UV inactivation and characteristics after photoreactivation of Escherichia coli with plasmid: health safety concern about UV disinfection.

Occurrence and degree of photoreactivation after ultraviolet (UV) exposure have been widely studied. However, the characteristics of photoreactivated microorganisms were rarely investigated. Hence, in this study, Escherichia coli with plasmids of ampicillin (amp)-resistance or fluorescence was used as indicators to examine the UV inactivation efficiencies and variations of characteristics of E. coli after subsequent photoreactivation. The experimental results indicate that the amp-resistant bacteria and the fluorescent bacteria used in this study had similar trends of UV dose-response curves. 3.5-log(10) and 3-log(10) reductions were achieved with a UV dose of 5 mJ/cm(2) for the amp-resistant and fluorescent E. coli, respectively. There was no significant difference in the UV inactivation behavior, as compared with common strains of E. coli. For the amp-resistant E. coli and the fluorescent E. coli, after exposures with UV doses of 5, 15, 25, 40 and 80 mJ/cm(2), the corresponding percent photoreactivations after a 4 h exposure to photoreactivating light were 1% and 46% respectively for a UV dose of 5 mJ/cm(2), and essentially negligible for all other UV doses. Furthermore, the photoreactivated amp-resistant bacteria still have the ability of amp-resistance. And the revived fluorescent E. coli showed similar fluorescent behavior, compared with the untreated bacteria. The experimental results imply that after UV inactivation and subsequent photoreactivation, the bacteria retained the initial characteristics coded in the plasmid. This reveals a possibility that some characteristics of bacteria can retain or recover through photoreactivation, and a safety concern about pathogenicity revival might need to be considered with UV disinfection and photoreactivation.