Effects of acetate on the growth and fermentation performance of Escherichia coli KO11.

Escherichia coli KO11, in which the genes pdc (pyruvate decarboxylase) and adh (alcohol dehydrogenase) encoding the ethanol pathway from Zymomonas mobilis were inserted into the chromosome, has been shown to metabolize all major sugars that are constituents of hemicellulosic hydrolysates to ethanol, in anaerobic conditions. However, the growth and fermentation performance of this recombinant bacteria may be affected by acetic acid, a potential inhibitor present in hemicellulose hydrolysates in a range of 2.0-15.0 g/L. It was observed that acetate affected the growth of E. coli KO11, prolonging the lag phase and inducing loss of biomass production and reduction of growth rate. At lower pH levels, the sensitivity to acetic acid was enhanced owing to the increased concentration of the protonated species. On the other hand, the recombinant bacteria showed a high tolerance to acetic acid regarding fermentative performance. In Luria broth medium with glucose or xylose as a single sugar source, it was observed that neither yield nor productivity was affected by the addition of acetate in a range of 2.0-12.0 g/L, suggesting some uncoupling of the growth vs ethanol production.