In vivo genetic analysis indicates that PhoP-PhoQ and the Salmonella pathogenicity island 2 type III secretion system contribute independently to Salmonella enterica serovar typhimurium virulence

Many virulence factors are required for Salmonella enterica serovar Typhimurium to replicate intracellularly and proliferate systemically within mice. In this work, we have carried out genetic analyses in vivo to determine the functional relationship between two major virulence factors necessary for systemic infection by S. enterica serovar Typhimurium: the Salmonella pathogenicity island 2 (SPI-2) type III secretion system (TTSS) and the PhoP-PhoQ two-component regulatory system. Although previous work suggested that PhoP-PhoQ regulates SPI-2 TTSS gene expression in vitro, in vivo competitive analysis of mutant strains indicates that these systems contribute independently to S. typhimurium virulence. Our results also suggest that mutation of phoP may compensate partially for defects in the SPI-2 TTSS by deregulating SPI-1 TTSS expression. These results provide an explanation for previous reports showing an apparent functional overlap between these two systems in vitro.