Impact of reattaching various Aeromonas salmonicida A-layer proteins on vaccine efficacy in Atlantic cod (Gadus morhua)

Atypical Aeromonas salmonicida causes atypical furunculosis in a whole range of farmed fish species. The bacterium comprises a heterogeneous group differing in surface components such as the A-layer protein and O-chain polysaccharide structures. Previously, the A-layer protein was shown to contribute to protective immunity as a vaccine with A. salmonicida cells with reattached A-layer protein protecting significantly better than the corresponding A-layer deficient bacteria used in the Atlantic cod (Gadus morhua) vaccine. In the present study, genetically different A. salmonicida A-proteins, either as preformed A-layer sheets from culture supernatants or as purified preparations, were shown to attach to A-layer deficient isolates with a different O-chain structure. Only vaccines containing A. salmonicida cells with reattached A-protein genetically homologous to the challenge isolate, elicited protection comparable to that of the homologous vaccine.