FACTORS INFLUENCING THE CLEARANCE OF A GENETICALLY ATTENUATED (DELTA-AROA) STRAIN OF AEROMONAS-SALMONICIDA FROM RAINBOW-TROUT ONCORHYNCHUS-MYKISS

Clearance of an aroA deletion mutant of Aeromonas salmonicida from tis sues of rainbow trout Oncorhynchus mykiss was studied under a number o f conditions. Following intraperitoneal injection into trout, the bact eria were rapidly found in all tissues/body fluids examined (kidney, s pleen, 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 degrees C regardless of the inoculum dose. However, with the highest dose (2 x 10(9) bacte ria per fish) used some fish mortalities occurred. Temperature was sho wn to influence clearance, with low water temperatures significantly d elaying clearance. Whole body homogenates confirmed that some increase in bacterial 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 c learance. These results are discussed with respect to the possible use of this deletion mutant as a live vaccine in aquaculture.