Pyruvate flux distribution in NADH-oxidase-overproducing Lactococcus lactis strain as a function of culture conditions

The influence of growth conditions on product formation from glucose by Lac tococcus lactis strain NZ9800 engineered for NADH-oxidase overproduction wa s examined, In aerobic batch cultures, a large production of acetoin and di acetyl was found at acidic pH under pH-unregulated conditions, However, pyr uvate flux was mainly driven towards lactate production when these cells we re grown under strictly pH-controlled conditions, A decreased NADH-oxidase overproduction accompanied the homolactic fermentation, suggesting that the cellular energy was used with preference to maintain cellular homeostasis rather than for NADH-oxidase overproduction, The end product formation and NADH-oxidase activity were also studied in cells grown in aerobic continuou s cultures under acidic conditions. A homoacetic type of fermentation as we ll as a low NADH-oxidase overproduction were observed at low dilution rates . NADH-oxidase was efficiently overproduced as the dilution rate was increa sed and consequently metabolic flux through lactate dehydrogenase drastical ly decreased. Under these conditions the flux limitation via pyruvate dehyd rogenase was relieved and this enzymatic complex accommodated most of the p yruvate flux, Pyruvate was also significantly converted to acetoin and diac etyl via a-acetolactate synthase, At higher dilution rates, acetate product ion declined and the cultures turned to mixed-acid fermentation. These resu lts suggest that the need to maintain the cellular homeostasis influenced N ADH-oxidase overproduction and consequently the end product formation from glucose in these engineered strains. (C) 1999 Federation of European Microb iological Societies. Published by Elsevier Science B.V. All rights reserved .