Differentiation of the effects of lethal pH and water activity: food safety implications

The influence of a second lethal stress (SLS) was investigated when populations of Escherichia coli M23 OR.H- were exposed to either a low water activity (aw) of 0.90 or a pH of 3.50 after 24 h at 25 degrees C. Regardless of the initial stress, E. coli M23 OR.H- populations initially demonstrated biphasic inactivation kinetics consisting of a rapid first phase of death followed by a slower second phase. When cultures initially exposed to aw 0.90 experienced an SLS of pH 3.50, a second rapid inactivation period was observed before a subpopulation of more resistant cells emerged. This subpopulation was able to persist for approximately 50 h after imposition of the SLS. In contrast, E. coli M23 OR.H- cells first exposed to a pH of 3.50 were inactivated rapidly to levels below the limits of detection upon imposition of an SLS of aw 0.90. It is hypothesized that pH stress constitutes a large energy drain on the cell and subsequently sensitizes it to other environmental constraints requiring expenditure of metabolic energy.