Influence of blue light on Streptococcus mutans re-organization in biofilm

Our aim was to examine the viability and structure of new biofilm formed by Streptococcus mutans that was previously exposed to blue light. S. mutans bacteria were grown to form a mature biofilm, that was exposed to blue light (wavelengths, 400–500 nm) for 1–10 min (equivalent to 68–680 J/cm2). Biofilm was dispersed by sonication, and then the suspended bacteria were grown to re-organize as a new biofilm. Biofilm formation after 2, 4, and 6 h, was examined by viable counts and by confocal laser scanning microscopy using live/dead bacterial staining. A significant decrease in bacterial viability was found in the 6 h biofilms formed by bacteria that had been previously exposed to blue light for 7 or 10 min. Confocal microscopy images showed a decrease in the live/dead bacterial ratio after 3–10 min of light exposures. Dead bacteria were mainly at the outer layers of the biofilm. Exposure of S. mutans in biofilm to blue light affected the re-formation of a new biofilm, showing an increase in the amount of dead bacteria. This phenomenon suggests that blue light has a delayed antibacterial effect, although it does not interfere with bacterial capability to reform an initial biofilm.