Effect of visible light exposure on E. coli treated with pulsed UV-rich light

Despite the significant advances that have been made toward a better understanding of microbial transmission and pathogenicity, the levels of food/water-borne infection still remain high. Concern over this situation has led to a search for alternative technologies including the use of pulsed power applications, such as pulsed UV-rich light treatment, for sterilisation and disinfection purposes. Pulsed Ultra Violet (PUV) treatment involves the use of intense and short-duration pulses of UV-rich light to inactivate microorganisms. This method has an advantage over traditional UV delivery systems where applications are limited due to the long exposure times required with the low intensity emissions which are produced. The antimicrobial effect of pulsed light is primarily due to the ultraviolet content, which causes transformations of the pyrimidine bases of microbial nucleic acids. However, a drawback with pulsed light treatment is that many microorganisms are known to possess photo-repair mechanisms that facilitate recovery following sub-lethal exposure to continuous UV rich light. Both light and dark repair mechanisms are known to exist, but the extent to which microorganisms can exhibit photo-repair following pulsed light exposure is not clear. Experiments were carried out in which E. coli was exposed to pulsed UV-rich light from a low-pressure (450 torr) xenon flashlamp (Samtech Ltd). Following exposure to pulsed light, the treated cells were kept for varying periods, either in light (various intensities) or in dark conditions, and the surviving populations enumerated. Results showed little evidence of dark repair, whereas substantial photo-repair took place (2.1 log increase) when cells were exposed to visible light following treatment. The results also show that when the intensity of visible light is increased the greater the level of photoreactivation that occurs.