Glucosyltransferase-modulated Streptococcus mutans adhesion to different surfaces involved in biofilm formation by atomic force microscopy.

Biofilm on dental restorative materials is an important determinant in the etiology of secondary caries development. Formation of biofilm involves adhesion of bacteria onto substrate, bacterial cell, and biofilm surfaces. Glucosyltransferase B and C (GtfB and GtfC) are essential factors for regulation of Streptococcus mutans biofilm formation, but the mechanisms involving different kinds of bacterial adhesion still lack detailed description. In this study, nanoscale adhesion force measurement was performed using atomic force microscopy. Bacteria-coated cantilevers were used to probe S. mutans adhesion to substrates, bacterial cells, and early biofilms. Two representative dental materials, glass ionomer cement (GIC) and composite resin, served as substrates. It was found that deletion of gtfB and gtfC genes both reduced adhesion forces of S. mutans toward substrate and bacterial cell surfaces (P < 0.05). Notably, reduction of the gtfB gene remarkably decreased bacterial adhesion to biofilm surfaces (P < 0.05), while gtfC showed no obvious effect during this stage. Biofilms cultured on GIG further decreased cell-biofilm adhesion, compared with those on resin (P < 0.05). Confocal fluorescence images and scanning electron microscopy images showed that deletion of gtfB lead to reduced microcolony formation and less production of exopolysaccharides (EPSs) in the biofilm, and after bacterial culturing on GIC, the EPS content was further decreased. Our findings suggest that EPSs mainly mediate bacterial adhesion to early biofilm surface. Deletion of gtfB and coculture with GIC could significantly reduce the cell-biofilm adhesion, which is probably through decreasing of EPS production. gtfB exerts a critical role in the bacterial adhesion for the whole process of biofilm development, while gtfC possibly works only in the early stages.