Effects of organic acid anions on growth, glycolysis, and intracellular pHof oral streptococci

Oral streptococci produce large quantities of organic acids as the end-prod ucts of carbohydrate fermentation. In an approach to determine if oral stre ptococci exhibit differential sensitivities to organic acid anions, we dete rmined the effects of formate, lactate, and acetate on intracellular pH mai ntenance, glycolysis, and growth of Streptococcus mutans and Streptococcus sanguis. Growth was determined as maximum culture optical density in the pr esence of the organic acid anions at pH 7.1, 6.7, 6.3, and 6.1, and the eff ects of the anions on glycolytic activity and intracellular pH were determi ned at pH 7.0 and 5.0. At pH 7.1, the organic acid anions had little effect on growth of either species. At the lower pH values, all of the anions red uced the maximum culture optical density of both species in a pH- and conce ntration-dependent manner, with S. sanguis being more sensitive to growth i nhitition than S. mutans. The organic acid anions had little or no effect o n glycolytic activity of either species at pH 7.0. However, all of the orga nic acid anions tested reduced glycolytic activity at pH 5.0 in a concentra tion-dependent manner, with S. sanguis being more sensitive than S. mutans. The inhibition of glycolysis could be related to the pK(a) of the organic acid, with formate and lactate being mon inhibitory than acetate. The organ ic acid anions decreased the intracellular pH of S. mutans and S. sanguis, glycolyzing at an extracellular pll of 5.0, such that the reduction in glyc olytic activity caused by the organic acid anions could be directly attribu ted to the fall in intracellular pH. In conclusion, the production of lacti c acid in plaque would not only lower pH, thereby having a disadvantageous effect on less aciduric oral streptococci, such as S. sanguis, but would al so increase their sensitivity to the effects of low pH, helping S. mutans t o become more dominant.