Photoreactivation of E. coli depending on light intensity after UV irradiation

Interest in the disinfection of drinking water by UV rays has been growing in recent years. Since the beginning of the 40's it has been known that, by means of certain repair mechanism, microorganisms are able to restore lesions on DNA. These processes are more or less clarified. However there is no secure knowledge about the degree of possible repair. Photoreactivation induced by light of 350-600 nm is one of these repair mechanisms. Thus the effect of light exposure on disinfection results with E. coli after UV disinfection was examined. E. coli ATCC 11229 were cultivated, washed, suspended and irradiated by means of UV rays from a mercury low-pressure lamp. After UV disinfection, the bacterial suspension was illuminated by fluorescent light. The degree of photoreactivation could be influenced by illumination intensity. The speed of reactivation decreased with the distance of the probes to the illumination source and increased with the period of time of illumination. After a 4 log reduction of the colony count (10(6)/ml-10(2)/ml) through UV rays, the reduction rate could be decreased to only 1 log (10(5)/ml) after UV disinfection and illumination. In probes kept in darkness for the same period there was no change in the count of reproductive bacteria. It seems to be necessary to take photoreactivation into consideration with the UV disinfection of drinking water.