Enteric micro-organisms have developed several inducible mechanisms for surviving transient periods of extreme acid stress. Salmonella typhimurium possesses an acid tolerance response (ATR) induced in minimal medium by short exposures to mild acid stress. More than 50 acid shock proteins (ASPs) are induced during adaptation. Eight ASPs are regulated by the major iron regulatory protein, Fur, in an unusual iron-independent manner. The two-component regulator, PhoP, is an autoinduced ASP that controls the induction of three additional ASPs. The stress sigma factor sigma S is an ASP that regulates induction of eight ASPs. Acid induction of sigma S is due to its decreased proteolytic turnover via the ClpXP protease in conjunction with the two-component-type response regulator MviA (RssB in Escherichia coli). Mutations in any of these three regulators leads to a defective ATR. Repair of pH stress-induced DNA damage appears to require the Ada protein (O6-methylguanine methyltransferase) since an ada mutant is both acid and alkaline sensitive. In contrast to S. typhimurium, E. coli and Shigella have acid resistance systems induced in complex media that include a glucose-repressed system protective at pH 2.5 without amino acid supplementation, a glutamate decarboxylase system that requires glutamate and an arginine decarboxylase system unique to E. coli.