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 mon itoring the specific activity of acid phosphatase. Anaerobic growth of O157:H7 strain 43895 in synthetic rumen fluid resulted in earlier dev elopment of acid tolerance than aerobic growth. However, stationary-ph ase cells of both aerobic and anaerobic cultures had an equivalent deg ree of acid tolerance that was greater than that achieved in log-phase cultures grown anaerobically. These results are consistent with the g rowth-phase regulation of acid tolerance by the stationary-phase sigma factor sigma(38). The addition of NaCl (1%) also enhanced acid tolera nce of log-phase but not stationary-phase cells of strain 43895. Growt h temperature influenced the acid tolerance with progressively greater D-pH1.5 values obtained at 15, 25 and 37 degrees C, in both log and s tationary phase. Therefore, the influence of temperature on the subseq uent survival and acid tolerance of E. coli O157:H7 strain 43895 in gr ound 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 r esults indicate that the processing temperature can influence acid tol erance in E. coli O157:H7. (C) 1998 Academic Press Limited.