Pathogenicity islands and phages in Vibrio cholerae evolution

The identification of accessory genetic elements (plasmids, phages and chromosomal 'pathogenicity islands') encoding virulence-associated genes has facilitated our efforts to understand the origination of pathogenic microorganisms. Toxigenic Vibrio cholerae, the etiologic agent of cholera, represents a paradigm for this process in that this organism evolved from environmental nonpathogenic V. cholerae by acquisition of virulence genes. The major virulence genes in V. cholerae, which are clustered in several chromosomal regions, appear to have been recently acquired from phages or through undefined horizontal gene transfer events. Evidence is accumulating that the interactions of phages with each other can also influence the emergence of pathogenic clones of V. cholerae. Therefore, to track the evolution of pathogens from their nonpathogenic progenitors, it is also…