Study on dynamic change of sugar metabolism by microorganisms in dental plaque due to the drastic change of oral environments.

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The environment of oral cavity changes greatly from time to time. Thus acid production In dental plaque, an inevitable event for the initiation and progression of dental caries, is affected by this change of environment. We studied the effect of possible change of oral environment on the acid production of oral microorganisms, such as streptococcal, actinomyces and lactobacilli.The pH in dental plaque drops during meals due to the uptake of sugars. At neural pH streptococci have been known to produce formic andacetic acids in addition to ethanol when supply of sugar is limited. But at pH 5.5 streptococci produced mainly lactate even if supply on sugar is limited and even under strictly anaerobic conditions. The lactate dehydrogenase (LDH), a key enzyme of lactate production , of streptococci was found to be induced at low pH, while pyruvate formate-lyase (PFL), a key enzyme of formic and acetic acids production under strictly anaerobic conditions, was not induced. The optimum PB of the LDH was lower than that of PFL. Thus the low pH in the cells of streptococci was also considered responsible for this change into lactate fermentation at low pH. PFL is activated under anaerobic conditions in the presence of fermentable sugar. For this activation NADPH was found to be an essential factor. This means that the activation PFL, an important enzyme of anaerobic sugar metabolism, is finely regulated during sugar metabolism of streptococci. A presence and a similar regulation of PFL were also found in Actinomyces naeslundii, Actinomyces viscosus. Lactobacillus cases and Streptococcus lactis.