Factors influencing the clearance of a genetically attenuated (ΔaroA) strain of Aeromonas salmonicida from rainbow trout Oncorhynchus mykiss

Clearance of an aroA deletion mutant of Aeromonas salmonicida from, tissues of rainbow trout Oncorhynchus mykiss was studied under a number of conditions. Following intraperitoneal injection into trout, the bacteria were rapidly found in all tissues/body fluids examined (kidney, spleen, liver, heart, muscle, blood and peritoneal fluid), with highest recoveries in the spleen and peritoneal fluid Whilst in some tissues numbers increased over the first few days post-injection, in general clearance had been effected within 7 to 9 d at 16°C regardless of the inoculum, dose. However, with the highest dose (2 × 10⁹ bacteria per fish) used some fish mortalities occurred. Temperature was shown to influence clearance, with low water temperatures significantly delaying clearance. Whole body homogenates confirmed that some increase in bactenal numbers occurred in vivo, after an initial and immediate dramatic drop in bacterial viability. Finally, a 2 h crowding stress which significantly elevated blood glucose levels had no effect upon clearance. These results are discussed with respect to the possible use of this deletion mutant as a live vaccine in aquaculture.