Summary: When the Arrhenius equation is applied to microbial growth rates, a temperature characteristic (μ) represents the activation energy. While these μ values describe the energetics favouring growth, there is still no convincing evidence from empirical data to support the idea of a widely applicable relationship between μ values and optimal growth temperatures, which may serve as a basis for the classification of psychrophiles, psychrotrophs, mesophiles and thermophiles. The μ values for the exponential growth rates of 16 psychrophilic and psychrotrophic bacterial strains, including 9 from the literature, varied from 3 to 33 (kcal mol). A plot of μ versus growth temperature optimum deviated considerably from empirical equations reported recently (Mohr & Krawiec, 1980) for a different selection of strains. Moreover, Arrhenius profiles with two distinct slopes at suboptimal temperatures were not restricted to bacteria with growth temperature optima higher than 37 °C, as had been suggested in that report. A psychrotrophic strain of with a growth temperature optimum of 23-25 °C was characterized by two different temperature characteristics. Furthermore, in the upper range of suboptimal temperatures (11-24 °C), temperature characteristics (μ) were also affected by temperature acclimation of the inoculum.


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