Lactic acid fermentation is the main aerobic metabolic pathway in Enterococcus faecalis metabolizing a high concentration of glycerol

Bacterial lactate production by fermentative lactate dehydrogenase is generally activated under anaerobic conditions. However, when the Enterococcus faecalis strain W11 was cultured with a high concentration of glycerol (1.0M), but not with glucose, up to 0.95M L-lactate was produced from 1.0M glycerol via the fermentative L-lactate dehydrogenase (LdhL1) reaction despite the abundant supply of oxygen. To understand the underlying mechanism, transcription, metabolic products, and intracellular NADH/total NAD (tNAD) ratio of W11 were analyzed. The ldhL1 transcription was constitutive and not markedly affected by dissolved oxygen level or NADH/tNAD ratio. Conversely, gene transcription and enzyme activities related to other pyruvate metabolic pathways, namely the acetoin, acetate, and ethanol production pathways, in W11 utilizing 1.0M glycerol tended to be downregulated or inactivated by a high NADH/tNAD ratio or supply of oxygen. These findings indicated that the amount and yield of L-lactate depended on the activity levels of other metabolic pathways and that lactic acid fermentation was the main aerobic metabolic pathway in W11 utilizing glycerol at a high concentration. This phenomenon enabled W11 to produce up to 1.50M and 1.15M L-lactate from glycerol and crude glycerol with a high yield, respectively, and these production amounts were higher than those in previous studies.