Bactericidal effects of hematoporphyrin monomethyl ether-mediated blue-light photodynamic therapy against Staphylococcus aureus

The implementation of photodynamic therapy (PDT) usually uses red light as the excitation source to obtain a deeper penetration depth. However, for some superficial infectious diseases, using red-light PDT may damage the normal tissues underneath. If we choose a shorter wavelength light, then the effect of PDT can be limited to the superficial region. This study assessed the effect of blue-light PDT against Staphylococcus aureus. The absorption of hematoporphyrin monomethyl ether (HMME) by S. aureus was investigated using fluorescence spectroscopy. The bactericidal effects of HMME, light alone, and PDT using blue light (405 nm) on S. aureus were studied. The results indicate that the HMME uptake by S. aureus rapidly reached a certain value, then steadily increased with time in the range of 0-80 min, and thenreached a plateau at 80 min before a slow decline afterward. Without light irradiation, less than 2 μg ml⁻¹ HMME showed no bactericidal effect on S. aureus. Without HMME, blue-light at a power density of 20 mW cm⁻² had no significant bactericidal effect for 0.5 min to 10 min. When 2 μg ml⁻¹ of HMME was combined with blue-light (20 mW cm⁻²), the bactericidal effect showed a reduction of 3 log₁₀ with the extension of irradiation time. These results demonstrated that bacteria have the ability to absorb HMME, and HMME-mediated blue-light PDT can effectively kill the bacteria, which laid the foundation for blue-light PDT as a non-invasive treatment for superficial infectious diseases.