Abstract: Objective　To investigate the sterilizing effect of photothermal therapy (PTT) based on indocyanine green (ICG) on methicillin-resistant Staphylococcus Aureus. Methods　Ultraviolet and fluorescence spectra were used to characterize the photophysical property of ICG. The photothermal heating curves of ICG with different concentrations (0, 12.5, 25, 50, 100 and 200 mg/L) were determined to evaluate the photothermal conversion performance of ICG. Mouse breast cancer 4T1 cells were mixed with different concentrations of ICG solution (0, 50, 100, 200 and 500 mg/L), and the cytotoxicity of ICG was measured by MTT assay. The concentration of methicillin-resistant Staphylococcus Aureus was 1×1010 CFU/mL, and the experiment was divided into the 7 groups: the PBS group, the ICG group (500 mg/L), the laser irradiation group (808 nm laser irradiation of 1.5 W/cm2 for 10 min), the ICG+0.33 W /cm2 laser irradiation group, the ICG+0.66 W/cm2 laser irradiation group, the ICG+1 W/cm2 laser irradiation group and the ICG+ 1.5 W/cm2 laser irradiation group. The number of surviving colonies was calculated and the sterilizing effect of ICG-based PTT was analyzed. Results The ultraviolet absorption curve showed that ICG had strong absorption band at 808 nm. The temperature of ICG solution (200 mg/L) can be increased by 23.3 ℃ after irradiation by 808 nm near-infrared laser with 1.5 W/cm2 for 10 min, while the temperature of pure water only increased by only 4.8 ℃ at the same experimental condition. The relative survival rates of ICG groups with different concentrations (50 mg/L, 100 mg/L, 200 mg/L and 500 mg/L) were 81.87%±2.69%, 81.47%±2.94%, 82.92%±2.61% and 80.44%±2.98%, respectively. There were no significant differences between the groups (F=0.402, P＞0.05). The survival rates of different laser irradiation groups (0.33, 0.66, 1 and 1.5 W/cm2) were 94.92%±1.98%, 93.89%±1.38% and 93.03%±2.31% respectively, and there was no significant difference between the groups (F=2.864, P＞0.05). In the sterilization experiment, compared with the PBS group, the number of surviving bacteria significantly decreased gradually with the increase of the laser power density in the ICG+ laser irradiation group (F=133.800, P＜0.01). Conclusion　ICG has a great biocompatibility and an excellent photothermal sterilizing effect under near-infrared laser irradiation to methicillin-resistant Staphylococcus Aureus, which exhibits a potential clinical application.

Key words: lasers, methicillin-resistant staphylococcus aureus, materials testing, photothermal therapy, indocyanine green, sterilization