Pulsed-field gel electrophoresis (PFGE) pattern analysis with XbaI restriction enzyme was used
to study the genetic heterogeneity of 88 atypical Aeromonas salmonicida strains which were
earlier or during this study characterized phenotypically, by ribotyping (ClaI/PstI) and by
plasmid profile analysis. The strains of certain ribotypes were also analysed by digestion with
SpeI. The strains represented different geographic locations: Finland (72 strains), Iceland (5
strains), Norway (5 strains), Sweden (4 strains) and Denmark (2 strains), and they were from
17 fish species during 1981–97. Thirty-one PFGE genotypes found among these strains
correlated well with the ribotypes, and in most cases PFGE pattern analysis subdivided
ribotypes into several PFGE genotypes, and further within a PFGE genotype into subtypes.
XbaI and SpeI digests produced concordant results. In most cases, PFGE patterns of strains
with the same ribotype shared many fragments, suggesting genetic relatedness. PFGE patterns
of most Norwegian and Icelandic strains isolated during an approximately 10-year period had
the same ribotype and their PFGE patterns shared most fragments, suggesting close genetic
relatedness. Moreover, atypical strains of ribotypes B/B and H/H isolated from the same
Finnish fish farms had closely related patterns suggesting genetic stability and persistence of
these genotypes. Genotype 29 of Achromogenic strains was strongly associated with disease of
Finnish arctic char and grayling. PFGE was shown to be a distinguishing method to study the
genetic heterogeneity of atypical A. salmonicida. This method is applicable to studies of the
epidemiology of these infections.