Effect of bacterial bioburden and light intensity on the efficacy of antimicrobial 405-nm light

Antimicrobial 405-nm light has recently been developed for various infection control applications including continuous decontamination of the clinical environment, where low bacterial populations (~10² CFU cm^-2) are typically expected, and wound decontamination, where a microbial load of >10⁵ CFU g^-1 is considered the clinical threshold for diagnosing infection.
This study investigates the bactericidal efficacy of high versus low-irradiance 405-nm light on Staphylococcus aureus at various population densities to establish the impact of bacterial load and light intensity on inactivation efficacy.
Liquid-suspended S. aureus (10³-10⁹ CFU ml^-1) were exposed to increasing doses of 405-nm light using irradiances of 5 and 150 mW cm^-2. Post-exposure, inactivation kinetics at each irradiance and bacterial density were established and susceptibility at equivalent light doses compared.
Results demonstrate S. aureus susceptibility to a fixed dose of 405-nm light was significantly enhanced when exposed at low-irradiance for bacterial densities ≤10⁷ CFU ml^-1 (2-4 times greater dose was required for complete inactivation using 150 mW cm^-2 compared to 5 mW cm^-2). This enhancement was not observed at a 10⁹ CFU ml^-1 density, with significantly greater inactivation achieved using 150 mW cm^-2 compared to 5 mW cm^-2 (P<0.05).
This study demonstrates the enhanced germicidal efficiency of low-irradiance 405-nm light for the inactivation of nosocomial bacteria at densities ≤10⁷ CFU ml^-1, further supporting its use for clinical environment decontamination, and identifies that higher irradiance applications may require consideration where greater contamination levels are expected. These findings establish a basis for further investigation into associated photochemical mechanisms to better understand why bacterial density and light intensity are influential in 405-nm light inactivation.