Protection of Oral Streptococci Against Acid Damage by Malolactic Fermentation

Objectives: Oral streptococci are protected against acid stress at low pH values in cariogenic dental plaque partly by alkali producing systems, such as the arginine deiminase system (ADS) of Streptococcus sanguinis, agmatine deiminase of S. mutans or urease of S. salivarius. Recent findings indicate that these organisms also have the capacity for malolactic fermentation (MLF) involving degradation of L-malate to L-lactate and carbon dioxide with associated alkalinization and ATP production by F-ATPase. Our objectives were to determine if MLF works in consort with other alkali-generating systems for protection against lethal acid damage. Methods: Standard enzymatic methods for assessing metabolism of malate, arginine, urea and agmatine by cells in suspensions and fed-batch biofilms were used along with standard plate counting for viability determinations. Results: Alkali production by the ADS of S. sanguinis ATCC 10556 at pH 5.0 in suspension cells and intact biofilms was not inhibited by concomitant MLF, and the two systems acted additively to increase alkali production to rates as high as 8.0 ± 0.7 microequivalents/mg cell dry weight/h at pH 5.0. Conjoint alkali production resulted in major protection of the organism against acid killing at pH below 3 with both alkali-producing systems active at potentially lethal pH values. The agmatine deiminase system of S. mutans UA159 was considerably less potent than MLF for alkali generation, and protective effects against acid killing by the combined systems were mainly those of MLF. For S. salivarius ATCC13419, MLF was more potent for acid protection than urease, although both systems were effective for alkali production at lethal pH values. Conclusion: MLF can act in conjunction with other alkali-generating systems to protect oral streptococci against acid damage and to increase acid tolerance. This work was supported by awards DE06127 and DE13683 from the U. S. National Institute of Dental and Craniofacial Research.