Cybernetic models, developed earlier by the authors, have been evaluated experimentally for the growth of Klebsiella oxytoca in batch cultures using mixed substrates from glucose, xylose, arabinose, lactose, and fructose. Based entirely on information procured from batch growth on single substrates, the models accurately predict without further parameter fitting, diauxic growth on mixed substrates, automatically predicting the order in which the substrates are consumed. Even triauxic growth on a mixture of glucose, xylose, and lactose is predicted by the model based on single substrate data. Growth on glucose-fructose mixtures appears to need a slightly modified strategy for cybernetic variables.

It is known that 2,3-butanediol is a potentially valuable chemical feedstock that can be produced from the sugars present in hemicellulose and celluose hydrolysates. Klebsiella oxytoca is able to ferment most pentoses, hexoses, and disaccharides. Butanediol appears to be a primary metabolite, excreted as a product of energy methabolism. The theoretical maximum yield of butanediol from monosaccharides is 0.50 g/g. This article describes the effects of pH, xylose concentration, and the oxygen transfer rate on the bioconversion of D-xylose to 2,3-butanediol. Product inhibition by butanediol is also examined. The most important variable affecting the kinetics of this system appears to be the oxygen transfer rate. A higher oxygen supply favors the formation of cell mass at the expense of butanediol. Decreasing the oxygen supply rate increases the butanediol yield, but decreases the overall conversion rate due to a lower cell concentration.

Bacillus polymyxa produces (R, R)-2,3-butanediol from a variety of carbohydrates. Other metabolites are also produced including acetoin, acetate, lactate, and ethanol. The excretion of each metabolite was found to depend on the relative availability of oxygen to the culture. When the relative oxygen uptake rate was high, enhanced yields of acetate and acetoin were noted. At an intermediate oxygen availability, the butanediol yield was maximal. When the availability of oxygen was more restricted, higher yields of lactate and ethanol occurred. The cells appeared to regulate themselves such that energy generation is optimal subject to the constraint that the cells do not produce more reducing equivalents than can be oxidized by the electron transport system. The dependence of each product yield on the relative oxygen availability was determined, and this knowledge was used to carry out a fed-batch fermentation that attained a final butanediol concentration of over 40 g/L in 50 h.

A cybernetic model to predict the low-growth-rate behavior of bacteria in mixed-substrate environment is presented. Using only growth and maintenance parameters from single-substrate experiments, the model accurately predicts the simultaneous substrate utilization and maintenance energy effects in constant fed-batch cultures of Klebsiella oxytoca. The robustness of the model was examined more rigorously by perturbing glucose-limited fed-batch cultures with additions of arabinose, xylose, and fructose. In all cases, reasonable agreement of the model prediction with the experimental data was observed.

No abstract