[Virulence plasmids of Salmonella enterica--incidence and properties]

Salmonella virulence plasmids (SVPs) are large and closely related low-copy plasmids harbored by certain serovars of Salmonella enterica subspecies enterica. These serovars not only comprise those of veterinary significance like Abortusequi, Abortusovis, Choleraesuis, Dublin and Gallinarum/Pullorum, but also Typhimurium and Enteritidis which currently are the most prevalent serotypes in humans and food animals. Experiments with several animal species gave evidence that SVPs increase Salmonella strains' capabilities to replicate in extraintestinal organs of infected hosts thus leading to death of those hosts more frequently and rapidly. The common feature of all SVPs is the "Salmonella plasmid virulence" locus (spv-locus), a highly conserved 7.8 kbp region that is most responsible for the SVP-encoded virulence phenotype of Salmonella. Although functional characterisation of spv gene products has made some progress the molecular mechanism of spv-mediated virulence has not been fully elucidated yet. Some SVPs carry additional gene loci causatively related to Salmonella virulence like the pef-operon of Typhimurium and Enteritidis strains which encodes an adhesive type of fimbria, or genes traT, rsk and rck which are involved in serum resistance. The frequent occurrence of SVPs in host-adapted serovars suggests that SVP-encoded factors represented selective advantages to some Salmonella variants in their effort to colonize certain new niches during Salmonella evolution. This study provides an overview over current knowledge about the virulence plasmids of Salmonella enterica.