Dynamics of the bacterial genome: deletions and integrations as mechanisms of bacterial virulence modulation.

Bacterial virulence is a multifactorial phenomenon, arising from the coordinate action of special abilities of the infectious agents, termed as virulence factors, which is crucial for the infectious process. The genetic determinants encoding those factors are termed virulence associated genes, which can be located on the bacterial chromosome or on extrachromosomal elements (plasmids). Various examples have repeatedly demonstrated that bacterial genome dynamics contributes to virulence modulation. Strikingly, a reduced in vivo virulence of the pathogens was shown to be due to the spontaneous loss of virulence associated genes. The deletion events can involve chromosomal as well as plasmid regions. Also integration of plasmids into the chromosome are considered as dynamic events. The new genetic location of the formerly plasmid encoded virulence associated genes can result in an alteration of virulence expression.