Abstract
Although Salmonella typhimurium prefers neutral-pH environments, it can adapt to survive conditions of severe low-pH stress (pH 3.3). The process, termed the acid tolerance response (ATR), includes two distinct stages. The first stage, called pre-acid shock, is induced at pH 5.8 and involves the production of an inducible pH homeostasis system functional at external pH values below 4.0. The second stage occurs following an acid shock shift to pH 4.5 or below and is called the post-acid shock stage. During this stage of the ATR, 43 acid shock proteins (ASPs) are synthesized. The present data reveal that several ASPs important for pH 3.3 acid tolerance are only transiently produced. Their disappearance after 30 to 40 min of pH 4.4 acid shock coincides with an inability to survive subsequent pH 3.3 acid challenge. Clearly, an essential feature of inducible acid tolerance is an ability to synthesize these key ASPs. The pre-acid shock stage, with its inducible pH homeostasis system, offers the cell an enhanced ability to synthesize ASPs following rapid shifts to conditions below pH 4.0, an external pH that normally prevents ASP synthesis. The data also address possible signals for ASP synthesis. The inducing signal for 22 ASPs appears to be internal acidification, while external pH serves to induce 13 others. Of the 14 transient ASPs, 10 are induced in response to changes in internal pH. Mutations in the fur (ferric uptake regulator) locus that produce an Atr- acid-sensitive phenotype also eliminate induction of six transiently induced ASPs.
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