Plasmid-associated virulence of Salmonella typhimurium

We investigated the role of the 100-kilobase (kb) plasmid of Salmonella typhimurium in the virulence of this organism for mice. Three strains, LT2-Z, SR-11, and SL1344, which possessed 100-kb plasmids with identical restriction enzyme digestion profiles, were cured of their respective 100-kb plasmids after Tnmini-tet was used to label plasmids. Curing wild-type virulent strains SR-11 and SL1344 raised peroral 50% lethal doses from 3 x 10⁵ and 6 x 10⁴ CFU, respectively, to greater than 10⁸ CFU. Both wild-type strains had intraperitoneal 50% lethal doses of less than 50 CFU, whereas the intraperitoneal 50% lethal doses for cured SR-11 and SL1344 were less than 50 and 400 CFU, respectively. Reintroduction of the Tnmini-tet-labeled, 100-kb plasmid restored wild-type virulence. Invasion from Peyer's patches to mesenteric lymph nodes and spleens after peroral inoculation was the stage of pathogenesis most affected by curing S. typhimurium of the 100-kb plasmid. Wild-type S. typhimurium replicated in spleens of mice inoculated intravenously to a greater extent than did plasmid-cured derivatives. Wild-type and cured strains equally adhered to and invaded Henle-407, HEp-2, and CHO cells; furthermore, the presence of the 100-kb plasmid was not necessary for replication of S. typhimurium within CHO cells. The 100-kb plasmid had no effect on phagocytosis and killing of S. typhimurium by murine peritoneal macrophages in vitro and in vivo. Similarly, wild-type and plasmid-cured strains were resistant to killing by 90% normal human, rabbit, and guinea pig sera. All wild-type and plasmid-cured S. typhimurium strains possessed complete lipopolysaccharide, as determined by silver staining solubilized cells in sodium dodecyl sulfate-polyacrylamide gels. We have confirmed the role of the 100-kb plasmid of S. typhimurium in virulence primarily in invasion to mesenteric lymph nodes and spleens after peroral inoculation of mice. Involvement of the 100-kb plasmid in infection of mesenteric lymph nodes and spleens suggests a role for the plasmid in the complex interaction of S. typhimurium with cells of the reticuloendothelial system.