Acetoin production by wild-type strains and a lactate dehydrogenase-deficient mutant of Streptococcus mutans

Abstract

Eleven different laboratory strains of Streptococcus mutans representing the various serogroups were found to produce an average of 6.0 + 4.8 mM acetoin when grown in glucose-containing medium under aerobic conditions. None of the strains produced detectable acetoin when grown anaerobically. A lactate dehydrogenase-deficient mutant produced acetoin both aerobically and anaerobically and in substantially greater amounts than the wild-type strains did. Substitution of mannitol for glucose resulted in decreased acetoin production by wild-type strains and the lactate dehydrogenase-deficient mutant, indicating a role for NADH2 in the regulation of the acetoin pathway. Pyruvate incorporated into the growth medium of a wild-type strain caused acetoin to be produced anaerobically and stimulated acetoin production aerobically. Cell extracts of a wild-type S. mutans strain were capable of producing acetoin from pyruvate and were (partly) dependent on thiamine PP;. Extracts prepared from aerobically grown cells had approximately twice the acetoin-producing activity as did extracts prepared from anaerobically grown cells. The results indicate that acetoin production by S. mutans may represent an auxiliary reaction of pyruvate dehydrogenase in this organism.

Genetic and physiological analysis of the lethal effect of L-(+)-lactate dehydrogenase deficiency in Streptococcus mutans: complementation by alcohol dehydrogenase from Zymomonas mobilis

“…Stable growth of CH4ts at 42ЊC was achieved in continuous culture under glucose-limiting conditions. Under these conditions, CH4ts behaved like chemically derived LDH-deficient mutants of S. rattus (12) with regard to increased ethanol and acetoin production in place of lactic acid production. These results are in accord with fermentation and enzymology studies indicating that S. mutans has regulated, alternative pathways for pyruvate metabolism.…”

“…These results are in accord with fermentation and enzymology studies indicating that S. mutans has regulated, alternative pathways for pyruvate metabolism. As described above, under appropriate conditions of cultivation, wild-type S. mutans uses pyruvate formate-lyase or pyruvate dehydrogenase to produce substantial amounts of formate, acetate, ethanol, and acetoin (1,5,12). These alternative pathways are evidently sufficient to compensate for the absence of LDH when appropriate limitation of glucose is provided but are apparently insufficient in the presence of high glucose concentrations.…”

“…Cells were removed by centrifugation at 4ЊC, and the resulting cell-free liquors were assayed enzymatically (Sigma Chemical Co., St. Louis, Mo.) for glucose, ethanol, and lactic acid and by gas-liquid chromatography (12) for acetoin. The data presented in the figures are the averages of duplicate experiments.…”

“…Anaerobically, the activity of pyruvate formate-lyase accounts for the observed production of formate, acetate, and ethanol (1). In an aerobic environment, pyruvate formate-lyase is inactivated by oxygen, and instead, pyruvate dehydrogenase activity can account for the observed production of acetate (5) and acetoin (12).…”

“…As was shown to be the case for the closely related Streptococcus rattus (12), it was initially presumed that these alternate pathways for pyruvate dissimilation would enable an LDHdeficient mutant of S. mutans to thrive under both aerobic and anaerobic cultivation conditions. The finding that LDH deficiency was lethal in S. mutans suggests that these pathways are less active than they are in S. rattus, resulting in growth inhibition from NAD-NADH imbalance and/or the accumulation of glycolytic intermediates.…”