Exponential-phase expression of spvA of the Salmonella typhimurium virulence plasmid: induction in intracellular salts medium and intracellularly in mice and cultured mammalian cells

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Abstract
Thespvgenes ofSalmonella typhimuriumand other non-typhoidalSalmonellaserovars 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 inLbroth but are induced when the bacteria enter the stationary phase of growth. Using RNase protection analysis to directly measurespvAmRNA from the virulence plasmid ofS. typhimurium,we found thatspvAwas 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 clonedspvA-lacZoperon fusion inS. typhimuriumwas induced intracellularly in peritoneal cells of mice, correlatingin vivointracellular gene expression with intracellular function of thespvgenes in infected mice.spvAwas also induced intracellularlyin vitrowithin 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 cytochalasinDinhibitedspvAinduction within tissue culture cells, indicating that salmonellae must be internalized forspvAto be induced. ThespvA-lacZfusion 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.