In
Salmonella typhimurium, the transcription of several virulence genes including
spvB is regulated by the PhoP/PhoQ regulatory system. To further examine the relationship between the PhoP/PhoQ and Spv systems for virulence in mice, we examined a non-polar
phoP mutation combined with different virulence plasmid genotypes for effects on virulence of
S. typhimurium in the mouse model. PhoP
-/Spv
+ and PhoP
-/Spv
- mutants were not detectably recovered from the spleens of subcutaneously or orally inoculated mice. The
phoP gene constitutively expressed from the
lacZ promoter of a low copy number vector (
phoPC) only partially complemented the non-polar
phoP mutation for mouse-virulence in both the Spv
+ and Spv
- backgrounds; both PhoP
C strains exhibited virulence equal only to a PhoP
+/Spv
- strain. Interestingly, in a PhoP
+ background, the
phoPC gene reduced splenic infection of the Spv
+ but not Spv
- salmonellae after subcutaneous or oral inoculation compared with the PhoP
+ parents. Additionally, the
phoPC gene in an Spv
+ background reduced the net growth of salmonellae in macrophages
in vitro;
phoPC in an Spv
- background was without effect. These data suggest that the constitutive expression of the
phoP gene attenuates the virulence of
S. typhimurium in mice in an Spv-dependent manner.
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