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 system that can minimize the lethal effects of extreme low pH (pH 3). The data presented illustrate that the ATR can also defend against weak acids such as butyric, acetic or propionic acids. Although an acid shock 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 tolerance to weak acids. An mutant lacking the alternative sigma factor s failed to protect cells against weak acids as well as extreme acid pH. The (ferric uptake regulator) and (Mg-dependent ATPase) mutants defective in extreme acid tolerance showed no defects in their tolerance to weak acids. Curiously, the mutant that exhibits increased tolerance to extreme acid pH proved sensitive to weak acids. Several insertions that rendered cells sensitive to organic acids were isolated, all of which proved to be linked to the locus.


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