Bacterial Genetic Variation, Gene Transfer and Evolution of Virulence I

Question 1

Describe the mechanisms that generate genetic diversity within a bacterial species and how these contribute to the evolution of virulence

Answer 1

• Spontaneous Mutation
• Recombination
• Acquisition of new DNA segments
• –acquisition of transposable elements (transposon, IS and CT)
• –bacteriophage conversion
• –acquisition of plasmids
• –acquisition of “pathogenicity islands”

Question 2

Describe spontaneous mutation and how it contribute to the evolution of virulence

Answer 2

Single base changes, deletions and insertions occur spontaneously within a population. Under appropriate selective pressure, the preferential growth of a pre-existing mutant within a population is selected.

1. Mutation rates. the rate of spontaneous mutation in bacteria is very low
2. Ex. Of medical importance: i) Increased resistance to antimicrobials in Pseudomonas and Mycobacterium tuberculosis, and ii) Streptococcus pyogenes strains with an increased likelihood of causing invasive disease due to a single amino acid change in pyogenic exotoxin B.

Question 3

Describe recombination and how it contribute to the evolution of virulence

Answer 3

Either site-specific or homologous recombination within a particular organism, or genetic exchange or recombination between closely related organisms can contribute to the emergence of strains with new properties.

1. Neisseria gonorrhoeae: phase variation involves the successive alternation between several antigenic forms of pili expressed on the cell surface. Recombination between variant pilin genes also produces essentially hybrid genes that encode pilin with new unique antigenic properties.
2. Standard genetic exchange between closely related but non-identical strains can produce recombinants that have a new constellation of phenotypic traits.

Question 4

Describe acquisition of new DNA segments and how it contribute to the evolution of virulence

Answer 4

Acquisition of new genes may alter the virulence potential, survival characteristics or antimicrobial resistance of the microorganism.

• –acquisition of transposable elements (transposon, IS and CT)
• –bacteriophage conversion
• –acquisition of plasmids
• –acquisition of “pathogenicity islands”

Question 5

Acquisition of transposable elements:

Answer 5

A transposable element (transposon) is a discrete segment of DNA which is capable of moving itself (or a copy of itself) from one chromosomal location to a new location within the cell. A transposon that is introduced into a cell, (e.g., as a component of a bacteriophage or plasmid) may transpose and become stabley and permanently integrated into the bacterial chromosome.
“Insertion Sequences” (IS elements) are transposons that simply encode transposase. Play a role in genome evolution by inactivating genes into which they transpose, or turning on expression of adjacent genes.
“Complex transposons” carry additional genes such as those encoding antibiotic resistance, toxins, adhesins and other virulence factors.

Question 6

Bacteriophage conversion:

Answer 6

Certain virulence genes (including those encoding diphtheria toxin, cholera toxin, streptococcal pyrogenic toxins, botulism toxins and certain LPS antigens) are carried on bacteriophage and are not a “normal” component of the respective bacterial genome. Therefore, the respective virulence factor is only carried and expressed by bacterial strains that have become lysogenized (see below) and the bacteriophage genome is stably maintained by the bacterium

Question 7

Acquisition of Plasmids:

Answer 7

Bacterial plasmids are autonomously replicating, usually circular, extrachromosomal DNA’s ranging in size from a couple genes to a few percent the size of the bacterial chromosome. Often they can be transferred from one bacterium to another by conjugation or transduction. Plasmids can carry virulence genes and genes conferring antibiotic resistance

Question 8

Acquisition of “Pathogenicity Islands”:

Answer 8

Pathogenicity Islands are generally relatively large segments of DNA present in the chromosome of some, but not all strains of a particular bacterial species. PIs encode genes that contribute to the virulence of these isolates. Bacterial isolates that lack the PI may be avirulent, or have a somewhat different disease-causing potential

Question 9

Discuss how spontaneous mutation and selection can interact to determine the genetic composition of bacterial populations

Answer 9

Single base changes, deletions and insertions occur spontaneously within a population. Under appropriate selective pressure (e.g., a patient receiving streptomycin), the preferential growth of a pre-existing mutant within a population is selected

Question 10

Transformation:

Answer 10

crude extracts, and ultimately pure DNA, taken from virulent, encapsulated strains of the pneumococcus (S forms) could convert avirulent, nonencapsulated strains (R form) to the virulent phenotype

• –active component is naked DNA
• –transformable species become competent for DNA uptake at only certain points of the growth cycle, and competence requires synthesis of specialized proteins to mediate the uptake
• –in natures, normally occurs between members of the same species

Question 11

Transduction:

Answer 11

gene transfer mediated by a bacteriophage. In transduction, bacterial viruses (bacteriophages) transfer segments of DNA (a couple genes up to a couple hundred genes) from one cell to another.
-Mechanism of Transduction: segment of the donor cell genome (chromosome or resident plasmids) may be passed into another cell

Question 12

Conjugation:

Answer 12

form of genetic transfer that is dependent upon physical contact between the donor and recipient cells, and is usually mediated by certain types of bacterial plasmids
-plasmids: replicating, usually circular, extrachromosomal elements ranging in size from a couple genes to a few percent the size of the bacterial chromosome. Often encode abx resistance and virulence factors. Can acquire new genetic material through transposition

Question 13

Distinguish between the lytic and lysogenic state.

Answer 13

Temperate phages may elicit a lytic or lysogenic response in which the host cell remains viable and the infecting phage DNA is maintained by the host cell in a noninfectious state known as prophage

• Lytic state: phage multiplication and host cell lysis
• Lysogenic state: the host cell remains viable and the infecting phage DNA is maintained by the host cell in a noninfectious state known as prophage

Question 14

Describe how errors in bacteriophage development can lead to phage-mediated gene transfer

Answer 14

Generalized transducing phages are formed as a consequence of errors in DNA packaging during phage assembly. Occasionally, the phage’s packaging system will insert a “headful”-sized piece of bacterial DNA into a maturing phage capsid in place of a normal phage DNA molecule. These errors occur at a frequency ca 10-3, and the amount of DNA incorporated may represent 1-2% of the bacterial genome. These transducing particles contain no viral genetic information, but they are still able to attach to other host cells and inject the bacterial DNA which they contain. The injected DNA may then recombine with homologous segments in the recipient genome to produce a genetic recombinant, or transductant

Question 15

Define lysogenic conversion. Distinguish between lysogenic conversion and generalized transduction

Answer 15

• Lysogenic conversion: Certain temperate bacteriophage encodes gene(s) which may be expressed during the lysogenic state and cause the appearance of a new phenotypic trait (e.g. toxin production in C. diptheriae) in the lysogenic host. The converting genes are not found alone as normal constituents of the bacterial genome. Diphtheria toxin, scarlet fever toxin, cholera toxin and certain types of botulism toxin are all bacteriophage-encoded toxins and produced by strains of bacteria that have become lysogenized by the respective bacteriophage.
• Generalized transduction: In transduction, bacterial viruses (bacteriophages) transfer segments of DNA (a couple genes up to a couple hundred genes) from one cell to another.