넙치 연쇄구균병 예방을 위한 Streptococcus parauberis의 포르말린 사균 및 M-like protein (SimA) 혼합항원의 효능에 관한 연구

Streptococcus parauberis is a Gram-positive, alpha-hemolytic coccus, that has recently emerged as the leading cause of mortality in farmed olive flounder (Paralichthys olivaceus) in South Korea. In this study, the efficacy of M-like protein (SimA), a major outer membrane protein known to de- termine the pathogenicity of S. parauberis, was evaluated as an adjuvant when combined with a formalin-killed vaccine (FKC). SimA, produced using an Escherichia coli expression system, was inactivated by formalin and ultrasonic disruption before being intraperitoneally route injected to olive flounder (average length: 12.06±0.13 cm; average weight: 17.1±2.27 g). Five experimental groups were established as follows: Group 1 (PBS), Group 2 (FKC, 1 mg/fish), Group 3 (FKC/E.coli, 2 mg/ fish), Group 4 (FKC/Freund’s incomplete adjuvant (FIA), 2 mg/fish), and Group 5 (FKC/SimA, 2 mg/fish). We evaluated antigen-specific antibody production following vaccination, protective efficacy against S. parauberis infection, and the safety of inactivated SimA as mixed antigens. ELISA results showed that Group 5 exhibited significantly higher antigen-specific antibody titers compared to the other groups up to 3 weeks post-vaccination. At 3 weeks post-vaccination, the fish were challenged subcutaneously with S. parauberis (4.86 × 10⁷ CFU/fish), and mortality was monitored for 28 days. Relative survival rates compared to Group 1 were 25% in Group 2, 25% in Group 3, 40% in Group 4, and 45% in Group 5. Additionally, at 4 weeks post-vaccination, blood biochemical analysis (glucose; GLU, total protein; TP, aspartate aminotransferase; AST, and alanine aminotransferase; ALT) indicated that Group 5 showed levels comparable to those of Group 1, suggesting no cytotoxic effects associated with SimA administration.