ACID AND BASE RESISTANCE IN ESCHERICHIA-COLI AND SHIGELLA-FLEXNERI - ROLE OF RPOS AND GROWTH PH

Escherichia coli K-12 strains and Shigella flexneri grown to stationar y phase can sun ive 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 rp oS, the growth phase-dependent sigma factor sigma(38). The clone also conferred acid resistance on S. flexneri rpoS::Tn10 but not on Salmone lla typhimurium. E. coli and S. flexneri strains containing wild-type rpoS maintained greater internal pH in the face of a low external pH t han strains lacking functional rpoS, but the ability to sun ive at low pH did not require maintenance of a high transmembrane pH difference. Aerobic stationary-phase cultures of E. coli MC4100 and S. flexneri 3 136, 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 th was increased to pH 8.0. Extended growth in log ph ase also decreased acid resistance substantially. Strains containing r poS::Tn10 showed ed partial acid resistance when grown at pH 5 to stat ionary phase; log-phase cultures showed <0.01% acid resistance. When g rown anaerobically at low pH, however, the rpoS::Tn10 strains were aci d resistant. E. coli MC4100 also showed resistance at alkaline pH outs ide the growth range (base resistance). Significant base resistance wa s observed up to pH 10.2. Base resistance was diminished by rpoS::Tn10 and by the presence of Na+. Base resistance was increased by an order of magnitude for stationary-phase cultures grown in moderate base (pH 8) compared with those grown in moderate acid (pH 5). Anaerobic growt h partly restored base resistance in cultures grown at pH 5 but not in those grown at pH 8. Thus, both acid resistance and base resistance s how dependence on growth pH and are regulated by rpoS under certain co nditions. For acid resistance, and in part for base resistance, the rp oS requirement can be overcome by anaerobic growth in moderate acid.