Acid and Base Resistance in Escherichia coli and Shigella flexneri: Role of rpoS and Growth pH

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Escherichia coli K-12 strains and Shigellaflexneri grown to stationary phase can survive several hours at pH 2 to 3, which is considerably lower than the acid limit for growth (about pH 4.5). A 1.3-kb fragment cloned from S. flexneri conferred acid resistance on acid-sensitive E. coli HB101; sequence data identified the fragment as a homolog of rpoS, the growth phase-dependent sigma factor Ci38. The clone also conferred acid resistance on S.flexneri rpoS::TnlO but not on Salmonella typhimurium. E. coli and S.flexneri strains containing wild-type rpoS maintained greater internal pH in the face of a low external pH than strains lacking functional rpoS, but the ability to survive at low pH did not require maintenance of a high transmembrane pH difference. Aerobic stationary-phase cultures of E. coli MC4100 and S. flexneri 3136, grown initially at an external pH range of 5 to 8, were 100% acid resistant (surviving 2 h at pH 2.5). Aerobic log-phase cultures grown at pH 5.0 were acid resistant; survival decreased 10- to 100-fold as the pH of growth was increased to pH 8.0. Extended growth in log phase also decreased acid resistance substantially. Strains containing rpoS::TnJO showed partial acid resistance when grown at pH 5 to stationary phase; log-phase cultures showed


Journal of Bacteriology





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