Mechanism of inhibition of acid production in Streptococcus mutans by sodium ions under strictly anaerobic conditions

Acids excreted and intracellular levels of glycolytic intermediates during glucose metabolism in Streptococcus mutans NCTC 10449 under strictly anaerobic conditions were quantified in an attempt to understand the effect of sodium ions on bacterial acid production. In the presence of NaCl (0.15–0.30 M), the total amount of individual carboxylic acids excreted was inhibited by up to 31%. The intracellular level of fructose 1,6-bisphosphate increased by 58% and levels of 3-phosphoglycerate and pyruvate decreased by 46% and 12%, respectively. Sodium ions directly inhibited the activities of fructose 1,6-phosphate aldolase and triose phosphate isomerase. This indicated that the glycolytic enzymes responsible for the catalysis of fructose 1,6-bisphosphate to 3-phosphoglycerate were inhibited. However, in spite of the expected reduction in acid production intracellularly, the intracellular pH actually decreased in the presence of sodium ions. It is possible that the low intracellular pH inhibits the activity of the glycolytic enzymes involved in the breakdown of fructose 1,6-bisphosphate to 3-phosphoglycerate.