Influence of the growth rate calculation on the relationship between growth rate and temperature

Three calculations of the growth rate (e.g. slope of a plot of the log(10) of cfu ml(-1) vs time, mu(m) of the Gompertz equation and the reciprocal of time to obtain 10(8) cfu ml(-1)) were compared for Escherichia coli TG1 gr owing in tryptone soy broth medium at temperatures ranging from 14 to 39 de grees C. Up to now, the influence of using such different definitions on th e relationship between microbial growth rate and temperature has never been investigated. In order to compare these calculation procedures, a dimensio nless analysis based on the following normalized variables, mu(dim) = mu/mu (opt) and T-dim = [T-T-min]/[T-opt-T-min], was used (Dantigny 1998). The in fluence of suboptimal temperatures on the growth rate was represented by me ans of a Belehradek-type model based on a power function law: [mu(dim)] = [ T-dim](alpha). The influence of the different growth rate calculations on t he model constants was assessed. Despite the great dependence of the raw gr owth rate values on the calculation procedure, the dimensionless analysis d emonstrated that the alpha-value is independent of the growth rate definiti on. This result suggests that any definition for the growth rate can be uti lized in studies aimed at determining the influence of temperature on micro bial growth and highlights the interest of using dimensionless variables to overcome differences in the order of magnitude of the growth rate data and to avoid confusion between definitions.