Three isogenic strains of Lactococcus lactis with different levels of H 2 O-forming NADH oxidase activity were used to study the effect of oxygen on glucose metabolism: the parent strain L. lactis MG1363, a NOX ؊ strain harboring a deletion of the gene coding for H 2 O-forming NADH oxidase, and a NOX ؉ strain with the NADH oxidase activity enhanced by about 100-fold. A comprehensive description of the metabolic events was obtained by using 13 C nuclear magnetic resonance in vivo. The most noticeable results of this study are as follows: (i) under aerobic conditions the level of fructose 1,6-bisphosphate [Fru(1,6)P 2 ] was lower than the level under anaerobic conditions, and the rate of Fru(1,6)P 2 depletion was very high; (ii) the levels of 3-phosphoglycerate and phosphoenolpyruvate were considerably enhanced under aerobic conditions and significantly lower in the NOX ؊ strain; and (iii) the glycolytic flux decreased in the presence of saturating levels of oxygen, but it was not altered in response to changes in the NADH oxidase activity. In particular, the observation that the glycolytic flux was not enhanced in the NOX ؉ strain indicated that glycolytic flux was not primarily determined by the level of NADH in the cell. The patterns of end products were identical for the NOX ؊ and parent strains; in the NOX ؉ strain the carbon flux was diverted to the production of ␣-acetolactate-derived compounds, and at a low pH this strain produced diacetyl at concentrations up to 1.6 mM. The data were integrated with the goal of identifying the main regulatory aspects of glucose metabolism in the presence of oxygen.Lactococcus lactis is a facultatively anaerobic bacterium that converts more than 90% of milk sugar to lactate, preventing spoilage of fermented foods. Although this organism exhibits relatively simple carbohydrate metabolism, primarily designed for energy conservation, considerable versatility with respect to end product formation has been described (26). The pattern of end products resulting from glucose metabolism under anaerobic conditions, characterized by the production of large amounts of lactate and only trace amounts of acetate, ethanol, formate, and 2,3-butanediol, is markedly different from that observed in the presence of oxygen. In fact, when the organism is switched from an anaerobic atmosphere to an aerobic atmosphere, a mixture of lactate and acetate is produced, and enhancement of the flux through ␣-acetolactate synthase is observed, resulting in acetoin-diacetyl production (7,8,28,40). Diacetyl is a by-product of aerobic metabolism with industrial relevance, since it confers the butter-like taste characteristic of many dairy foods; this trait was the basis for several previous studies that focused on strategies to enhance diacetyl production (16,22). Nevertheless, the mechanisms involved in regulation of the metabolic switch from homolactic fermentation to mixed acid and aroma fermentation under aerobic conditions are not completely understood.The ability of L. lactis to grow under aerobic cond...
