Abstract : The experimental background and the biological underlying assumptions of Monod's bacterial growth model are presented. The success of this model, its main fields of application, the variability of its parameter values, and the growth of its impact are documented. There is a strong feeling that Monod's model cannot be justified in the same way as the traditional Michaelis and Menten's model for enzyme kinetics.
These problems led us to propose an alternative justification of Monod's model based on the introduction of a delay into a simple differential equation corresponding to the logistic growth model. This predicts that parameter <> should increase with parameter <>. A re-examination of data from literature shows that <> usually increases with <> though this fact was overlooked. However, as parameter values are almost always reported without their confidence limits, it is impossible to estimate the significance of this link.
In order to test the increase of <> with <>, Escherichia coli K12 CIP 54117 was grown in two different synthetic growth media with glucose as limiting nitrient. Experimental data are not in contradiction with an increase of <> with <>. Hence, the parameter <> may no longuer be considered to be a physiological constant. Healey's ratio <> should therefore be used to characterize the affinity of a bacteria for its substrate.
The maintenance requirement of a microbial population can be determined using Monod's relationship from batch culture data. It can be used to characterize the effect of sub-inhibitory concentrations of antibiotics on microbial growth. An exponential increase of maintenance requirements with antibiotic concentration is typically observed.