Non-pigment-producing isolates of Aeromonas salmonicida subspecies salmonicida: isolation, identification, transmission and pathogenicity in Atlanticsalmon, Salmo salar L.

Four non-pigment-producing isolates and two pigment-producing isolates of A eromonas salmonicida sp. salmonicida were isolated from the head-kidney of diseased farmed Atlantic salmon, Salmo salar L. The cultural, morphological and biochemical features of the isolates were compared with those of refer ence strains. Injection and cohabitation experiments were performed. The on ly difference between the non-pigment-producing isolates and the pigment pr oducing reference strains of A. salmonicida ssp. salmonicida was the inabil ity of the former to produce pigment. In the injection experiments, the inv estigated non-pigment-producing isolate produced a significantly higher mor tality compared with the mortality caused by the reference strain, whereas no difference in mortality was detected in the cohabitation experiments.