THE ACID TOLERANCE RESPONSE OF SALMONELLA-TYPHIMURIUM PROVIDES PROTECTION AGAINST ORGANIC-ACIDS

Salmonella typhimurium encounters a variety of acid stress situations during pathogenesis and in the natural environment. These include the extreme low ph encountered in the stomach and a less acidic intestinal environment containing large amounts of organic weak acids (volatile fatty acids). The acid tolerance response (ATR) is a complex defence s ystem that can minimize the lethal effects of extreme low pH (pH 3). T he data presented illustrate that the ATR can also defend against weak acids such as butyric, acetic or propionic acids. Although an acid sh ock of pH 4.4 induced the ATR, growth in subinhibitory concentrations of weak acids did not. Various mutations shown to affect tolerance to extreme acid conditions (pH 3) were tested for their effects on tolera nce to weak acids. An rpoS mutant lacking the alternative sigma factor sigma(s) failed to protect cells against weak acids as well as extrem e acid ph. The fur (ferric uptake regulator) and atp (Mg2+-dependent A TPase) mutants defective in extreme acid tolerance showed no defects i n their tolerance to weak acids. Curiously, the atbR mutant that exhib its increased tolerance to extreme acid ph proved sensitive to weak ac ids. Several insertions that rendered cells sensitive to organic acids were isolated, all of which proved to be linked to the rpoS locus.