FIG 1 .

Pyruvate catabolism in S. agalactiae. One molecule of glucose will produce two of pyruvate via glycolysis, with net reduction of 2 NADH molecules. Fermentation of pyruvate, will allow NADH reoxidation for its recycling and use in a new round of glycolysis. (A) In the absence of oxygen, acetyl coenzyme A (acetyl-CoA) is made from pyruvate by pyruvate formate lyase (PFL), allowing synthesis of fatty acids. The main fermentation product is lactate, but significant amounts of ethanol, acetate, acetoin, and diacetyl are also found (21). Under aerobic conditions, production of acetoin, diacetyl, and acetate is prevalent over that of ethanol. (B) In the presence of oxygen without addition of external heme or quinone sources (respiration nonpermissive condition), lactate is still the main fermentation product, which leads to a significant decrease in pH. (C) Upon addition of external heme and quinone, in the presence of oxygen (respiration permissive condition), the respiratory chain (NDH-2/cyt bd) becomes functional. Metabolism is shifted toward production of acetate, acetoin, and diacetyl, reducing the amount of pyruvate available to be converted into lactate. This results in less acidification of the medium in the stationary phase. Higher growth yield is achieved as a result of enhanced ATP formation via the Pta-Ack pathway and possibly oxidative phosphorylation by the electron transport chain. Boxes indicate the main products for each condition. ACK, acetate kinase; ALS, acetolactate synthase; ALDB, 2-acetolactate decarboxylase; LDH, lactate dehydrogenase; PDH, pyruvate dehydrogenase; PTA, phosphate transacetylase; n.e., nonenzymatic reaction. (Adapted with data from references 21 to 23 and 66.)