EFFECTS OF PH AND ACID END-PRODUCTS ON ACID PRODUCTION IN ORAL STREPTOCOCCI

Mechanisms of acid end-product export are poorly characterised for ora l streptococci. This study considered the effects of external pH and t wo acid end-products on acid production from glucose, in a pH stat, fo r three species of oral streptococci: Streptococcus downei MFe28, Stre ptococcus mutans R9 and Streptococcus oralis SF572. Kinetics of acid p roduction were determined by fitting results to the Michaelis-Menten e quation, 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 str ains was found to fall progressively with falling external pH. At neut ral pH the acid:glucose molar ratios were consistent with lactic acid being the sole fermentation product in S. downei and S. mutans, but th e ratios decreased with falling external pH, accompanied by formation of intracellular polysaccharide (IFS). The acid:glucose molar ratio in S. oralis was about three at neutral pH, possibly because of mixed ac id fermentation, but decreased to about two with falling external pH a nd no IFS formation was detected. V-app decreased in the presence of l actic acid in both S. mutans and S. downei, while the K-M was unchange d, indicating non-competitive inhibition. Both species also exhibited competitive inhibition by acetic acid. Both lactic and acetic acid cau sed total, irreversible inhibition of acid production in S. oralis. Th ese results support the hypothesis that lactate is exported as the pro tonated species, driven by the lactic acid gradient. Furthermore, comp etitive inhibition of lactic acid production by acetic acid strongly s uggests a common export system for acid end-products.