Computing and interpreting specific production rates in a chemostat in steady state according to the Luedeking-Piret model.

The Luedeking-Piret model is an empirical relationship which is very widely used in cell cultures to evaluate specific production rates of some products (metabolites or others). It constitutes a very common method of calculation as much in fundamental as in applied research and especially for designing and optimizing industrial processes in very varied fields. However, this model appears to be frequently deficient and has to be greatly adapted, practically, one might say, for each individual case. Obviously, this is a very great drawback, requiring a great deal of time spent on it and one that greatly lessens the 'universality' of the model. This work reveals that it is possible to give the initial Luedeking-Piret model a much more general scope. The used method revealed metabolic switches that have never been suspected until now. Confirmation of the method would certainly give a precious general tool both to optimize production processes and to increase understanding of some physiological states of cells in chemostat.

Modeling of competitive mutualistic relationships. Application to cellulose degradation by Streptomyces sp. strains.

A "cascade" model depicts microbial degradation of a complex nutrient/substrate through a succession of intermediate compounds. Each stage is characterized by a particular species producing a typical degradation enzyme induced by its own degradation product. The final compound of the cascade consists of a single assimilable substrate used by all species. This results in a competition situation, whereas the contribution of all strains to the production of a complete set of efficient enzymes generates a mutualistic relationship. The model was shown to be appropriate to describe degradation of cellulose by a consortium of Streptomyces sp. strains. The simplicity and the model capacity for generalization are promising and could be used for various degradation processes both at laboratory and environmental scales.

Possible occurrence of a Crabtree effect in the production of lactic and butyric acids by a floc-forming bacterial consortium.

Lactic and butyric acid production by bacterial flocs in a continuous culture obeyed different physiological constraints. The butyric acid rate of production was constant and independent of the growth rate [0.012 +/- 0.001 gBUT/(L.h)], whereas lactic fermentation occurred only beyond a critical growth rate (0.25 +/- 0.05 h(-1)) and was apparently associated with an abrupt drop in biomass. Principles of modeling used to describe a Crabtree effect in Saccharomyces cerevisiae were found to apply to lactic acid production by flocs. A rank of "physiological unit" (or "metabolic unit") can be attributed to the bacterial floc. From a practical point of view, the production of fermentation products by stable flocs, naturally resistant to contamination, opens the possibility of industrial production by continuous cultivation by using flocs-forming consortia.

Modeling threshold phenomena, metabolic pathways switches and signals in chemostat-cultivated cells: the Crabtree effect in Saccharomyces cerevisiae.

The long-term Crabtree effect in Saccharomyces cerevisiae cultivated in aerobic chemostat at steady state has been studied for three different substrate concentrations in the feed of the bioreactor (data: J. Gen. Microbiol., 129 (1983) 653). We have shown that a model using two ways of transport/metabolization (T/M) of hyperbolic form, with high and low affinity for the substrate, allowed to represent correctly the main characteristics of the phenomenon. The model is based on an explicit form of the T/M kinetics when the bioreactor is considered as a polyphasic dispersed system (PDS). Mass balances analysis also allows to quantify the critical dilution rate value (threshold), Dc, of the transition between respiratory and respirofermentative mode, for which ethanol is produced. A good approximation for the threshold is Dc = V(S)0 Y(Xc, S) where Y(Xc,S) is the average yield coefficient before transition and V(S)0, the maximum specific rate of high affinity T/M pathway. The theoretical value is 0.3 h(-1), and is equal to the experimental value. We thus show in a quantitative way that the transition depends both on culture conditions (global characteristic of the system) and on strain properties (intrinsic characteristic of the microorganism as well). Using two different methods to calculate the residual substrate has carried out the comparison between the simulations end the experimental data. This allowed showing that the latter is not well represented by Monod's model and has confirmed that the affinity for the substrate varies according to the biomass. We have then shown how to calculate the most important specific rates (or metabolic flux) related to biomass, ethanol, oxygen, hydrogen, respiratory and fermentative CO(2) and H(2)O within the cellular phase. It has appeared that the oxygen uptake rate directly depends on high-affinity T/M pathway. This let us think that the regulation of the Crabtree effect in S. cerevisiae depends on the saturation of some glucose metabolization and transport pathways rather than on saturation of the respiratory chains. The specific rates analysis has also allowed us to show, at least in this case, that the metabolization rate (biosynthesis+fueling) had its maximum value on the whole dilution rates interval; metabolites excretion (ethanol and fermentative CO(2)) only intervenes to drain a "surplus" glucose flux. As a consequence, the transport capacity must be higher than the one of metabolization. Maximization of the metabolization specific rate could then be used as an optimization criterion in the stoichiometric calculation of metabolic flux (and not the specific growth rate maximization because growth is limited in a chemostat (mu = D)). We have also shown that the mass balances based on the T/M processes are in agreement with molar and elementary balances of the general stoichiometric equation for glucose respiration and fermentation under aerobic conditions. Thanks to the specific rates calculating the stoichiometric coefficients has done this. The total mass balance difference does not exceed 4%, which is compatible with the experimental carbon balance. Finally, we have emphasized that the ratio of biosynthesis flux and metabolization flux is constant before and after transition. This observation could be applied as soon as the free substrate concentration in the cellular phase is low. The paper succinctly describes the former theoretical results on which the model is built and sufficiently explains the algorithm for straightforward implementation.