Effect of suspension media on nonthermal inactivation of Escherichia coli

Aims: To investigate the influence of suspension media on the survival of Escherichia coli M23 exposed to nonthermal, lethal stresses. Methods and Results: Populations of E. coli M23 suspended in minimal medium (MM) or in different nutrient-rich broths were exposed to water activity 0.90 and/or pH 3.5 and inactivation was determined by culture-based enumeration. In response to the osmotic or acid challenges, E. coli M23 displayed enhanced survival in MM rather than in complex broth. That trend was reversed when populations were exposed to low water activity in combination with low pH. Comparison of microbial survival in three complex media indicated that even relatively small differences in composition influenced inactivation. In most media the combination of lethal stresses resulted in a synergism, which enhanced bacterial inactivation; however, an exception (tryptone soya broth) was observed. Conclusions: The suspension medium strongly influences the inactivation of E. coli M23 by osmotic and/or acid stresses. This should be considered when comparing studies of microbial survival that use different media and when broth-derived data are intended to represent specific environments (e.g. food matrices). Significance and Impact of the Study: The specific effects of synthetic media need to be appreciated when studying bacterial inactivation in conditions relevant to food-manufacturing regimes. © 2006 The Authors.