Atypical Aeromonas salmonicida strains comprise a heterogeneous group in terms of molecular and phenotypic characteristics. They cause various conditions of
ulcer diseases or atypical
furunculosis and are being isolated in increasing number from various fish species and geographical areas. Several marine fish species susceptible to atypical A. salmonicida, including spotted wolffish Anarhichas minor O., are now being farmed and new
vaccines may be needed. A commercial
furunculosis vaccine for salmon is reported to protect wolffish poorly against experimental challenge with atypical A. salmonicida. The protective
antigen(s) in
furunculosis vaccines is still unclear, but in oil-adjuvanted
vaccine for Atlantic salmon Salmo salar L., the surface A-layer was shown to be important for protection. In spotted wolffish, the efficacy of atypical
furunculosis vaccines seems to vary with the atypical A. salmonicida strains used as
bacterin in the
vaccine. In the present study we investigated whether differences in the A-layer
protein among atypical strains might be responsible for the observed variation in
vaccine efficacy. Atypical A. salmonicida strains from 16 fish species in 11 countries were compared by genome polymorphism analysis using amplified fragment length polymorphism (AFLP) fingerprinting and by comparative sequencing of the vapA genes encoding the A-
protein. The A-
protein sequences appeared to be highly conserved except for a variable region between Residues 90 to 170. Surprisingly, the grouping of strains based on AFLP- or A-
protein sequence similarities was consistent. In addition, serological differences in the A-
protein among the strains were demonstrated by an A-
protein-specific
monoclonal antibody.
Vaccines based on atypical A. salmonicida strains possessing genetically and serologically different A-layer
proteins were shown to result in significantly different protection in spotted wolffish.