Lactococcus lactis plays an important role in dairy fermentations, mainly in the production of cheeses. In these fermentation processes, lactose is present at high concentrations (50 g·L−1) and is converted through glycolysis to lactic acid, with minor amounts of other compounds being produced in addition (homolactic fermentation). The resulting low pH contributes to the texture and flavor of cheeses and inhibits the growth of other bacterial species. Under conditions where sugar becomes limiting for growth of Lactococcus lactis, the metabolism shifts to mixed-acid products, i.e. formate, acetate and ethanol along with smaller amounts of lactate [1,2].
Work has been performed in the past to study the mechanisms involved in the shift between the two different fermentation modes in L. lactis. In the presence of excess sugar, the concentration of fructose 1,6-bisphosphate, the triose-phosphates, pyruvate, and the NADH/NAD+ ratio are high, whereas the concentration of phosphoenolpyruvate and inorganic phosphate are relative low [3–6]. In contrast, when sugar is limiting the concentration of these metabolites and cofactors are reversed to the opposite, high or low level. Particularly, the level of fructose-1,6-bisphosphate, which is known to activate both pyruvate kinase and lactate dehydrogenase, has been suggested to play a key role in the regulation of the fermentation mode .
Work has also been performed to determine the factors that control the flux through glycolysis by applying metabolic control analysis [7,8]. Based on inhibitor titration, Poolman et al.  suggested that glyceraldehyde 3-phosphate dehydrogenase had a large amount of control over the glycolytic flux during nongrowing conditions. However, it is still unclear whether the enzyme is important when the flux through glycolysis is high.
Recently, we showed that a mere twofold reduction of the activity of phosphofructokinase in L. lactis had a strong negative influence on the growth rate and the glycolytic flux . Lactate dehydrogenase is the last enzyme in the pathway converting sugar to lactate in L. lactis. As expected, disruption of the ldh gene in L. lactis MG1363  or in the derivative NZ3900  diverts the majority of pyruvate towards mixed-acid products. However, it is still unclear to what extent this enzyme controls the metabolic fluxes in the wild-type L. lactis cells.
In this study, we use a new method for subtle and stable modulations of lactate dehydrogenase activity around wild-type levels in growing cells and we quantify the control exerted by lactate dehydrogenase on the growth rate and the metabolic fluxes in L. lactis. We show that lactate dehydrogenase has virtually no control on the growth rate, the glycolytic flux, and the flux to lactate, but the enzyme has a high negative control on the flux towards formate.