A generalized form of Monod kinetics is proposed to account for all kinds of product, cell, and substrate inhibition. This model assumes that there exists a critical inhibitor concentration above which cells cannot grow, and that the constants of the Monod equation are functions of this limiting inhibitor concentration. Methods for evaluating the constants of this rate form are presented. Finally the proposed kinetic form is compared with the available data in the literature, which unfortunately is very sparse. In all cases, this equation form fitted the data very well.

Acetic acid formation in Escherichia coli fermentation has been studied in continuous cultures. Experimental results suggest that the limited capacity of the oxidative metabolism (perhaps the limited capacity of TCA cycle) may be responsible for acetic acid formation. At low growth rates, both anabolic and catabolic requirements may be satisfied by the oxidative metabolism. However, at high growth rates these two demands may exceed the capacity of the oxidative metabolism alone. It is proposed that under these circumstances, E. coli reorganizes the oxidative metabolism to first meet the anabolic requisition and then supply the necessary amount of energy using both the remaining capacity of the oxidative metabolism and acetic acid formation metabolism. Escherichia coli selects acetic acid synthesis as the aerobic energy source because it generates the second largest amount of ATP and NADH(2). According to our proposition, acetic acid formation could be reduced by decreasing the anabolic requirement, i.e., reducing glucose uptake, or by increasing the capacity of the oxidative metabolism. These two approaches were experimentally confirmed by observing reduced acetic acid formation by reducing the glucose uptake with a yeast extract addition and enhancing the capacity of oxidative metabolism with a methionine addition.

Among amino acids screened for their potential to relieve wild and recombinant Escherichia coli from the negative effects of acetic acid, glycine, and methionine showed a sparing effect. In the presence of 2 g/L of acetic acid, addition of 0.5 g/L of glycine or methionine resulted in either a complete recovery or a further enhancement in the specific growth rate, while the enhancement was significant but not fully complete in the presence of 4 g/L of acetic acid. The addition of 0.5 g/L of methionine alleviated the negative effect of acetic acid on recombinant E. Coli growth to produce more beta-lactamase, which was encoded by plasmid pUC18. In continuous fermentation the methionine effect on recombinant. E. coli metabolism depended on dilution rate; at high dilution rates, above 0.4 h(-1), the methionine addition enhanced beta-lactamase production and reduced acetic acid formation, while at low dilution rates, below 0.3 h (-1), the effect was reversed. In def-batch fermentation with wild-type E. Coli, cell growth rate and cell yield from glucose were enhanced with methionine addition, while the acetic acid concentration reached over 4 g/L.