AN ALTERED RPOS ALLELE CONTRIBUTES TO THE AVIRULENCE OF SALMONELLA-TYPHIMURIUM LT2

Virulent Salmonella typhimurium strains differ from the attenuated lab oratory strain LT2 at the rpoS locus, It was previously shown that the rpoS gene in strain LT2 contains a rare UUG start codon (I, S, Lee, J , Lin, H. K, Hall, B, Bearson, and J, W, Foster, Mel. Microbiol, 17:15 5-167, 1995), This difference is responsible for the inability of LT2 to display a sustained log-phase acid tolerance response. We show that the altered rpoS allele (rpoS(LT2)) also affects the stationary-phase acid tolerance response in Salmonella. By transducing the rpoS(LT2) a llele into virulent strain backgrounds and crossing wild-type rpoS all ele into strain LT2, we demonstrate that the rpoS(LT2) allele contribu tes to the attenuation of strain LT2, We examined the effect of the rp oS allele on invasion and found that the rpoS status of the cell had n o effect on the ability of the strains to invade intestinal epithelial cells in tissue culture, Enumeration of bacteria from tissues of infe cted mice indicated that the presence of the rpoS(LT2) allele affected the ability of S, typhimurium to reach the liver and spleen and to pe rsist in several tissues at 6 days postinfection. This is likely doe, at least in part, to a decrease in spy gene expression in these mutant s, We demonstrate that strains containing the rpoS(LT2) allele are not only sensitive to pH 3.0 (acid stress) but are also sensitive to the DNA-damaging agent methyl methanesulfonate, However, these strains app ear to survive stationary-phase and oxidative stresses as well as stra ins containing a wild-type rpoS allele, Despite an increased sensitivi ty to acid stress and DNA damage, strains containing either an rpoS-nu ll mutation or the rpoS(LT2) allele survived in J774 cells and bone ma rrow-derived macrophages as well as did otherwise isogenic strains wit h a wild-type rpoS allele.