THE STEADY-STATES OF MICROBIAL-GROWTH ON MIXTURES OF SUBSTITUTABLE SUBSTRATES IN A CHEMOSTAT

Microbes growing on mixtures of substrates in a chemostat exhibit diff erent substrate utilization patterns, depending on the dilution rate a nd feed concentrations. For instance, when supplied with high feed con centrations of a binary mixture, both substrates are consumed at low d ilution rates, but only one of the substrates is consumed at high dilu tion rates. The goal of this work is to explain the onset of such tran sitions, which play a very significant role in ecology and bioengineer ing. In previous work, we formulated a mathematical model of mixed-sub strate growth in batch cultures. We use the extension of this model to continuous cultures as the framework for understanding substrate util ization patterns in continuous cultures. Our explanation rests upon th e existence of two special types of dilution rates predicted by the mo del. The first is the so-called critical dilution rate at which the gr owth rate becomes zero, leading to cell washout. The existence of the critical dilution rate obtains from the simplest models of microbial g rowth, and is rooted in the fact that growth is inherently autocatalyt ic. The second type of special dilution rate, a unique feature of our model, stems from the recognition that synthesis of the enzymes cataly sing the uptake of substrates is also autocatalytic. Hence, associated with each substrate is a transition dilution rate at which the synthe sis rate of the transport enzyme becomes zero. We show that: (1) the s ubstrate utilization patterns in continuous cultures are completely de termined by the relative magnitudes of the critical and transition dil ution rates; and (2) the critical and transition dilution rates are in turn determined by the feed concentrations. This allows us to constru ct an operating diagram, which yields the substrate utilization patter n for any given dilution rate and feed concentrations. The theory expl ains most of the mixed-substrate phenomena summarized in a recent revi ew article by Egli (1995, Adv. Microbiol. Ecol. 14, 305-386). (C) 1998 Academic Press Limited.