Toxicity of Aeromonas salmonicida cells to Atlantic salmon Salmo salar peritoneal macrophages.

Several strains of Aeromonas salmonicida were toxic to cultured peritoneal macrophages of Atlantic salmon, Salmo salar, in minimum doses of 5 x 10(6) CFU, whereas other strains were not. There was no correlation between cytotoxicity and in vivo virulence of the bacteria, the presence or absence of the two major surface components of A. salmonicida (namely, the A-layer and the O-polysaccharide chain of the LPS) nor, finally, with the ability of the strains to produce the following enzymes in vitro: protease, hemolysin, elastase and lecithinase. Toxicity was only observed with metabolically active bacteria and not with formalin- or heat-killed bacteria. The exact nature of the toxic factor remains unknown but is most likely associated with the surface of the bacteria. It is not extracellular since 24 and 48 h culture supernatants of the cytotoxic strains had no apparent effect on macrophages. The cytotoxic effect was found to be severe and rapid, it is likely a major virulence factor of A. salmonicida, but the exact role of such a potent toxin in the pathology of furunculosis has yet to be clarified.