Pathogenic bacteria are thought to emerge when groups of virulence genes are transferred together into a previously avirulent bacterium by a process called horizontal gene transfer (to distinguish it from vertical gene transfer from parent to offspring). Horizontal transfer can occur by one of three mechanisms: natural transformation by released naked DNA, transduction by bacteriophages, or sexual exchange by conjugation (Figure 23–4B and Movie 23.1). Sequencing the genomes of large numbers of pathogenic and nonpathogenic bacteria has indicated that horizontal gene transfer has made important contributions to bacterial evolution, enabling species to inhabit new ecological and nutritional niches, as well as to cause disease. Even within a single bacterial species, the amount of chromosomal
E. coli Shigella fexneri Salmonella enterica
Figure 23–4 Genetic differences between pathogenic and nonpathogenic bacteria. (A) Genetic differences between nonpathogenic E. coli and two closely related food-borne pathogens—Shigella flexneri, which causes dysentery, and Salmonella enterica, a common cause of food poisoning. Nonpathogenic E. coli has a single circular chromosome. The chromosome of S. flexneri differs from that of E. coli in a limited number of locations; most of the genes required for pathogenesis (virulence genes) are carried on an extrachromosomal virulence plasmid. The chromosome of S. enterica carries two large inserts (pathogenicity islands) not found in the E. coli chromosome; these inserts each contain many virulence genes.
(B) Bacterial pathogens evolve by horizontal gene transfer. This can occur by three mechanisms: natural transformation, in which naked DNA is taken in by competent bacteria; transduction, in which bacterial viruses (bacteriophages) transfer DNA from one bacterium into another; and conjugation, during which plasmid DNA, and even chromosomal DNA, is transferred from a donor to a recipient bacterium.
V.cholerae
V. cholerae O1 serogroup V. cholerae O1 serogroup Classical V. cholerae O139 serogroup VSP1 VSP2 CTX˜Classical CTX˜El Tor RS1˜O139 antigen SXT 1st through 6th pandemics 7th pandemic
V.cholerae variation is astonishing; the genomes of different strains of Escherichia coli can differ by as much as 25%. Such variation has led to the concept that a bacterial species has both a core genome common to all isolates within the species and a larger pan-genome consisting of all genes present in the full spectrum of isolates.