Photoinactivation of Planktonic Cutibacterium acnes subsp. elongatum: A Study on Wavelength-Dependent Effects

Bacterial inflammatory skin conditions, especially those caused by Cutibacterium acnes (C. acnes), are among the most common skin diseases globally. While C. acnes is a gram-positive bacterium found in healthy human skin, its overgrowth or reduced phylotype diversity have been associated with inflammatory conditions such as acne vulgaris. Traditional treatments often apply antibiotics. Alternatives, like photodynamic therapy (PDT) and intense pulsed light (IPL) therapy, apply visible light irradiation. These therapies utilize photosensitizers, such as coproporphyrin III (CP III), to generate reactive oxygen species (ROS) under specific wavelengths that inactivate bacteria. Compounds like aminolevulinic acid (ALA) enhance therapeutic effectiveness. In this study planktonic cultures of C. acnes subsp. elongatum were irradiated at wavelengths matched to the CP III absorbance spectrum, including rarely applied green and yellow light: 405 nm (violet), 496 nm (cyan), 547 nm (lime), 594 nm (yellow/amber), and 636 nm (red). To target flavins, another group of photosensitizers, C. acnes was irradiated at 456 nm (blue), resulting in bacterial reductions comparable to violet light. Results revealed a lethal dose to inactivate 90% (LD90) of 83.3 J/cm² at 405 nm. Higher doses were required for longer wavelengths with lower CP III absorbance. The addition of ALA improved the 636 nm LD90 dose from 2666 J/cm² to 1369 J/cm². This efficacy was comparable to green or yellow irradiation. These findings suggest that certain wavelengths, even without additives, may offer a competitive and simpler therapeutic option for irradiation therapies. Convolution of the applied LED spectra with the CP III absorbance spectra indicates a mathematical relationship between applied wavelength and bacterial reduction.