COMPARATIVE-ANALYSIS OF EXTREME ACID SURVIVAL IN SALMONELLA-TYPHIMURIUM, SHIGELLA-FLEXNERI, AND ESCHERICHIA-COLI

Several members of the family Enterobacteriaceae were examined for dif ferences in extreme acid survival strategies. A surprising degree of v ariety was found between three related genera. The minimum growth pH o f Salmonella typhimurium was shown to be significantly lower (pH 4.0) than that of either Escherichia coli (pH 4.4) or Shigella flexneri (pH 4.8), yet E. coli and S. flexneri both survive exposure to lower pH l evels (2 to 2.5) than S. typhimurium (pH 3.0) in complex medium. S. ty phimurium and E. coli but not S. flexneri expressed low-pH-inducible l og-phase and stationary-phase acid tolerance response (ATR) systems th at function in minimal or complex medium to protect cells to pH 3.0. A ll of the organisms also expressed a pa-independent general stress res istance system that contributed to acid survival during stationary pha se. E. coli and S. flexneri possessed several acid survival systems (t ermed acid resistance [AR]) that were not demonstrable in S. typhimuri um. These additional AR systems protected cells to pH 2.5 and below bu t required supplementation of minimal medium for either induction or f unction. One acid-inducible AR system required oxidative growth in com plex medium for expression hut successfully protected cells to pH 2.5 in unsupplemented minimal medium, while two other AR systems important for fermentatively grown cells required the addition of either glutam ate or arginine during pH 2.5 acid challenge. The arginine AR system w as only observed in E. coli and required stationary-phase induction in acidified complex medium. The product of the adi locus, arginine deca rboxylase, was responsible for arginine-based acid survival.