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


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Krist, K 1997 , 'Description and mechanisms of bacterial growth responses to water activity and compatible solutes', PhD thesis, University of Tasmania.

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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.

Item Type: Thesis - PhD
Authors/Creators:Krist, K
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