Effects of high intensity light on ultraviolet-irradiated and non-irradiated fish pathogenic bacteria

The viability of the fish pathogenic bacterium Aeromonas salmonicida was rapidly reduced when exposed to sunlight at about 105 lx in river water. A 99.9% inactivation was observed after 2 h exposure. Artificial light at 28,000 lx induced inactivation at a lowered rate. If pre-exposed to disinfecting doses of ultraviolet C irradiation in the river water, sunlight illumination induced rapid initial photorecovery followed by inactivation as outlined above. After reducing the number of colony-forming units to 0.08% of the initial count in an experimental UV system, a 160-fold recovery within 20 min of sunlight exposure was observed. However, after 2 h of sunlight exposure, the viable count was down again to 0.07% of the initial value. The presence of humic acid accelerated the photoinactivating effect while sodium chloride at 0.9% concentration increased survivability of both light-stressed and dark-incubated bacteria. The marine bacterium Vibrio anguillarum showed a slower response than A. salmonicida, both with respect to sunlight-induced inactivation and photorepair. In non-irradiated cells, the viable count was reduced by 97.5% after 3 h, while 1 h was needed to obtain a maximum of 19-fold photorecovery in sunlight after UV-C irradiation. It is concluded that sunlight induced processes should be taken into consideration when assessing the survival of fish pathogens in surface waters and the UV disinfection efficiency towards inlet and effluent water of aquacultural installations.