Characterization and Vaccine Development of Vibrio anguillarum, Aeromonas salmonicida salmonicida and Aeromonas salmonicida masoucida Isolated from Salmonids in Republic of Korea

<b>Background/Objectives</b>: This study aimed to characterize <i>Vibrio anguillarum</i> strain 23FBVib0271 (VA) isolated from rainbow trout and <i>Aeromonas salmonicida salmonicida</i> strain 17FBASa0016 (ASS) and <i>A. salmonicida masoucida</i> strain 23FBAer0174 (ASM) isolated from Atlantic salmon in the Republic of Korea. Their physiological traits, pathogenicity, and antigenicity were examined, and formalin-inactivated vaccines were developed to evaluate safety and immunogenicity in Atlantic salmon (<i>Salmo salar</i>). <b>Methods</b>: Formalin-inactivated VA, ASS and ASM were administered intraperitoneally, and protective efficacy was determined after six weeks. Serum biochemical parameters (AST, ALT, ALP, glucose) and histopathology were analyzed for safety. ELISA and real-time PCR targeting IL-1β, BCL6, membrane-bound IgM, and secretory IgM were performed to assess immune responses. <b>Results:</b> Vaccination provided relative percent survivals of 100%, 75%, and 95% for VA, ASS and ASM, respectively, without adverse physiological or histological effects. Immunological analyses revealed strong antibody production and upregulation of immune-related genes. <b>Conclusions:</b> Formalin-inactivated vaccines from VA, ASS and ASM are safe and effectively induce protective humoral immunity in Atlantic salmon by promoting antigen-specific antibody responses and immune gene activation.