Influence of liquid holding recovery and photoreactivation on survival of ultraviolet-irradiated fish pathogenic bacteria

Survival of the fish pathogens Aeromonas salmonicida subsp. salmonicida, Vibrio anguillarum, and Yersinia ruckeri, when subjected to various post-u.v. irradiation treatments, was investigated in a laboratory model system. In A. salmonicida, 99.9% inactivation doses had to be increased from (a) 3.2 mWscm−2 in the case of no post-irradiation recovery treatment to (b) 8.1 mWscm−2 if liquid holding in nutrient-free buffer in the dark for 48 h was applied before plating, (c) 9.5 mWscm−2 with visible light illumination at 1500 lx for 8 h and (d) 10.6 mWscm−2 with illumination followed by liquid holding. The corresponding figures for V. anguillarum were 2.8, 8.1, 6.3 and 13.4 mWscm−2 and for Y. ruckeri 1.2, 5.3, 4.9 and 8.5 mWscm−2. On combined treatment, the latter two species showed distinct plateaus of no or very low sensitivities to increased irradiation doses, while this feature was not evident in A. salmonicida. Complete liquid holding recovery demanded from 48 to more than 72 h at 22°C, while photoreactivation was completed within 4–6 h in all three species at the same temperature. If illumination intensity was reduced from 1500 to 150 lx, recovery time was approximately doubled in A. salmonicida. During post-irradiation dark incubation photoreactivating capacity was lost within approximately 15 h. It is concluded that the potential of fish pathogenic bacteria for liquid holding recovery and photoreactivation should be taken into consideration when assessing the efficiency of u.v. disinfection of aquacultural water. Particular concern should be aimed at recovery processes after discharge to recipient water.