Acid tolerance of acid-adapted and nonacid-adapted Escherichia coli O157:H7 strains in beef decontamination runoff fluids or on beef tissue

Abstract

This study assessed the acid tolerance response (ATR) of stationary phase, acid-adapted (tryptic soy broth [TSB]+1% glucose) or nonacid-adapted (glucose-free TSB) Escherichia coli O157:H7 strains (ATCC43889, ATCC43895, ATCC51658 and EO139), grown individually or in a mixed culture, prior to inoculation of beef or meat decontamination runoff (washings) fluids (acidic [pH 4.95] or nonacidic [pH 7.01]). The inoculated beef was left untreated or treated by dipping for 30 s in hot water (75 °C) followed by 2% lactic acid (55 °C). Inoculated beef samples and washings were stored aerobically at 4 or 15 °C for 6 d, and at set intervals (0, 2, and 6 d) were exposed (for 0, 60, 120, and 180 min) to pH 3.5 (adjusted with lactic acid) TSB plus 0.6% yeast extract. Overall, there were no significant ( P ⩾ 0.05 ) differences in responses of cultures prepared as individual or mixed strains. Decontamination of meat did not affect the subsequent ATR of E. coli O157:H7 other than resulting in lower initial pathogen levels exposed to acidic conditions. In this study, E. coli O157:H7 appeared to become more tolerant to acid following incubation in acidic washings of sublethal pH (4.89–5.22) compared to nonacidic washings (pH 6.97–7.41) at 4 °C or in both types of washings incubated at 15 °C. The ATR of the pathogen inoculated into washings was enhanced when cells were previously acid-adapted and incubated at 4 °C. Similarly, the ATR on meat was increased by previous acid-adaptation of the inoculum in broth and enhanced by storage at 4 °C. Populations on treated meat were consistently lower than those on untreated meat during storage and following exposure to acid. Although on day-0 there were no significant ( P ⩾ 0.05 ) differences in ATR between acid-adapted and nonacid-adapted populations on meat, acid-adapted cells displayed consistently higher resistance through day-6. This suggests that acid-adapted E. coli O157:H7 introduced on meat may become resistant to subsequent lactic acid exposure following storage at 4 °C.