Microbial growth in a closed system can be described by a model based on the continuous logistic equation on which a Fermi's term is superimposed. The first component of the combined model accounts for growth and the second for decay. It has the form:
where N(t) and N o , are the instantaneous and initial numbers of cells, or an equivalent population size or density measure, and N , the medium's capacity; k and t, are rate constants and characteristic times with the subscript g or 1 signifying growth or decay, respectively. The model has the versatility to account for a short or long lag time, populations starting from a very small or substantial inoculum and asymmetric or symmetric growth curves with a broad or sharp peak. The model was tested with published experimental growth curves of bacteria and cell cultures grown under very different conditions and had a good fit in all the cases. Its parameters could be used for quantitative comparison between growth patterns and assessment of the relative overall role of growthpromoting and decay-causing agents that shape the growth curve. At least in principle, they can also be used to map the effects of factors such as temperature and pH algebraically or graphically.