Even though the bactericidal properties of photodynamic effects have been known for a long time, only recently has there been increased interest in their practical use17–20. The development of resistance to aPDT appears to be unlikely because in microbial cells, reactive oxygen species (ROS), such as singlet oxygen (1O2) and some free radicals, interact with several cell structures and different metabolic pathways. Furthermore, aPDT is equally effective against antibiotic-resistant and antibiotic-susceptible bacteria, and repeated photosensitization has not induced the selection of resistant strains21, 22. There is also interest in applying aPDT against P. gingivalis, and studies have been conducted using visible light with wavelengths of 600 nm or more and with toluidine blue or methylene blue as a photosensitizer18, 23, 24. Visible light of 600 nm or greater is excellent for tissue penetration compared with shorter wavelengths due to low light absorption and scattering by living tissue25. Additionally, toluidine blue and methylene blue stain living tissues and cells as well as bacteria26–28. Therefore, these techniques using several stains may damage biological tissues and cells, even if it is effective as sterilization for P. gingivalis.