Role of oral streptococci in the pH-dependent carious dentin

The aim of the present study was to investigate the extent to which oral streptococci or lactic acid producing bacteria are able to regulate pH value, especially at low pH values associated with dentinal carious activity using a PCR method. One millimeter-thick sections were sliced from 24 extracted carious human molars. The pH values on the tooth sections were evaluated using a pH-imaging microscope. A dentin sample (1×1×1mm) was prepared from the areas with the lowest or highest pH and homogenized to extract bacterial genomic DNA. Specific primers were used for nested PCR to mutans streptococci (MS: Streptococcus mutans and Streptococcus sobrinus). Conserved primers were also used for PCR to lactobacilli and gram positive bacteria. The PCR products were separated by electrophoresis, and then oral bacteria were identified. There was no significant difference between carious and intact dentin in MS identification. However, the frequency of the product amplified by the conserved primers in carious dentin (16/24, 66.7%) was significantly higher than that in intact dentin (2/24, 8.3%), and PCR products demonstrated, by sequence analysis, various bacteria, including oral streptococci. It was concluded that oral streptococci may be associated with the development of "low pH-carious dentin"