Exposure of Streptococcus mutans and Streptococcus sanguinis to blue light in an oral biofilm model

The potential anti-cariogenic effect of blue light was evaluated using an oral biofilm model. Two species, Streptococcus mutans and Streptococcus sanguinis, were cultivated ex vivo on bovine enamel blocks for 24 h, either separately or mixed together, then exposed to blue light (wavelengths 400-500 nm) using 112 J/cm². Twenty four or 48 h after exposure to light the biofilm structure and biomass were characterized and quantified using SEM and qPCR, respectively. Bacterial viability was analyzed by CLSM using live/dead bacterial staining. Gene expression was examined by RT-qPCR. After exposure to light, S. mutans biomass in mono-species biofilm was increased mainly by dead bacteria, relative to control. However, the bacterial biomass of S. mutans when grown in mixed biofilm and of S. sanguinis in mono-species biofilm was reduced after light exposure, with no significant change in viability when compared to control. Furthermore, when grown separately, an upregulation of gene expression related to biofilm formation of S. mutans, and downregulation of similar genes of S. sanguinis, were measured 24 h after exposure to blue light. However, in mixed biofilm, a downregulation of those genes in both species was observed, although not significant in S. mutans. In conclusion, blue light seems to effectively alter the bacterial biomass by reducing the viability and virulence characteristics in both bacterial species and may promote the anti-cariogenic balance between them, when grown in a mixed biofilm. Therefore, exposure of oral biofilm to blue light has the potential to serve as a complementary approach in preventive dentistry.