Genetic and physiological effects of noncoherent visible light combined with hydrogen peroxide on Streptococcus mutans in biofilm

Oral biofilms are associated with the most common infections of the oral cavity. Bacteria embedded in the biofilms are less sensitive to antibacterial agents than planktonic bacteria are. Recently, an antibacterial synergic effect of noncoherent blue light and hydrogen peroxide (H₂O₂) on planktonic Streptococcus mutans was demonstrated. In this study, we tested the effect of a combination of light and H₂O₂ on the vitality and gene expression of S. mutans embedded in biofilm. Biofilms of S. mutans were exposed to visible light (wavelengths, 400 to 500 nm) for 30 or 60 s (equivalent to 34 or 68 J/cm²) in the presence of 3 to 300 mM H ₂O₂. The antibacterial effect was assessed by microbial counts of each treated sample compared with that of the control. The effect of light combined with H₂O₂ on the different layers of the biofilm was evaluated by confocal laser scanning microscopy. Gene expression was determined by real-time reverse transcription-PCR. Our results show that noncoherent light, in combination with H₂O₂, has a synergistic antibacterial effect through all of the layers of the biofilm. Furthermore, this treatment was more effective against bacteria in biofilm than against planktonic bacteria. The combined light and H₂O₂ treatment up-regulated the expression of several genes such as gtfB, brp, smu630, and comDE but did not affect relA and ftf. The ability of noncoherent visible light in combination with H₂O₂ to affect bacteria in deep layers of the biofilm suggests that this treatment may be applied in biofilm-related diseases as a minimally invasive antibacterial procedure.