Bacterial Genetic Variation, Gene Transfer and Evolution of Virulence Flashcards
Describe the mechanisms that generate genetic diversity within a bacterial species and how these contribute to the evolution of virulence.
Spontaneous mutation: single base changes, deletions, and insertions. Under appropriate selective pressure, the preferential growth of a pre-existing mutant within a population is selected.
Recombination: Either site-specific or homologous recombination within a particular organism, or genetic exchange and recombination between closely related organisms. Can contribute to the emergence of strains with new properties.
Acquisition of new DNA segments: Lateral transfer from other bacteria, unrelated species, possibly eukaryotic organisms. May alter virulence potential, survival characteristics, or antimicrobial resistance. Can occur with transposons, IS elements, complex transposons. Bacteriophage conversion, acquisition of plasmids, or acquisition of pathogenicity islands (large segments of DNA present in the chromosome of some bacteria species that encode genes that contribute to the virulence of these isolates.)
Discuss how spontaneous mutation and selection can interact to determine the genetic composition of bacterial populations
In rare instance, mutation can confer a selective advantage, resulting in trait that makes bacteria more virulent (e.g. antibotic resistance).
Distinguish between transformation, transduction and conjugation as mechanisms of gene transfer. Identify the salient features of each mechanism
Transformation: genetetic transfer of naked DNA (usually DNA released from lysing cells), is uptaken by bacteria and is incorporated into the genome.
Transduction: gene transfer mediated by a bacteriophage–> bacteriophages transfer segments of DNA from one cell to another.
Virus enters bacteria, viral genome is replicated and virus genes are transcribe and translated. Viral components are being synthesized (latent period). At this point, virus can enter lytic cycle (phage multiplication and host cell lysis) or lysogenic cycle (host cell remains viable and the infecting phage DNA is maintained by the host cell in a noninfectious state known as prophage.)
Conjugation: form of genetic transfer that is dependent upon physical contact between donor and recipeint cells, usually mediated by certain types of bacterial plasmids.
