Photodynamic therapy was originally reported as a treatment of malignant tumors using photosensitive agents (photosensitizers) and visible light irradiation. When a tumor tissue containing a photosensitizer is irradiated with light of an appropriate wavelength and dose, a photochemical reaction is induced and the activated photosensitizer produces free radicals that damage cells and cause necrosis of the tumor. Recently, this technique has also been shown to be effective for inhibiting bacterial growth. Porphyromonas gingivalis, a periodontal pathogenic bacterium, produces endogenous protoporphyrin by degrading hemoglobin for its growth, which could make this bacterium photosensitive. Therefore, photodynamic therapy for periodontal diseases targeting this bacterium without using exogenous photosensitizer is expected. The effects of light irradiation on P. gingivalis have been reported, however the optimal irradiation condition remains to be determined in terms of the light sources, wavelength, and other parameters. Recently, we demonstrated that P. gingivalis growth was inhibited by visible laser light irradiation at 405 nm and an energy density of 15 J/cm2 without the use of any exogenous photosensitizers. These findings suggest that photodynamic therapy without any exogenous photosensitizer is a promising novel technology for the control of periodontal diseases
