| Summary: | 碩士 === 國立臺灣大學 === 口腔生物科學研究所 === 89 === Streptococcus mutans, a member of the viridans streptococci, is the primary aetiologic agent of dental caries. Acidogenesis through metabolism of multiple sugars and tolerance in the low pH are important virulent factors. S. mutans may enter the bloodstream during routine dental procedures and cause infective endocarditis in patients with heart valve abnormalities. The exopolysaccharide ( glucan ) formation through glucosyltransferases ( GTFs ) promotes tenacious adherence of S. mutans to the tooth surface in the oral cavity, and also enhances adherence to the vegetations present on the valves of the heart. Therefore, the regulation of GTF genes of S. mutans in response to environmental stress is hypothesized to be essential for virulence in vivo.
In this study, the transcriptional levels of the three genes ( gtfB, gtfC and gtfD ) in response to different stress challenges were analyzed by Northern blot in bacteria from exponential phase of growing. Differential expressions of gtfB, gtfC and gtfD at the transcriptional level were detected. Differential regulations of gtfC and gtfD at the transcriptional level were detected and were dependent on growth medium in addition to stress challenge. The gtfC but not gtfD gene was significantly up-regulated when S. mutans grown in defined media ( M4 media ) were exposed to acid stress. The gtfD gene was significantly up-regulated when S. mutans grown in rich media ( BHI media ) were exposed to acid stress. The expression of gtfC gene was subjected to catabolite repression by glucose in minimal media ( E media ) at pH 7.4 and pH 5.5. The expression of gtfD gene was subjected to catabolite repression by glucose only in minimal media ( E media ) at pH 5.5. The gtfD gene also was significantly up-regulated when S. mutans were treated with citric acid, serum, plasma, and in the presence of copper ion.
These results indicated that GTFs are subjected to complex regulation at transcriptional level in the presence of different nutritional factors and during stress response of S. mutans.
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