Inactivation of foodborne pathogens by UV-LED treatment

Abstract

Ultraviolet-C light is a widely used sterilization technology as type of lamp. But UV-lamps have several limitations including low activity at refrigeration temperatures, a long warm-up time, and risk of mercury exposure. As an alternative, UV-LEDs have been developed which can render desired wavelengths while UV-type lamps only emit light at 254 nm. In this study, I validated the inactivation efficacy of UV-LED with different wavelengths and compared results with those of conventional UV-lamps. Selective media inoculated with E. coli O157:H7, S. Typhimurium, and L. monocytogenes were irradiated with UV-LEDs of 266, 270, 275, and 279 nm in the UV-C spectrum at 0.1, 0.2, 0.5, and 0.7 mJ/cm2, respectively. The radiation intensity of UV-LEDs was about 4 μW/cm2, and UV-lamps were covered with polypropylene films to adjust the light intensity similar to those of UV-LEDs. Additionally, I applied UV-LED to sliced cheese at doses of 1, 2, and 3 mJ/cm2. My results showed that inactivation rates following UV-LED treatment were much higher than those of UV-lamps at a same intensity. On microbiological media, UV-LED treatment at 266 and 270 nm showed a slightly higher inactivation effect than for other high wavelength modules, but there were no critical differences by wavelength. For sliced cheeses, 4 to 5 log reductions occurred after treatment at a level of 3 mJ/cm2 for all three pathogens with negligible generation of injured cells. To determine primary factors affecting reduction trends shown in several bacterial groups, I investigated the efficacy of UV-LED to Gram negative (GN) or Gram positive bacteria (GP). Four major foodborne pathogens (E. coli O157:H7, Salmonella spp. L. monocytogenes, and S. aureus) were inoculated onto selective and non-selective media in order to investigate reduction tendencies at 4 different peak wavelengths (266 to 279 nm). As irradiation dose increased, inactivation levels for every microorganism were enhanced, but there were different UV-sensitivities in GN and GP. By using fluorescence dyes (DiBAC4(3), PI, and Hoechst 33258), the effects of UVC-LED treatment on cell membranes or DNA were investigated. Loss of membrane potential measured by DiBAC4(3) increased as peak wavelength increased for every bacteria studied. Similar results were observed in membrane integrity measured by PI. However, there were contrasting results which showed that greater DNA damage occurred at a lower peak wavelength as measured by Hoechst 33258. The level of DNA damage was strongly related to trends of microbial inactivation. This study showed that even though membrane damage was present in every bacterium studied, DNA damage was the primary factor for inactivating microorganisms through UVC-LED treatment.