Evaluation of profiles of Aeromonas salmonicida as epidemiological markers of furunculosis infections in fish

. Strains of Aeromonas salmonicida subsp. salmonicida, the agent of furunculosis disease of salmonid fish, have fairly uniform plasmid patterns. Of 35 strains examined by agarose gel electrophoresis, 28 had a pattern consisting of four small plasmids (4.2, 3.6, 3.5, 3.3 Mda) and a larger plasmid. The larger plasmid was most often 50–56 Mda, but it was larger in some strains. In the remaining seven strains, the same general profile was seen, but one of the small plasmids was missing. An additional plasmid was present in six strains. The pattern seen in 30 strains collected from Ontario fish over an 8‐year period did not differ significantly from five reference isolates from other locations. Plasmid profiles of A. salmonicida strains appear too uniform to provide a useful epidemiological tool. The non‐pigmented. atypical strains of A. salmonicida subsp. achromogenes and A. salmonicida subsp. masoucida, A. media, and brown‐pigmented strains of A. hydrophila had different plasmid DNA profiles, which were distinct from those of typical isolates of A. salmonicida subsp. salmonicida. Antibiotic susceptibility patterns, determined by the agar dilution method, were uniform for most typical strains. A non‐transferable resistance to tetracyclmes was found in two Ontario isolates, but antibiotic resistance was relatively uncommon among the Ontario isolates.