Near-UVA Radiation at 405 nm: Germicidal Effect With and Without Sub-Lethal Conditions on Gram-Positive and Gram-Negative Bacteria

TL;DR

This study shows that 405 nm near-UVA light can effectively kill bacteria, especially when combined with stress conditions or porphyrins, offering a new disinfection method for healthcare.

Contribution

The study demonstrates the enhanced germicidal effect of near-UVA radiation when combined with sub-lethal stress and porphyrins.

Findings

P. aeruginosa showed the highest reduction (2.4 log10) under near-UVA alone.

Osmotic stress combined with light achieved up to 5.46 log10 reduction in E. faecalis.

Exogenous porphyrins significantly improved disinfection for S. aureus and P. aeruginosa.

Abstract

The need for contrasting Healthcare-Associated Infections requires the promotion and support of alternative disinfection techniques. Due to the antimicrobial potential of UV, devices equipped with UVC, UVB and UVA lamps or LEDs have been developed in recent years for domestic, everyday use. In this study, four bacterial strains (S. aureus, E. faecalis, E. coli, and P. aeruginosa) were exposed to different doses of near-UVA radiation at 405 nm, with an average irradiance of 21 mW/cm2, using an experimental multi-LED device. Bacterial suspensions were irradiated under both sub-lethal and non-sub-lethal stress conditions. When using only near-UVA light, the highest abatement effect was observed on P. aeruginosa (2.4 log10). Treatment with osmotic stress, in combination with light irradiation, was effective on all bacterial strains (mean abatement of 2.76, 5.46, 5.31, and 1.5 log10 on E. coli, E. faecalis, P. aeruginosa, and S. aureus, respectively). In heat stress conditions at 4 °C, P. aeruginosa and S. aureus species were the most susceptible (2.76 and 5.5 log10), whereas at 45 °C all species, except E. faecalis (0.58 log10), achieved significant reduction. The addition of exogenous photosensitive porphyrins produced a reduction in total concentrations from the lowest doses for S. aureus and P. aeruginosa, while for E. coli and E. faecalis, the reductions did not exceed 1 log10 abatement. Near-UVA radiation at 405 nm has a high disinfectant potential when combined with certain sub-lethal stress conditions. The most significant germicidal effect was achieved with the use of exogenous porphyrins in S. aureus and P. aeruginosa species. This study opens perspectives on the possible future application of near-UVA radiation in disinfection in order to limit the spread of healthcare-related infections.

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