Abstract
Ultraviolet (UV) C light emitted by a krypton chloride (KrCl) lamp consists of mainly less harmful 222-nm Far-UVC (unfiltered 222-mm Far-UVC) compared with conventionally used 254-nm UVC. It also contains wavelengths that are harmful to mammalian cells. By contrast, UVC from a KrCl lamp with optical filter (filtered 222-nm Far-UVC) consists of much less harmful 222-nm Far-UVC and is available for sterilization of dwelling spaces. The germicidal mechanisms of the 254-nm UVC and unfiltered 222-nm Far-UVC have been partially elucidated; however, the mechanism of action of filtered 222-nm Far-UVC remains unknown. It is known that 254 nm UVC induces cyclobutene pyrimidine dimers (CPDs), which are DNA lesions in Escherichia coli (E. coli); however, the CPDs are repaired by photoreactivation. In the present study, it was demonstrated that filtered 222-nm Far-UVC also generated CPDs, which were not repaired by photoreactivation. Therefore, a germicidal mechanism of filtered 222-nm Far-UVC may be different from a 254-nm UVC. It was reported that unfiltered 222-nm Far-UVC induced reactive oxygen species (ROS) in E. coli. In the present study, filtered 222-nm Far-UVC also induced ROS production. In accordance with increased ROS production, the levels of carbonylated proteins were increased, and morphological alteration was observed in E. coli. From these results, it was suggested that ROS generated by filtered 222-nm Far-UVC inactivated ROS scavenger enzymes and the enzyme photolyase that is involved in photoreactivation. The increased ROS levels and unrepaired CPDs impaired photoreactivation in E. coli and may be involved in the germicidal mechanism of action of the filtered 222-nm Far-UVC.IMPORTANCEThe 222 nm Far-ultraviolet (UV) C light (UVC) emitted from a krypton chloride lamp with an optical filter is currently available for the sterilization of dwelling spaces. To use the filtered 222-nm Far-UVC more effectively and safely for sterilization, it is necessary to understand its germicidal mechanism. The present study suggests that the germicidal effect of filtered 222-nm Far-UVC on E. coli may not only involve CPD but also ROS. These results could be useful in establishing more effective preventive methods in dwelling spaces for infectious diseases by UVC irradiation.