Impact of pH on the cardinal temperatures of E. coli K12: Evaluation of the gamma hypothesis

Accurate description of the effect of (a combination of) environmental conditions on the microbial growth rate is of high importance for the predictive quality of models used in predictive microbiology. According to the previously defined gamma hypothesis, environmental factors act independently on the microbial growth dynamics. For temperature specifically, this concept implies that the cardinal temperatures (Tmin, Topt, Tmax) are only determined by the microbial temperature response and not influenced by other environmental conditions. In this research, it is evaluated if pH affects the values of the cardinal temperatures. Hereto, the parameters of the Cardinal Temperature Model with Inflection (CTMI, Rosso, Lobry, & Flandrois, 1993) have been derived from an extended experimental data set. Tmin and Tmax as a function of pH seem to follow a parabolic trend, which is in contradiction to the gamma hypothesis: the relation implies that pH does affect the cardinal temperatures. In contrast, the experimental data possibly show that the Tmax value is approximately constant at moderate pH values. This observation partially validates the gamma hypothesis. For Topt, no obvious trend could be observed. Independently of the exact relation, a combination of a pH stress and extreme temperature act synergistically on the microbial growth dynamics.