some microbes have ability to resist antibiotics due to production

ofBeta-Lactamase enzymes.


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Pyogenic Cocci: The Grams positive Cocci:

1-Staphylococci :

Morphology:It is a Gram positive Cocci, one micrometer in diameter, arranged in

clusters, non spore formers, and non-motile.

Staphylococcus aureus species are capsulated by a microcapsule.


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Cultural Characteristics:

-Staphylococcus species are facultative anaerobic bacteria.

-All species grow best on nutrient agar and blood agar, forms large

yellow to creamy colonies.

-Staphylococcus aureus is often hemolytic on blood agar.

- Mannitol salt agar is a selective media for Staphylococcus species.

-It can grow at a temperature

range of 15-45 C, and NaCl

concentration as high as 15%.


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Mannitol Fermentation:

Staphylococcus aureus ferments Mannitol.

Staphylococcus epidermidis is a non-Mannitol fermenter.


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Biochemical activity ofStaphylococci:

Staphylococcus are catalase positive bacteria.

Staphylococcus aureus produces Dnase, liquefy gelatin, and

coagulate blood plasma.


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Staphylococcus Virulence factors:

1- Enzymes: leukocidin, hemolysins, catalase, and leukotoxins.

2-Surface proteins and Microcapsule.

3-Toxins:

A- Exfoliative toxin (dermatotoxin): scaled skin syndrome.

B- Toxic shock toxin-1: toxic shock syndrome.

C- Enterotoxins: food poisoning.


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Streptococcus species:

- Gram positive cocci 0.5-1.0m in diameter, occur in pairs and in

short chains.

- None-motile, none- spore formers, some strains are capsulated.

- All species are catalase negative.

- Strict anaerobes and facultative anaerobes.


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Classification ofStreptococcus species:

Streptococcus species are classified according to hemolytic activity.


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Beta and Alpha hemolytic Streptococci:

Beta-Hemolytic: Alpha Hemolytic

Streptococcus pyogenes Streptococcus pneumoniae


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The Gram negative Diplococci:

Neissereiae meningitidis

Neissereiae gonorrhoeae

Microscopy:

Gram negative cocci 0.8 m in diameter, Kidney-shaped,

occur in pairs, in clinical specimens: present inside

the polymorphonuclear cells.Non-motile, non-sporing,N. meningitidis are capsulated.


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Cultural characteristics:

They grow best on Thayer-Martin Medium, Chocolate agar

supplemented by vancomycin and nystatin.

Optimum growth occur under microaerophilic conditions

( 5-10%) CO2 at 35-36C in a moist atmosphere.

-Both factor X and V (hemoglobin, NAD) are required for

Neissereiae species cultivation.

-Colonies ofN. meningitidis are grey, and slightly convex of0.5-1.0 mm in diameter.

In 8-24 hours old culture ,colonies ofN. gonorrhoeae are rough

with crenate margin.


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Cultural characteristics and Biochemical activities:

All Neissereriae species are oxidase positive.

N. gonorrhoeae ferments glucose,WhileN. meningitidis ferments

glucose and maltose.

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Microbial Genetics 6