Pathogenicity mechanisms of prokaryotic cells: an evolutionary view.

The success of pathogenic microbes depends on their ability to colonize host tissues and to counter host defense mechanisms. Microorganisms can produce overwhelming infection because of their relatively short generation times, and because they have evolved powerful mechanisms for generating phenotypic diversity as an efficient strategy for adapting to rapidly responding immune system defenses and the broad range of polymorphisms characteristic of different host tissues. Bacterial evolution may not be a continuous process, but more of a succession of temporally spaced major events. These events cause a non-gradual sequence of adaptations to a given environment. The pathogenicity islands are genetically unstable elements, and many of the genes coding for the adhesins, toxins and other virulence factors are present in pathogenicity islands, which almost certainly had former lives as accessory elements or as parts thereof, or were borne on functional accessory elements. Novel genes are also acquired by transduction (mediated by bacteriophages, plasmids or transposons), by conjugation (DNA transfer between cells) or by transformation (natural DNA uptake). Horizontal gene transfer from other species is a major source of variation and is fundamental to the genetic theory of adaptive evolution in prokaryotes.