7 Dynamics of pyruvate metabolism in Lactococcus lactis

The lactic acid bacterium Lactococcus lactis is commonly used in the dairy industry for the manufacture of fermented milk and cheese. This is mainly due to the ability of this organism to rapidly convert the sugars present in milk to lactic acid, whereby a preservative effect is obtained. L. lactis is generally considered homofermentative, i.e., lactic acid is produced as the major end product, and when grown on a rapidly fermentable sugar such as glucose, more than 90% of the glucose consumed is recovered in lactic acid. However, when grown on a less favourable sugar such as galactose, a significant fraction of the pyruvate pool is diverted from lactic acid towards the mixed acid fermentation products formate, acetate and ethanol. The primary metabolism of L. lactis is quite simple (Fig. 7.1), but the regulation of the metabolic shift from homolactic to mixed acid product formation is complex and has challenged researchers for decades. The glycolytic intermediates glyceraldehyde-3-phosphate (GAP) and dihydroxyacetone phosphate (DHAP) have been shown to allosterically inhibit the activity of the pyruvate formate-lyase (PFL) enzyme [8]. Furthermore, fructose-1,6-diphosphate (FDP) is a necessary activator of the lactate dehydrogenase (LDH) enzyme [4,9]. The intracellular concentrations of FDP, GAP and DHAP are high during growth in batch cultures on glucose [5], whereby LDH is activated and PFL is inhibited. In contrast, the levels of FDP, GAP and DHAP are low during growth on galactose [5], leading to alleviation of PFL inhibition and reduced activation of LDH. This model based on allosteric control is generally used to explain the metabolic shift, but it has recently been shown that the transcription of pfl gene, encoding PFL, is induced during growth on galactose