Factors affecting the extreme acid resistance of Escherichia coli O157 : H7

When stationary phase Escherichia coli O157:H7 cells were subjected to extr eme acid shock (pH 2.0, 6 h, 37 degrees C) cell survival was as great as 10 % but culture conditions greatly affected the acid resistance. Anaerobic cu ltures were more resistant to extreme acid shock if the glucose concentrati on of the growth medium was high, acids accumulated, and pH declined. By va rying pH and acetate concentration, it was possible to demonstrate a high c orrelation (R-2 = 0.86) between undissociated acetate and extreme acid resi stance. Because dissociated acetate and extreme acid resistance were poorly correlated (R-2 < 0.01), it appeared that the pH effects were being mediat ed via acetate dissociation. Propionate and butyrate were as effective as a cetate, 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 ail decr eased the intracellular pH of E. coli O157:H7, hut the correlation between intracellular pH and extreme acid resistance was low (R-2 < 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 a gent, cysteine, to anaerobic media made the stationary phase cells less res ponsive 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 acet ate were needed to promote even a small increase in viability. (C) 1999 Aca demic Press.