Submitted to: Food Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/22/1999
Publication Date: N/A
Interpretive Summary: O157:H7 is a highly toxic strain of Escherichia coli that kills more than 500 people per year. The virulence of O157:H7 is dependent on its ability to survive the low pH of the gastric stomach. The genetics of extreme acid resistance in E. coli was very poorly understood. Our experiments sought to define more precisely the roles of pH, fermentation acids, redox potential and RpoS in the acid resistance in E. coli O157:H7. Results indicated acetic acid promoted extreme acid resistance particularly when the pH was low. Oxygen decreased the ability of acetic acid to promote extreme acid resistance. This and subsequent work may provide means of preventing E. coli infection in humans.
Technical Abstract: The growth conditions of Escherichia coli O157:H7 greatly affected its ability to survive a subsequent acid shock (pH 2.0, 6 h, 37s C). When cultures were grown anaerobically with increasing amounts of glucose, acetate accumulated, and extracellular pH declined from 7.4 to 4.7, and the viable count of stationary phase cells after acid shock increased 1000-fold. When cultures were grown with a low concentration of glucose (2 mM) at pH values ranging from 7.0 to 5.6, the extracellular acetate concentration was always 3 mM, but viable count of stationary phase cells after acid shock increased 1000-fold. When cultures were grown at pH 7.0 with increasing amounts of acetate, a similar increase in viability was observed. These results indicated that acetate was operating in a pH-dependent fashion to promote extreme acid resistance, the viable cells number and undissociated acetate were highly correlated (r2 = 0.86). Propionate and butyrate were as effective as acetate, but formate, lactate, benzoate and the uncoupler, carbonylcyanide-m-chlorophenylhydrazone, did not promote extreme acid resistance. Organic acids decreased intracellular pH, but the correlation of intracellular pH and viability was very low (r2 < 0.01). Cultures that were grown aerobically needed less acetate to induce extreme acid resistance than those grown anaerobically, the addition of cysteine to cultures grown under strictly anaerobic conditions caused a further decrease in acetate sensitivity. An rpoS mutant of E. coli O157:H7 was always more sensitive to acid shock than wild-type, and large amounts of acetate were needed to promote even small increases in viability.