Pathogenicity of atypical Aeromonas salmonicida in Atlantic salmon compared with protease production

The pathogenicity of extracellular products (ECPs) from 24 atypical Aeromonas salmonicida strains was studied with respect to : lethality in Atlantic salmon, pathogenic effect in muscle, haemolytic activity, cytotoxicity in two fish cell lines and proteolytic activities. Furthermore, the relationship between lethality of ECPs and mortality caused by bacterial challenge was examined. Correlation was demonstrated between the pathogenic properties and proteolytic activities of the ECPs. Cytolytic (GCAT) activity comparable with that of the typical reference strain used (NCMB 1102) was not detected in ECPs of any of the atypical strain tested. An extracellular metallo-caseinase, AsaP1, was linked with lethal toxicity and a strong pathogenic effect. Furuncular-like lesions were produced by ECPs containing AsaP1 activity. One strain produced a lethal toxin which was neither caseinolytic nor with GCAT comparable activity. The examined atypical strains form at least three distinct groups based on different virulence mechanisms and extracellular proteases.