Role of Streptococcus mutans Glucan-Binding Protein A in Oral Biofilm Formation

Abstract

Glucan-binding proteins (Gbps) of Streptococcus mutans, a major pathogen of dental caries, mediate binding of glucans synthesized from sucrose by the action of glucosyltransferases (GTFs) encoded by gtfB, gtfC, and gtfD. Several stress proteins, including DnaK and GroEL, encoded by dnaK and groEL, respectively, are related to environmental stress tolerance. The contribution of Gbp expression to biofilm formation was analyzed by focusing on expression levels of genes encoding GTFs and stress proteins. Biofilm-forming assays were performed using GbpA-, GbpB-, and GbpC-deficient mutant strains as well as the parental strain MT8148. Expression levels of gtfB, gtfC, gtfD, dnaK, and groEL were evaluated using reverse transcription-quantitative polymerase chain reaction (RT-qPCR) assays. Furthermore, the structure of biofilms formed by these Gbp-deficient mutant strains was observed with confocal laser scanning microscopy (CLSM). Biofilm-forming assay findings demonstrated that the amount formed by the GbpA-deficient mutant strain (AD1) was nearly the same as that by the parental strain, while the GbpB- and GbpC-deficient mutant strains produced amounts lower than MT8148. Furthermore, RT-qPCR assay results showed that the expressions of gtfB, dnaK,and groEL in AD1 were elevated as compared with MT8148. CLSM also revealed that the structure of biofilm formed by AD1 was prominently different as compared with that formed by the parental strain. These results suggest that a defect in GbpA influences expression of genes controlling biofilm formation, indicating its importance as a protein for firm and stable biofilm formation.