Contribution of the PhoP/Q regulon to survival and replication of Salmonella enterica serovar Typhimurium in macrophages

The ability of serovars of Salmonella enterica to cause systemic disease is dependent upon their survival and replication within macrophages. To do this, bacteria must withstand or surmount bacteriostatic and bactericidal responses by the host cell, including the delivery of hydrolytic enzymes from lysosomes to the phagosome. The bacterial two-component regulatory system PhoP/Q has been implicated in avoidance of phagolysosomal fusion by S. enterica serovar Typhimurium (S. Typhimurium) in murine macrophages. In this study, the involvement of PhoP/Q-activated genes in avoidance of phagolysosomal fusion was analysed: of all the S. Typhimurium mutant strains tested, only an mgtC mutant strain partially reproduced the phenotype of the phoP mutant strain. As this gene is required for bacterial growth in magnesium-depleted conditions in vitro, the contributions of PhoP/Q to intramacrophage replication and survival were reappraised. Although PhoP/Q was required for both replication and survival of S. Typhimurium within murine macrophages, subsequent analysis of the kinetics of phagolysosomal fusion, taking account of differences in the replication rates of wild-type and phoP mutant strains, provided no evidence for a PhoP/Q-dependent role in this process. PhoP/Q appeared to act subsequent to the process of phagolysosomal avoidance and to promote replication of those bacteria that had already escaped a phagolysosomal fate. Therefore, we conclude that the PhoP/Q regulon enables S. Typhimurium to adapt to intramacrophage stresses other than phagolysosomal fusion.