Physiological properties of acetaldehyde production from ethanol by oral Streptococcus

Objectives: Acetaldehyde is known to have high carcinogenicity, and to be produced by oral bacteria from ethanol. Bacterial acetaldehyde production might contribute to initiating oral cancer. Therefore, we examined the effects of oral environmental factors on the acetaldehyde production by representative oral Streptococcus species and clarified its metabolic mechanisms.
Methods: Five representative oral Streptococcus species (S. mutans, NTCT 10449, S mitis JCM12971, S salivarius JCM 5707, S. gordonii ATCC 10558 and S. sanguinis ATCC 10556) were used. These bacteria were grown aerobically and anaerobically, and the aerobically-grown cells were incubated with 5% ethanol at pH 5.0-8.0 under aerobic conditions, while the anaerobically-grown cells were treated under anaerobic conditions The acetaldehyde production was measured using gas chromatography. Bacterial enzymatic activities (alcohol dehydrogenase and NADH oxidase) were also assessed.
Results: S. mitis, S. salivarius and S. gordonii produced a high amount of acetaldehyde (502-1400 nmol/mg of cells) at pH 7-8 under aerobic conditions, and the production was decreased at acidic pH and under anaerobic conditions. On the other hand, the acetaldehyde production by S. mutans and S. sanguinis was low (20.0-26.0 nmol/mg of cells) even at neutral pH under aerobic conditions. Alcohol dehydrogenase (0.28-90.2 mU/mg of protein) and NADH oxidase (198-769 mU/mg of protein) were detected in all of the bacteria.
Conclusions: The present study demonstrated that S. mitis, S. salivarius and S. gordonii are the major acetaldehyde producers among oral streptococci and that their acetaldehyde production prefers neutral-alkaline pH and aerobic conditions. Furthermore, alcohol dehydrogenase and NADH oxidase are thought to be responsible for the aerobic acetaldehyde production, since NADH produced by alcohol dehydrogenase has to be oxidized and recycled to NAD by NADH oxidase for continuous metabolic reaction. Mechanisms underlying the difference in acetaldehyde production among streptococcal species are remained to be solved.