Retention of acid tolerance and acid shock responses of Escherichia coli O157:H7 and non-O157:H7 isolates

Escherichia coli O157:H7 and non-O157:H7 survival due to induced acid tolerance or shock responses when exposed to lactic acid over time was studied. Induced acid tolerance or shock responses could allow pathogens, like E. coli O157:H7, to survive acidic conditions in foods during storage. Escherichia coli O157:H7 isolates 932 and E009 and a non-O157:H7 strain, 23716, were grown to stationary phase at 32 degrees C and exposed to one of two treatments: acid shock or acid adaption. Acid-shocked cells were exposed to lactic acid at pH 3.5 or 4.0. Acid-adapted cells were exposed to pH 5.5 for an adaptation period and then exposed to an acid challenge of pH 3.5 or 4.0. Samples were incubated at either 25 or 32 degrees C and survival of the isolates at 0, 3, 24, 48, 72, and 168 h (7 days), 336 h (14 days), and 504 h (21 days) was determined. All three isolates survived longer with larger populations at pH 4.0 and 25 degrees C compared to the other treatments. In cases where a difference was observed in the two responses, acid-shocked cells had a higher survival rate (typically less than 2 logs) than acid-adapted cells in most cases. Isolate differences were observed at the two pH and temperature levels. Isolate 932 was the most resistant to the acidic conditions during the incubation period, E009 intermediate, and strain 23716 was the most sensitive.