Genetic Variation, Gene Transfer, and the Evolution of Virulence Flashcards
What are three mechanisms that produce genetic variation in bacteria?
Spontaneous mutation, Recombination, Acquisition of new DNA segments.
How do spontaneous mutation and selection interact to determine the composition of bacterial populations?
Spontaneous mutations occur at a rate of 10^-6 to 10^-10 per cell generation. However, these mutations may produce a slight reproductive advantage in different environments. Appropriate selective pressure, such as a patient receiving streptomycin, the preferential growth of a pre-existing mutant allows it to become the dominant population.
How does genetic recombination occur in bacteria?
Homologous recombination within an organism or genetic exchange and recombination between closely related organisms can lead to new strains. Neisseria gonorrhea phase variation involves alternation between forms of pili. Recombination between the expressed pili and another unexpressed pili causes the creation of an entirely new, antigenically distinct pili.
What is involved in the acquisition of transposable elements such as transposons, IS elements, and “complex transposons”?
A transposon is a segment of DNA capable of moving itself from one chromosomal location to another, but cannot self-replicate. They usually encode for transposase, which mediates the tranposistion. IS elements merely code for the transposase and may activate or inactivate the genes they insert near or in. Complex transposons carry additional genes such as those encoding antibiotic resistance, toxins, adhesins, and other virulence factors.
What are “Pathogenicity Islands” and how do they represent the acquisition of new DNA?
Pathogenicity Islands (PIs) generally relatively large segments of DNA present in the chromosomes of some but not all strains of a particular species. They encode genes that contribute to the virulence of these strains. Acquisition from a heterologous organism is often implicated, and occasionally they contain site-specific recombination sequences similar to bacterial viruses suggesting viral transmission.
What is transformation?
Transformation is the uptake of “naked” DNA in an environment by competent bacteria. Competency is restricted to certain points in the life cycle or by chemical induction. It has been observed in Gram-positive and Gram-negative bacteria, and represents a mechanism of acquisition of new genes and genetic potential.
What is transduction?
Transduction is gene transfer mediated by a bacteriophage. This requires a temperate bacteriophage that inserts its genetic package into the host cell’s chromosome but does not immediately enter the lytic cycle. This results in a lysogenic response wherein the viral DNA is replicated during every cell division and is known as a prophage.
What is the mechanism of generalized transduction?
Generalized transducing phages are produced by errors in DNA packaging wherein a large portion of bacterial DNA is inserted into the phage. The transducing phage contains no viral DNA, but may still inject the bacterial DNA into another cell. The DNA may then recombine with homologous segments and produce a transductant. Bacterial plasmids may also be transferred by this method, particularly among Gram-positive bacteria.
What is lysogenic conversion, and how does it differ from generalized transduction?
Lysogenic conversion (aka Bacteriophage conversion), occurs when a temperate bacteriophage DNA encodes a gene that is then expressed during the lysogenic state (e.g., diphtheria toxin). The genes controlling the new phenotypic trait are found ONLY as a component of the phage genome, unlike generalized transduction which involves bacterial genes. Scarlet fever, cholera, and botulism toxins are all bacteriophage-encoded toxins.
What is conjugation?
Conjugation involves the transfer of genetic material from one bacteria to another via direct contact through sex pili. This material is contained on plasmids, which often contain genes for drug resistance or virulence factors. Transmission of the F plasmid is coupled to a special round of DNA synthesis which is initiated at a site called the oriT (origin of Transfer). The DNA is transferred as a single strand through a conjugation bridge (sex pilus).
What are conjugative transposons?
Conjugative transposons are mobile elements that mediate conjugation between pairs of cells in which the transferred element is the conjugative transposon itself. It then integrates into the chromosome of the new cell. It therefore encodes both the transfer (tra) and transposition genes. May encode antibiotic resistance, and may cause the widespread dissemination of the tetracycline resistance gene (tetM).
