Effect of pH-dependent,stationary phase acid resistance on the thermal tolerance of Escherichia coli O157:H7

The ability of pH-dependent, stationary phase acid resistance to cross-protect Escherichia coli O157:H7 against a subsequent lethal thermal stress was evaluated using microbiological media and three liquid foods. Three strains were grown for 18 h at 37°C in acidogenic (TSB+G, final pH 4·6–4·7) and non-acidogenic (TSB-G, final pH 7·0–7·2) media to provide stationary phase cells with and without induction of pH-dependent acid resistance. The cells were then heated in BHI broth (pH 6·0) at 58°C, using a submerged coil apparatus. The TSB+G grown strains had greatly increased heat resistance, with the heating time needed to achieve a five-log inactivation, being increased two- to four-fold. The z -values of TSB+G and TSB-G grown cells were 4·7°C and 4·3°C, respectively. Increases in heat resistance with TSB+G-grown E. coli O157:H7 were also observed using milk and chicken broth, but not with apple juice. However, cross-protection was restored if the pH of the apple juice was increased from 3·5 to 4·5. The data indicate that pH-dependent acid resistance provides E. coli O157:H7 with cross-protection against heat treatments, and that this factor must be considered to estimate this pathogen's thermal tolerance accurately.