The Origin of the Time Scale: A Crucial Issue for Predictive Microbiology

The collective behavior of microbial cells in a batch culture is the result of interactions among individuals and effects of the surrounding medium, which changes during the growth progress. A semi empirical model skips biological and physiological peculiarities of the microorganisms and focuses on the observed sigmoid shape of the growth curve that is a common feature of batch cultures of pro- and eukaryotic microorganisms. The model replaces the observed growth trend with the behavior of an ideal batch culture that undergoes an unperturbed duplication process. It leads one to recognize that: • the origin of the time scale for the microbes, , differs from that of the observer, t; • the absolute reference state for any batch culture is log (N) = 0 (no matter the log base) for  = 0; • the cell duplication occurs after an active latency gap, 0, that decreases with increasing inoculum population, log2(N0) and increasing temperature; • 0 substantially differs from the lag phase, , considered by most authors;
• the use of reduced variables allows gathering different growth curves in a single master plot; • the model applies to batch cultures which undergo change of the environmental conditions and predicts the width of the intermediate latency gap just after the change; • the expression for the decay trend of the microbial population allows definition of a parameter suitable to rank the effects of bactericidal drugs. The model justifies the demand of more restricted safety limits of microbial loads.