BIOLOGY AND CLINICAL-SIGNIFICANCE OF VIRULENCE PLASMIDS IN SALMONELLASEROVARS

Non-typhoid Salmonella strains containing virulence plasmids are highl y associated with bacteremia and disseminated infection in humans, The se plasmids are found in Salmonella serovars adapted to domestic anima ls, such as Salmonella dublin and Salmonella choleraesuis, as well as in the widely distributed pathogens Salmonella typhimurium and Salmone lla enteritidis, Although virulence plasmids differ between serovars, all contain a highly conserved 8-kb region containing the spy locus th at encodes the spvR regulatory gene and four structural spvABCD genes, Studies in mice suggest that the spy genes enhance the ability of Sal monella strains to grow within cells of the reticuloendothelial system , The spy genes are not expressed during exponential growth in vitro b ut are rapidly induced following entry of Salmonella strains into mamm alian cells, including macrophages, Transcription of the spy genes is controlled by the stationary-phase a factor RpoS, and mutations in Rpo S abolish virulence, These studies suggest that the ability of Salmone lla strains to respond to starvation stress in the host tissues is an essential component of virulence.