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

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C. SHADBOLT, T. ROSS AND T.A. MCMEEKIN. 2001 . 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 (a w ) of 0·90 or a pH of 3·50 after 24 h at 25°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 a w 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 a w 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.

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