Physiology of Malolactic Fermentation by Streptococcus mutans

Objectives: Streptococcus mutans is subjected regularly to acid stress in dental plaque and has developed an impressive array of responses to minimize damage caused by glycolytic acid production at low, cariogenic pH values. A recently identified system for reducing acid stress to oral lactic-acid bacteria involves malolactic fermentation (MLF) of L-malate, present in many fruits and vegetables, to L-lactic acid and carbon dioxide with associated alkalinization of the cytoplasm and production of ATP by F-ATPase acting as a synthase. A major objective of our research is to explore the physiology of MLF of S. mutans. Methods: Standard enzymatic methods for evaluating MLF in suspensions and fed-batch biofilms of S. mutans UA159 were used. Results: The gene cluster for MLF of S. mutans includes the genes: SMu0121 coding for a transcriptional regulator of the system, SMu0123 for malolactic enzyme and SMu0124 for malate permease. The system was fully induced by addition of L-, but not D-, malate to suspension cultures or biofilms, even in the presence of catabolite repressors such as glucose. Net growth did not appear to be required for induction. Optimal pH values for MLF by intact cells and biofilms were close to pH 4, a value well below the minimum for growth and close to the minimum for glycolysis. Metabolism of malate was sufficiently vigorous to offset pH drop associated with glycolysis in suspensions or biofilms, especially below about pH 5. The rate of MLF was set mainly by rate of malate uptake, and malate pooling by suspensions cells or biofilms was minimal. Conclusion: MLF is a major, inducible system for alkalinization of S. mutans suspensions or biofilms with potential for use in reducing cariogenicity of dental plaque. This work was supported by awards DE06127 and DE13683 from the U.S. National Institute of Dental and Craniofacial Research.