Effect of constitutively expressed phoP gene on the localization of Salmonella typhimurium within Mac-1 positive phagocytes

Intracellular localization of the wild-type (Spv(+)), the phoP-constitutive ly expressed strain (PhoP(c)), and the spy-deleted strain (Spv(-)) of Salmo nella typhimurium was examined by the use of confocal laser scanning micros copy analysis of immunostained sections of mouse spleens after oral or subc utaneous inoculation. Only 40% of salmonellae of both the PhoP(c) and the S pv(-) strains were detected intracellularly within Mac-1 positive cells at day five after oral or day four after subcutaneous inoculation. In contrast , over 85% of salmonellae of the Spv(+) strain were detected inside Mac-1 p ositive cells. In both inoculation trials, the splenic colony-forming unit values for the PhoP(c) and Spv(-) strains were significantly lower than the corresponding value for the Spv(+) strain. These findings suggest that the constitutively expressed phoP gene of S. typhimurium attenuated virulence by limiting intracellular proliferation within mouse spleen phagocytes, and that the lack of spv genes had the same effect.