Growth and processing conditions affecting acid tolerance in Escherichia coli O157:H7

The impact of growth conditions (anaerobiosis, growth phase, NaCl, pH, and temperature) on the development of acid tolerance in Escherichia coli O157:H7 was investigated directly (D-pH1.5) and indirectly by monitoring the specific activity of acid phosphatase. Anaerobic growth of O157:H7 strain 43895 in synthetic rumen fluid resulted in earlier development of acid tolerance than aerobic growth. However, stationary-phase cells of both aerobic and anaerobic cultures had an equivalent degree of acid tolerance that was greater than that achieved in log-phase cultures grown anaerobically. These results are consistent with the growth-phase regulation of acid tolerance by the stationary-phase sigma factor sigma(38). The addition of NaCl (1%) also enhanced acid tolerance of log-phase but not stationary-phase cells of strain 43895. Growth temperature influenced the acid tolerance with progressively greater D-pH1.5 values obtained at 15, 25 and 37 degrees C, in both log and stationary phase. Therefore, the influence of temperature on the subsequent survival and acid tolerance of E. coli O157:H7 strain 43895 in ground beef was evaluated. Numbers of strain 43895 decreased c. 1.4 log(10) cfu g(-1) in inoculated ground beef stored at 4 degrees C, whereas numbers remained essentially unchanged during storage at -20 degrees C. While pre-incubation at 15 degrees C for 4 h prior to storage at 4 or -20 degrees C did not influence survival, the acid tolerance of E. coli O157:H7 survivors was significantly decreased (P<0.0001). These results indicate that the processing temperature can influence acid tolerance in E. coli O157:H7. (C) 1998 Academic Press Limited.