Metabolism of biodiesel-derived glycerol in probiotic Lactobacillus strains
Three probiotic Lactobacillus strains, Lactobacillus acidophilus, Lactobacillus plantarum, and Lactobacillus delbrueckii, were tested for their ability to assimilate and metabolize glycerol. Biodiesel-derived glycerol was used as the main carbon and energy source in batch microaerobic growth. Here, we show that the tested strains were able to assimilate glycerol, consuming between 38 and 48 % in approximately 24 h. L. acidophilus and L. delbrueckii showed a similar growth, higher than L. plantarum. The highest biomass reached was 2.11 g L−1 for L. acidophilus, with a cell mass yield (YX/S) of 0.37 g g−1. L. delbrueckii and L. plantarum reached a biomass of 2.06 and 1.36 g L−1. All strains catabolize glycerol mainly through glycerol kinase (EC 22.214.171.124). For these lactobacillus species, kinetic parameters for glycerol kinase showed Michaelis–Menten constant (Km) ranging from 1.2 to 3.8 mM. The specific activities for glycerol kinase in these strains were in the range of 0.18 to 0.58 U mg protein−1, with L. acidophilus ATCC 4356 showing the maximum specific activity after 24 h of cultivation. Glycerol dehydrogenase activity was also detected in all strains studied but only for the reduction of glyceraldehyde with NADPH (Km for DL-glyceraldehyde ranging from 12.8 to 32.3 mM). This enzyme shows a very low oxidative activity with glycerol and NADP+ and, most likely, under physiological conditions, the oxidative reaction does not occur, supporting the assumption that the main metabolic flux concerning glycerol metabolism is through the glycerol kinase pathway.