Effects of pH and Acid End-Products on Acid Production in Oral Streptococci

Abstract

Mechanisms of acid end-product export are poorly characterised for oral streptococci. This study considered the effects of external pH and two acid end-products on acid production from glucose, in a pH stat, for three species of oral streptococci: Streptococcus downei MFe28, Streptococcus mutans R9 and Streptococcus oralis SF572. Kinetics of acid production were determined by fitting results to the Michaelis-Menten equation, for S. mutans and S. downei, or by half-time analysis for S. oralis. The apparent V_max (V_app) of acid production in all three strains was found to fall progressively with falling external pH. At neutral pH the acid:glucose molar ratios were consistent with lactic acid being the sole fermentation product in S. downei and S. mutans, but the ratios decreased with falling external pH, accompanied by formation of intracellular polysaccharide (IPS). The acid:glucose molar ratio in S. oralis was about three at neutral pH, possibly because of mixed acid fermentation, but decreased to about two with falling external pH and no IPS formation was detected. V_app decreased in the presence of lactic acid in both S. mutans and S. downei, while the K_M was unchanged, indicating non-competitive inhibition. Both species also exhibited competitive inhibition by acetic acid. Both lactic and acetic acid caused total, irreversible inhibition of acid production in S. oralis. These results support the hypothesis that lactate is exported as the protonated species, driven by the lactic acid gradient. Furthermore, competitive inhibition of lactic acid production by acetic acid strongly suggests a common export system for acid end-products.

Key Words:

• Streptococci
• lactic acid
• end-products
• inhibition
• export