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 (1.0 M), but not with glucose, up to 0.95 M L-lactate was produced from 1.0 M 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 1.0 M 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 1.50 M and 1.15 M L-lactate from glycerol and crude glycerol with a high yield, respectively, and these production amounts were higher than those in previous studies.