Antibody-mediated bacterial killing of Ichthyobacterium seriolicida in Japanese amberjack.

•Fish recovered from Ichthyobacterium seriolicida infection acquire immune defense.•Mortality was reduced by passive immunization with convalescent serum.•Bacteria were killed when exposed to convalescent serum but not serum from naïve fish.•Naïve serum became bactericidal by addition of purified IgM from convalescent serum.•Convalescent fish serum thus protects against reinfection via humoral immunity.