人工齲蝕象牙質におけるStreptococcus mutansの動態に関する研究

When Streptococcus mutans adheres to the surface of teeth, it begins to proliferate and form biofilm by 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. However, little is known about the ecology of S. mutans in carious lesions. 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 MT8148. Dentin slices and S. mutans MT8148 were incubated in brain heart infusion broth at 37℃, pH 7.5 for 14 days, and subsequently, the culture condition was changed to 42℃ or to pH 5.5 as environmental changes. Number of recovered viable cells was determined by colony forming units. Also the expression of heat shock protein 70, gtf genes, and dexA gene were examined using RT-PCR method. The growth rate of S. mutans MT8148 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 level in adhered cells. TEM 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. Especially changing to pH 5.5, more intra-cellular polysaccharide positive cells were observed, although the cell density in the tubles decreased. 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. Furthermore, synthesis of intracellular polysaccharides may be important to resist to physical and chemical stresses.

公開日:2010年1月24日で登録したコンテンツは、国立情報学研究所において電子化したものです。