Comparison of the antimicrobial efficacy and germicidal efficiency of 405-nm light for surface decontamination

Background
The persistence of infectious organisms on hospital surfaces presents a significant challenge to healthcare environments. Low irradiance visible 405-nm light has recently been developed as a method for environmental decontamination, with studies demonstrating successful reductions of environmental bacteria in wards and operating theatres. This study investigates the antimicrobial efficacy of 405-nm light for decontamination of surfaces, and how the dose-response kinetics are affected by use of differing light irradiances.

Methods
Surface-seeded Staphylococcus aureus and Pseudomonas aeruginosa (selected as model Gram-positive and Gram-negative species) were exposed to increasing doses of 405-nm light (≤ 90 Jcm-2) at three discrete irradiances (0.5, 5 and 50 mWcm-2). For both species, inactivation kinetics at each respective irradiance was established and susceptibility at equivalent light doses compared.

Results
Results demonstrate increased bacterial susceptibility to 405-nm light inactivation when exposed at lower irradiance treatments. For both species, 3 Jcm-2 was required when exposed using 0.5 mWcm-2 irradiance to achieve significant bacterial inactivation (P < 0.05; 26.7-73.7% reduction). When exposed at 5 mWcm-2, double the energy (6 Jcm-2) was required to achieve similar reductions. Exposure at the highest irradiance (50 mWcm-2) required 3-5 times greater dose (9-15 Jcm-2) to achieve similar reductions to the lowest irradiance tested (0.5 mWcm-2).

Conclusion
This study provides evidence of the enhanced germicidal efficiency of low irradiance 405-nm light, highlighting its efficacy for continuous environmental decontamination applications. Further investigation into the photo-chemical inactivation mechanisms will be crucial for its optimisation for a range of infection control applications.