Molecular and phenotypic methods for the characterization of atypical Aeromonas salmonicida.

Atypical Aeromonas salmonicida form a taxonomically diverse group among the psychrophilic A. salmonicida. Characteristics of 53 atypical A. salmonicida strains originating from Finland, Denmark, Norway and Sweden were studied using 60 phenotypic tests. Ribopattern analysis and plasmid profiles were used as genetic methods. The production of brown pigment on the furunculosis agar containing L-tyrosine divided the atypical oxidase-positive strains into two groups: pigment-producing (n = 35) and achromogenic (n = 16). PstI differentiated all the atypical A. salmonicida from A. salmonicida subsp. salmonicida. Combined ribopatterns ClaI/PstI divided pigment producing atypical strains into four major groups B/B, G/G, G/T, F/F. Most of the achromogenic oxidase-positive strains belonged to two major groups H/H or U/U. Cluster analysis of ribopatterns and plasmid profile analysis also supported the division of atypical oxidase-positive A. salmonicida into pigment-producing and achromogenic groups. The oxidase-negative strains formed a distinct group which differed from oxidase-positive atypical A. salmonicida type biochemically and in terms of ribopatterns. Our results also support the use of ribopattern analysis as a valid method to study the epidemiology of infections caused by atypical A. salmonicida on fish farms.