Florian Sommerfeld

To control the spread of bacteria and viruses on surfaces in medical environments and everyday life, suitable disinfection methods are required. Visible radiation in the violet or blue spectral range is known to exhibit an antimicrobial impact on microorganisms. However, so far most published studies were performed on liquids. In contrast, the sensitivity of microorganisms to visible radiation on surfaces was only investigated in a few studies. In order to transfer possible conclusions from irradiation in media to irradiation on surfaces and to apply visible light as a possible valid alternative for common disinfection methods, the log reduction doses for surfaces and liquids were compared in this study. The non-pathogenic Staphylococcus carnosus was selected as a surrogate for the ESKAPE pathogen Staphylococcus aureus, as the experiments were performed in an S1 laboratory. The irradiations were performed with wavelengths of 403 nm (violet) and 453 nm (blue). The observed log reduction doses in liquids (literature with the same strain and setup) and on surfaces (this investigation) were 101.8 J/cm² and 14.0 J/cm² at 403 nm and 374.3 J/cm² and 112.8 J/cm² at 453 nm, respectively. The results suggest that the photosensitivity of S. carnosus on surfaces is much higher than in that liquid with a ratio of 7.3 (violet) to 3.3 (blue). On the one hand, this demonstrates that irradiation on surfaces is more efficient than that in liquids, especially in the violet spectral range. On the other hand, depending on the strength of the irradiation source, disinfection with visible irradiation is a useful alternative to conventional disinfection methods.

(1) Background: Applications using touch screens are increasingly deployed in medical facilities, as well as in public areas. When touching the display with fingers, potentially pathogenic microorganisms such as methicillin-resistant Staphylococcus aureus (MRSA) can be transmitted. An automated process to decontaminate the device in between users would be highly useful. (2) Methods: Thin glass plates were superficially contaminated with the non-pathogenic Staphylococcus carnosus in a controlled manner. Subsequently, UVA radiation of 400 or 380 nm was laterally coupled into the glass plate, which acted as a light guide. Contact agar plates recorded the change in the staphylococci concentration over time. Additionally, the UVA radiation emitted by the glass plates was measured and the potential risk to humans assessed. (3) Results: Staphylococci concentration decreased as a result of UVA radiation for both wavelengths. At 400 nm, it took about 7.5 h and at 380 nm about 1 h until a reduction of 90% was reached. To meet higher disinfection requirements, disproportionately longer irradiation times were necessary. The potential UVA irradiation of humans in front of the glass pane was about 35 µW/cm² or less and posed no risk to humans. (4) Conclusions: Side-coupled UVA radiation is in principle capable of safely automatically disinfecting microorganisms on touch screens. However, the required irradiation times are still in the hour range, so that a rapid disinfection within a minute or less is not yet possible with the presented setup. However, higher UVA intensities might reduce the current disinfection durations.