Description and mechanisms of bacterial growth responses to water activity and compatible solutes

Bacterial growth is inhibited by unfavourably low water activity conditions; however, the inhibition is partly alleviated by exogenous provision of compatible solutes. As compatible solutes exist in all food systems, describing and understanding this bacterial growth response is useful for food microbiologists concerned with limiting microbial growth on foods using water activity stress. In the flrst application of predictive microbiology techniques to compatible solute growth responses, the four parameter square root model (Ratkowsky et aI., 1983) successfully described growth rate data collected for Escherichia coli, Paracoccus halodenitrificans and Halomonas elongata. The value of the parameter T min was independent of the exogenous provision of compatible solutes, but aw min values and the observed minimum temperature for growth were lower where compatible solutes were provided. Despite their accurate description of growth rate data, empirical square root models do not improve mechanistic understanding. A mechanistic explanation for the bacterial growth response to water activity and compatible solutes was examined using a substrate-limited batch culture technique, developed to measure cell yield. The cell yield of E. coli did not vary signiflcantly with extracellular water activity or compatible solutes, except at water activity values close to the growth/no growth interface, indicating that water activity challenge is not an energetic burden for bacterial cells. Therefore, energetic limitation of growth was eliminated as a possible mechanistic explanation. Cell yield was also independent of incubation temperature, over most of the biokinetic range. The cell yield responses with water activity and temperature conditions were similar and consistent with a mechanistic, thermodynamic model (McMeekin et al., 1993~ Ross, 1993), thus the influence of water activity on microbial growth may be explained in terms of the thermodynamics of protein folding. This mechanism is consistent with contemporary models for the effect of compatible solutes on water structure close to the surface of macromolecules (Wiggins, 1990). Examination of novel and published data using the thermodynamic model revealed a possible mechanism for the temperature and water activity limits for microbial growth. Large increases in the activation energy, close to the boundary between growth permissible and non-growth conditions, suggest a possible universal limiting activation energy for bacterial growth. This flnding may provide a mechanistic basis for, currently empirical, growth/no growth models.