Isolation and characterization of Vibrio toranzoniae from adult Pacific oysters (Crassostrea gigas) in South Korea: Insights into pathogenicity and virulence factors

The Pacific oyster (Crassostrea gigas) is an important species in global aquaculture, particularly in South Korea, where oyster farming constitutes a significant economic activity. However, oyster health and aquaculture sustainability face substantial threats from bacterial infections, particularly those caused by Vibrio spp. In this study, we aimed to isolate Vibrio toranzoniae from adult C. gigas in Tongyeong, South Korea, and evaluate its pathogenicity under different environmental conditions. Phylogenetic analysis confirmed that the isolated strain (V. toranzoniae SKTY) belonged to the splendidus clade. Pathogenicity assays revealed a 50 % lethal dose (LD₅₀) of approximately 1.3 × 10⁸ CFU/mL, with mortality rates increasing significantly at elevated temperatures. Nitric oxide production in oyster hemocytes was examined as an immune response indicator, exhibiting a dose-dependent increase upon bacterial exposure. The isolate exhibited enzymatic activities (lipase, phospholipase, caseinase, and gelatinase) and hemolytic activity, further supporting its virulence potential. Antibiotic susceptibility testing showed that V. toranzoniae SKTY was susceptible to all tested antibiotics, with no observed resistance. Growth kinetics analyses demonstrated optimal bacterial proliferation at 28°C, with enhanced virulence at higher temperatures. Our findings highlight the potential threat of V. toranzoniae to oyster aquaculture, particularly under climate-induced temperature stress. Hence, continuous monitoring and targeted mitigation strategies are required to ensure sustainable oyster production in South Korea.