The effect of acid treatment on survival and protein expression of a laboratory K-12 strain Escherichia coli.

Pre-exposure of log phase enteric bacteria to nonlethal acidic pH induces phenotypic changes that protect the organisms against subsequent lethal acidity. Studies have revealed that when Salmonella typhimurium is grown in minimal medium at pH 5.5 and 4.3 the organism develops a biphasic acid tolerance. This two-stage response has not been reported at present in Escherichia coli; rather it is thought that when this organism is grown in rich medium there is a single stress response throughout the pH range of 4 to 6. We believe that the evidence for such a report is lacking; therefore, in this study the acid response of log phase E. coli was examined in rich medium (LB). The pH 3.0 acid survival assays of a laboratory strain of E. coli K-12 MG1655, after cultures had been exposed to LB acidified to pH 5.5 or pH 4.3 indicate that like S. typhimurium, E. coli shows both an acid tolerance and an acid-shock response to pH 5.5 and 4.3 exposure, respectively. It was consistently found, however, that longer pre-exposure (60 min rather than 15 min) at either pH afforded better protection against the lethal pH 3.0 challenge. Analysis of polypeptide induction at pH 5.5 and 4.3 by two-dimensional gel electrophoresis clearly shows different profiles. Together the results show that in E. coli, pre-treatment between pH 4 and 6 does not result in a flat protective response.