Study on Streptococcus mutans sucrose-dependent adherence and cariogenesis.

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When Streptococcus mutans adheres to the surface of teeth, it begins to proliferate and form biofilm synthesizing the water-soluble (WSG) and -insoluble glucan (WIG) from sucrose. It is well known that these glucans are synthesized by three kinds of glucosyltransferases, GTFB / C / D, coded by gtfB / C / D gene, and are degraded by dextranase (DexA) coded by dexA gene. S. mutans in carious lesions may modulate glucan metabolism to adapt to the micro-environment surrounding cells. The purpose of this study was to investigate the ecological changes of S. mutans in biofilm or dentinal tubules when the environmental changes occur. We developed an in vitro artificial carious model composed of bovine dentin slices and S. mutans or deficient mutant strains. Dentin slices and S. mutans were incubated under vaious condition (temperature, pH or concentration of sucrose). The expression of gtf genes, and dexA gene were examined using PCR method. The growth rate of S. mutans MT(wild type) adhered to a dentin plate was not reduced by the environmental changes, while that of non-adhered cells decreased. The levels of gtf gene expression of adhered cells were higher than those of non-adhered cells. Moreover, gtfC gene showed the highest expression in adhered cells. TEM and LM observation revealed that S. mutans invaded most of dentinal tubules of bovine dentin in this model. When the culture condition was changed, the cells began to synthesize the intra-cellular polysaccharide. The gtfC deficient mutant strain produced lower amount of WIG that was adhered to the dentin plate compared with other gene deficient mutants. These results suggest that S. mutans adhered to the tooth surface changes the amount of GTFs and DexA, by synthesizing the glucan, or by adapting to the environmental changes.