Atypical furunculosis vaccines for Atlantic cod Gadhus morhua: impact of reattached Aeromonas salmonicida A-layer protein on vaccine efficacy.

Atypical furunculosis caused by atypical Aeromonas salmonicida bacteria is reported as an increasing problem in farmed Atlantic cod Gadus morhua in Norway. At present, furunculosis vaccines adapted for cod or other marine fish species are not available. To identify bacterial components important for inducing protection in cod, we compared oil-adjuvanted vaccines based on A. salmonicida isolates phenotypically differing in their major cell surface constituents, such as the A-layer protein and lipopolysaccharide O-chains. Also included was an A-layer-deficient isolate with physically reattached A-layer protein. Vaccines containing A. salmonicida A-layer-producing cells elicited significantly better protection than vaccines with A-layer-deficient cells or with a supernatant with secreted A-layer protein. The A. salmonicida cells with reattached A-layer-protein resulted in significant and equal protection to the A-layer-producing cells and protected significantly better than the A-layer-deficient isolate. These results indicate that the A-layer protein when attached to the cell surface plays a role in inducing protective immunity in cod.