Streptococcus mutans GcrR Regulates Oral Biofilm from a Cariogenic State to a Non-Cariogenic State by Affecting Exopolysaccharide Production and Biofilm Homeostasis

Introduction: Dental caries is a noncommunicable disease caused by dysbiosis of a dental biofilm. Streptococcus mutans is considered the major pathogen. The orphan response regulator GcrR negatively regulates exopolysaccharide (EPS) synthesis in S. mutans. We aimed to investigate the effect of GcrR on the cariogenicity of oral biofilms. A triple-species biofilm model was constructed, including S. mutans, Streptococcus sanguinis, and Streptococcus gordonii. Methods: The morphology of triple-species biofilms was detected through scanning electron microscopy, and the structure was observed using confocal laser scanning microscopy. The microbial composition was measured by fluorescence in situ hybridization and qPCR. The expression of genes was detected by quantitative reverse transcription-PCR. A specific pathogen-free rat model was used to assess the cariogenicity of the triple-species biofilms. Results: The architecture of the biofilm was significantly impaired when gcrR-overexpressed S. mutans were incubated with S. sanguinis and S. gordonii (SmugcrR⁺+S.s+S.g). The biofilm exhibited a decrease in the production of water-insoluble glucans and water-soluble glucans, consistent with a decreased expression of EPS synthesis-related genes. The SmugcrR⁺+S.s+S.g biofilm exhibited an increase in non-cariogenic species with lower lactic acid production. Furthermore, the SmugcrR⁺+S.s+S.g biofilm exhibited reduced cariogenicity. Conclusion: The biofilm cariogenicity could be shifted to a less cariogenic state by an increased expression of the GcrR regulator.