Visible light-induced killing of bacteria as a function of wavelength: Implication for wound healing

Background and objective: Visible light (400-800 nm) at high intensity was previously found to kill bacteria that are frequently found in infected wounds, while low-power white light enhances bacterial proliferation. The phototoxic effect was found to involve induction of reactive oxygen species (ROS) production by the bacteria. The aim of the present study was to identify the most effective wavelengths in the visible range for inducing a bactericidal effect. Experimental: ROS production in Staphylococcus aureus and Escherichia coli as a function of wavelengths in the visible range (400-500, 500-800, 415, and 455 nm) was studied using the electron paramagnetic resonance (EPR) spin trapping technique. The phototoxicity of 415 and 455nm light at different fluencies on the survival of S. aureus and E. coli was assessed by colony count of the bacteria following irradiation. Results: ROS production following blue (400-500 nm) light illumination was found to be higher than that of red (500-800 nm). Within the blue range, light of 415nm induced more ROS than 455 nm, which correlated with results obtained for the reduction in colony count of S. aureus and E. coli following illumination using equal intensities of these two wavelengths. At low fluencies, both 415 and 455nm enhanced proliferation of S. aureus but reduced viability of E. coli. Conclusion: Intense blue light, preferably at 415 nm, could be used for bacterial eradication. However, it should be noted that low intensity of visible light can be dangerous since it may promote proliferation of the microorganisms.