P. gingivalis requires iron and PpIX for growth30–32. The absorption spectrum of PpIX includes a maximal peak at 410 nm (Soret band) and four smaller peaks (Q-bands) from 500 to 630 nm33. Blue light at wavelengths of 400–500 nm is used for oral treatments such as resin restoration and tooth whitening34–36. The fluorescence wavelength peak (Fig. 1B) of PpIX excited with blue light (Fig. 1A) was consistent with the fluorescence wavelength for photodynamic diagnoses used in cancer therapy (600–740 nm)37, 38.