Bacterial virulence gene regulation: An evolutionary perspective

Coevolution between bacteria and their plant or animal hosts determines cha racteristics of the interaction, the bacterial virulence genes involved, an d the regulatory systems controlling expression of virulence genes. The lon g-standing association between Salmonellae and their animal hosts has resul ted in the acquisition by Salmonella subspecies of a variety of virulence g enes and the evolution of complex regulatory networks. The particular reper toire of virulence genes acquired by different Salmonella enterica subspeci es and the regulatory systems that control them dictate subspecies-specific infection characteristics. Although the association between Vibrio cholera e and humans appears to be more recent, to reflect a simpler pathogenic str ategy, and to involve fewer virulence genes than that of Salmonellae, compl ex virulence-regulatory networks have nonetheless evolved. In contrast, the re is no evidence for acquisition of virulence genes by horizontal gene tra nsfer in bordetellae, and their virulence regulon is less complex in overal l structure than those of salmonellae and Vibrio cholerae. In Bordetellae, subspecies-specific differences in pathogenic strategy appear to result fro m differential gene expression within and across Bordetella subspecies.