When stationary phase Escherichia coli 0157:H7 cells were subjected to extreme acid shock (pH 2.0, 6 h, 37°C) cell survival was as great as 10%, but culture conditions greatly affected the acid resistance. Anaerobic cultures were more resistant to extreme acid shock if the glucose concentration of the growth medium was high, acids accumulated, and pH declined. By varying pH and acetate concentration, it was possible to demonstrate a high correlation (R2 = 0.86) between undissociated acetate and extreme acid resistance. Because dissociated acetate and extreme acid resistance were poorly correlated (R2 < 0.01), it appeared that the pH effects were being mediated via acetate dissociation. Propionate and butyrate were as effective as acetate, but formate, lactate, benzoate and the uncoupler, carbonylcyanide m-chlorophenylhydrazone (CCCP), were much less effective in promoting extreme acid-resistance. Acetate, propionate, butyrate, benzoate and CCCP all decreased the intracellular pH of E. coli 0157:H7, but the correlation between intracellular pH and extreme acid resistance was low (R2 < 0.01). Cultures grown aerobically only needed half as much acetate to induce extreme acid resistance as those grown anaerobically, and the addition of the reducing agent, cysteine, to anaerobic media made the stationary phase cells less responsive to acetate. An rpoS mutant of E. coli O157:H7 was at least 100-fold more sensitive to acid shock than the wild-type, and large amounts of acetate were needed to promote even a small increase in viability.