Control of acid resistance in Escherichia coli

Acid resistance (AR) in Escherichia coil is defined as the ability to withs tand an acid challenge of pH 2.5 or less and is a trait generally restricte d to stationary-phase cells. Earlier reports described three AR systems in E. coli. In the present study, the genetics and control of these three syst ems have been more clearly defined, Expression of the first AR system (desi gnated the oxidative or glucose-repressed AR system) was previously shown t o require the alternative sigma factor RpoS. Consistent with glucose repres sion, this system also proved to be dependent in many situations on the cyc lic AMP receptor protein. The second AR system required the addition of arg inine during pH 2.5 acid challenge, the structural gene for arginine decarb oxylase (adiA), and the regulator cysB, confirming earlier reports. The thi rd AR system required glutamate for protection at pH 2.5, one of two genes encoding glutamate decarboxylase (gadA or gadB), and the gene encoding the putative glutamate:gamma-aminobutyric acid antiporter (gadC). Only one of t he two glutamate decarboxylases was needed for protection at pH 2.5. Howeve r, survival at pH 2 required both glutamate decarboxylase isozymes. Station ary phase and acid pH regulation of the gad genes proved separable. Station ary-phase induction of gadA and gadB required the alternative sigma factor sigma(S) encoded by rpoS. However, acid induction of these enzymes, which w as demonstrated to occur in exponential- and stationary-phase cells, proved to be sigma(S) independent. Neither gad gene required the presence of vola tile fatty acids for induction. The data also indicate that AR via the amin o acid decarboxylase systems requires more than an inducible decarboxylase and antiporter. Another surprising finding was that the sigma(S)-dependent oxidative system, originally thought to be acid induced, actually proved to be induced following entry into stationary phase regardless of the pH, How ever, an inhibitor produced at pH 8 somehow interferes with the activity of this system, giving the illusion of acid induction. The results also revea led that the AR system affording the most effective protection at pH 2 in c omplex medium (either Luria-Bertani broth or brain heart infusion broth plu s 0.4% glucose) is the glutamate-dependent GAD system. Thus, E. coli posses ses three overlapping acid survival systems whose various levels of control and differing requirements for activity ensure that at least one system wi ll be available to protect the stationary-phase cell under naturally occurr ing acidic environments.