Efficacy of specific antibody against agglutinating Aeromonas salmonicida strains on infectivity and vaccination with inactivated protease

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Abstract
Efficacy of specific antibody on serum resistance and adhesion was investigated using a pathogenic strain of Aeromonas salmonicida A‐7301 (which was autoagglutinative, haemagglutinative and protease production positive), a protease‐deficient, non‐pathogenic mutant NTG‐1 induced from A‐7301 (autoagglutinative and haemagglutinative), and a non‐pathogenic strain GH‐7501 (non‐agglutinative, non‐haemagglutinative and protease positive). A‐7301 could grow and produce protease extracellularly in the presence of rainbow trout anti‐A‐7301 serum, resulting in a considerable reduction of the antibody titre. NTG‐1 similarly grew, but the titre scarcely decreased. GH‐7501 could not survive in this medium. A‐7301 and NTG‐1 possessed a high capacity to adhere to the surface of fish monolayer cell cultures, whereas GH‐7501 lacked the capacity. The capacity for adhesion was not inhibited by the antibody. Although live NTG‐1 cells were ineffective as a live vaccine, sockeye salmon receiving protease fraction (obtained from extracellular products of A‐7301 by DEAE‐cellulose column chromatography) inactivated with normal serum, suffered only a low mortality when challenged with A‐7301. Thus, although the antibody specific to autoagglutinating cells showed no effects on serum resistance and adhesion, which are involved in the infectivity of this pathogen, the possibility of protease as an effective protective antigen was demonstrated.