We investigated the effect of photodynamic therapy on P. gingivalis using blue light irradiation. As shown in Fig. 4, according to the logarithm of colony forming units (CFU) per mL, the viability of P. gingivalis was significantly suppressed in the 10J and 100J blue light irradiation groups compared with the non-irradiated (0J) group. However, no difference in survival was observed between the non-irradiated group and the 1J blue light irradiation group (Fig. 4).Figure 4Effect of blue light irradiation on the viability of P. gingivalis. The data are expressed as means ± SD (n = 3). Significant differences are expressed with * for p < 0.001 and as † for p < 0.0001.