Glycerol metabolism and its regulation in lactic acid bacteria.

Glycerol is one of the most important substrates involved in phospholipid biosynthesis, along with dihydroxyacetone phosphate (DHAP) as an intermediate of glycolysis. Organisms produce glycerol 3-phosphate (G3P) from endogenous DHAP and/or exogenous glycerol to synthesize glycerophospholipids from G3P. On the other hand, organisms can utilize glycerol as a carbon source to generate adenosine triphosphate (ATP). Glycerol metabolism in microorganisms has been investigated for > 50 years. The main research targets have been four bacteria that can utilize glycerol efficiently: Escherichia coli, Enterobacter aerogenes, Klebsiella pneumoniae, and Enterococcus faecalis. E. coli, E. aerogenes, and K. pneumoniae are gram-negative bacteria in the Enterobacteriales order of the class γ-Proteobacteria. In contrast, E. faecalis is a gram-positive bacterium in the Lactobacillales order of the class Bacilli, which are well-known lactic acid bacteria (LAB). Therefore, the glycerol metabolism of E. faecalis is characterized by the properties of both gram-positive bacteria and LAB, which substantially differ from the other three bacteria. As glycerophospholipids are essential for LAB, genes encoding the enzyme for glycerol metabolism (including G3P synthesis) are broadly detected from various LAB. However, these LAB's classification and trend remained unclear until now, along with each LAB's ability to utilize glycerol. Hence, the present review summarizes LAB's glycerol metabolic pathway and regulation mechanism based on the distribution of the genes involved in those and discusses the peculiarities of glycerol metabolism in E. faecalis.