Photoinactivation and Photoreactivation Responses by Bacterial Pathogens after Exposure to Pulsed UV-Light

UV inactivation using a pulsed xenon flashlamp is an emerging non-thermal treatment technology that has the potential to inactivate (kill) microorganisms very rapidly in air, water and on exposed surfaces. In this study, a solid-state pulsed power source was used to transfer stored electrical energy to a xenon flashlamp, which emits light pulses with a broad spectral band extending from the ultraviolet to the infrared. The germicidal effects are caused mainly by the UV-wavelengths around 260 nm which are absorbed into DNA molecules, severely damaging these so that cellular functions are compromised leading to cell death. A potential limitation of UV-based inactivation technologies is that some microorganisms possess a photorepair mechanism (photoreactivation) to repair UV-damaged DNA and therefore facilitate recovery. Photoreactivation is an enzyme-mediated repair mechanism that occurs when the UV-damaged microbes are exposed to light of wavelengths between 300 and 500 nm. Suspensions of test bacteria, Staphylococcus aureus and Listeria monocytogenes, were exposed to pulsed UV (PUV) light treatment and germicidal efficiency was assessed by plotting log number of survivors against number of pulses. The results show that both S. aureus and L. monocytogenes are highly susceptible to PUV with a 6-7 log reduction achieved using less than 10 pulses. Further experiments were carried out to investigate the potential of S. aureus and L. monocytogenes to exhibit photoreactivation after PUV treatment. Both pathogens demonstrate a photoreactivation response when PUV treated cells are exposed to light in the wavelength range 300-500 nm. In experiments using a series of optical filters and different light sources it was established that the optimum wavelength for photoreactivation recovery of both S. aureus and L. monocytogenes, is in the region of 360-380 nm.