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

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 PPi. 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.