Expression of the Salmonella virulence plasmid gene spvB in cultured macrophages and nonphagocytic cells.

Certain serotypes of salmonellae carry virulence plasmids that greatly enhance the pathogenicity of these bacteria in experimentally infected mice. This phenotype is largely attributable to the 8-kb spv regulon. However, spv genes are not expressed while bacteria grow in vitro. We now show that spvB, which is required for virulence, is expressed rapidly after Salmonella dublin is ingested by cultured J774 and murine peritoneal macrophages and that expression is not affected by the alkalinization of intracellular vesicles. The level of induction of spvB is reduced when macrophages are pretreated with gamma interferon. spvB is also expressed in human and canine epithelial cell lines and a human hepatoma cell line. In all cases, spvB expression is dependent on the spvR gene, just as it is in stationary-phase cultures in vitro. These data suggest that spv virulence genes are expressed by intracellular salmonellae in vivo in response to a signal that is common to the intracellular compartments of cells that are invaded by salmonellae.