The genes of and other non-typhoidal serovars are essential for efficient systemic infection beyond the intestines in orally inoculated mice as a model for enteric fever. These virulence genes are not significantly expressed by salmonellae during exponential growth in broth but are induced when the bacteria enter the stationary phase of growth. Using RNase protection analysis to directly measure mRNA from the virulence plasmid of we found that was maximally induced in an SpvR- and RpoS-dependent manner during exponential growth in Intracellular Salts Medium, which mimics the intracellular environment of mammalian cells. A cloned operon fusion in was induced intracellularly in peritoneal cells of mice, correlating intracellular gene expression with intracellular function of the genes in infected mice. was also induced intracellularly within both Henle-407 intestinal epithelial cells and J774.A1 macrophage-like cells when the bacteria were replicating with exponential kinetics. Prevention of invasion of salmonellae with cytochalasin inhibited induction within tissue culture cells, indicating that salmonellae must be internalized for to be induced. The fusion was not induced by salmonellae in extracellular fluid of the peritoneal cavity or in serum. Since induction of the spv genes occurs intracellularly during exponential growth of salmonellae, cessation of growth may not be the most relevant inducing signal for spv gene expression.


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