Relationships between selected strains of typical and atypical Aeromonas salmonicida, Aeromonas hydrophila, and Haemophilus piscium.

Serological analyses of typical and atypical Aeromonas salmonicida divided the representative strains into two distinguishable groups based on their antigenic composition. However, the serological groupings do not coincide with divisions constructed on the basis of physiological reactions. All of the A. salmonicida strains and the two A. hydrophila strains examined share at least one antigen and are sensitive to A. salmonicida bacteriophage 40 RR2.8t. These data reinforce the reasons for placing these two species in the genus Aeromonas. Haemophilus piscium, the causative agent of ulcer disease, was serologically indistinguishable from A. salmonicida and sensitive to A. salmonicida bacteriophages. This information, in addition to published H. piscium DNA studies, suggests this organism is actually an atypical A. salmonicida; more H. piscium isolates should be characterized to confirm this identification.