Studies on the oxygen-impairment of the sugar metabolism of micro-organisms in dental plaque and on the biochemical mechanism of its recovery from the impairment.

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The acids generated through the fermentation of sugar by the microorganisms in dental plaque are directly involved in the initiation and progression of dental caries. The sugar metabolism by these microorganisms is also affected greatly by the contact of the organisms to air. The environmental conditions in dental plaque vary greatly during the cycle of the daily life, especially by meals. Thus, we studied the biochemical mechanism of the recovery of the sugar metabolism from the impairment by oxygen.We found that the sugar metabolism of Streptococcus mutans was more sensitive than that of Streptococcus sanguis. The low sensitivity of the sugar metabolism of S. sanguis stems from the interconversion of pyruvate formate-lyase between the two forms of this enzyme, while S. mutans cannot do this interconversion. The active pyruvate formate-lyase converted into an inactive form of the enzyme(R-form) when the intact cells were incubated anaerobically without sugar. The R-form enzyme was reactivated in the cells when sugar was available under strictly anaerobic conditions. The R-form of the enzyme was not sensitive to oxygen, while the active enzyme was extremely sensitive. The contact to oxygen converted the active enzyme into another form of an inactive enzyme that was not reactivated. The intact cells of S. mutans had no such a system to convert an active enzyme into the R-form. This could be a reason why S. mutans is sensitive to oxygen.