Stress induction of the virulence proteins (SpvA, -B, and -C) from native plasmid pSDL2 of Salmonella dublin.

The virulence region of the wild-type plasmid pSDL2 contained in Salmonella dublin is highly conserved among plasmids from several nontyphoid Salmonella serotypes and is essential for the development of systemic infection in BALB/c mice. Polyclonal antibodies against three proteins (SpvA, -B, and -C) expressed from a 4.1-kb EcoRI subclone of the plasmid virulence region were generated. These antibodies were used to detect expression of the Spv proteins when S. dublin was grown in vitro under stress-inducing conditions, such as nutrient deprivation and increased temperature, that the bacteria may encounter during the course of infection within the host. Glucose starvation resulted in expression of all three proteins shortly after the lag phase. When the bacteria were grown to the late-log phase without glucose, heat shock strongly induced expression of SpvA but not SpvB or SpvC. The addition of 0.2% glucose to the medium resulted in loss of expression of the proteins until the late-log to stationary phase. Iron limitation or lowered pH induced expression of the proteins during exponential growth even in the presence of glucose. Insertion mutations into the positive regulator gene spvR upstream from spvABC and insertions into spvA and spvC resulted in loss of expression of SpvA, -B, and -C, suggesting a complex regulation of expression. These studies define a variety of environmental conditions that induce expression of the Spv virulence proteins from the wild-type plasmid pSDL2 in S. dublin in vitro.