Role of the acid tolerance response in virulence of Salmonella typhimurium

During its life cycle, Salmonella typhimurium is exposed to a variety of acidic conditions. Survival in the acidic environments within the host may require the adaptive acid tolerance response (ATR), which is characterized by the induction of several Salmonella proteins upon exposure to mildly acidic conditions. These induced proteins protect the bacterium from death under severe acid challenge. The goal of this study was to examine the role of the ATR in Salmonella pathogenesis. Initially, we observed that differences exist between the virulent S. typhimurium strains and the laboratory S. typhimurium strain LT2 with respect to their ATR. Mutations affecting the ATR of S. typhimurium LT2, including atrB, atrC (polA), atrD, atbR, and fur, were crossed into virulent Salmonella strains, and the resultant transductants were screened for virulence in mice and acid sensitivity. Surprisingly, with the exception of the fur mutation, none of the mutations had a major effect on acid resistance or virulence in the pathogenic strains. The fur mutants showed a 1- to 3-log increase in the 50% lethal dose; however, the magnitude of its effect was dependent on the strain background. Strains containing two or three different air mutations were constructed, and these were also examined for acid sensitivity and virulence. The double and triple mutants that contained an atrC mutation no longer displayed an ATR. Those mutants which were more acid sensitive were also highly attenuated, suggesting a strong correlation between the ability to mount an ATR and virulence in S. typhimurium. Comparison of the ability of the various atr single, double, and triple mutants to survive within macrophages showed that strains containing an atrC mutation survived much less well than the wild type in bone marrow- derived macrophages. No differences in survival within J774 macrophage-like cells were detected.