Photoinactivation of Streptococcus cristatus and Streptococcus vestibularis by 222 and 254 nm UVC Irradiation

Streptococcus pneumoniae is one of the world’s most important pathogens, causing over 800,000 deaths annually without being the only pathogenic Streptococcus species. This motivates research in strep¬tococci disinfection approaches, including UVC and Far-UVC irradiation, and identifying suitable sur¬rogates for the development and testing of disinfection measures and systems. Therefore, the UVC and Far-UVC sensitivity of two non-pathogenic Streptococcus strains should be investigated and, if possible, assessed for their suitability as S. pneumoniae surrogates. The non-pathogenic streptococci Streptococcus vestibularis and Streptococcus cristatus were irradiated with Far-UVC (222 nm) and conventional UVC (254 nm) in phosphate-buffered saline. Exponential photoinactivation was observed for both strains and wavelengths. The determined log-reduction doses at 222 / 254 nm for S. vestibularis and S. cristatus were 1.8/1.7 mJ/cm2 and 5.2/4.2 mJ/cm2, respectively. Both species were slightly more sensitive to 254 nm irra¬diation. However, their photosensitivity differed by more than a factor of two at both wavelengths. Never¬theless, UVC and Far-UVC are very effective against streptococci, and the observed log-reduction doses fit very well in the broad range of the few so far published values. However, as no reliable log-reduction doses of S. pneumoniae were identified, no assessment of the suitability of S. vestibularis or S. cristatus as S. pneumoniae surrogate could be performed. Considering the many fatal streptococci infections, more in¬vestigations into their UVC and Far-UVC sensitivities should be performed with a focus on S. pneumoniae.