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

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 (<i>S. aureus</i>, <i>E. faecalis</i>, <i>E. coli</i>, and <i>P. aeruginosa</i>) 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 <i>P. aeruginosa</i> (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 <i>E. coli</i>, <i>E. faecalis</i>, <i>P. aeruginosa</i>, and <i>S. aureus</i>, respectively). In heat stress conditions at 4 °C, <i>P. aeruginosa</i> and <i>S. aureus</i> species were the most susceptible (2.76 and 5.5 log₁₀), whereas at 45 °C all species, except <i>E. faecalis</i> (0.58 log₁₀), achieved significant reduction. The addition of exogenous photosensitive porphyrins produced a reduction in total concentrations from the lowest doses for <i>S. aureus</i> and <i>P. aeruginosa</i>, while for <i>E. coli</i> and <i>E. faecalis</i>, 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 <i>S. aureus</i> and <i>P. aeruginosa</i> 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.