Genomic and Pathogenic Characterization of Vibrio alginolyticus CG-1 Isolated from Crassostrea gigas.

The Pacific oyster (Crassostrea gigas) is a species of significant economic importance in global aquaculture. Vibrio alginolyticus, a halophilic marine bacterium widely distributed in coastal environments, is recognized as an opportunistic pathogen in both humans and marine organisms. However, the genomic determinants of V. alginolyticus pathogenicity in oysters remain poorly understood. The strain CG-1 was isolated from diseased Pacific oysters collected during a mortality event in Tongyeong, South Korea. Here, we report the complete genome sequence of strain CG-1, which comprises two circular chromosomes measuring 3,535,696 bp (chromosome I) and 1,806,773 bp (chromosome II), with GC contents of 44.61% (chromosome I) and 44.66% (chromosome II). The genome contains 4,785 protein-coding sequences, 127 tRNA genes, 37 rRNA genes, and one tmRNA. Plasmids were not identified. Functional annotation assigned 96.21% of the coding sequences to clusters of orthologous groups (COG) categories primarily related to metabolism, transcription, and inorganic ion transport. The genome encodes multiple virulence-associated genes including components of the type III secretion system, extracellular enzymes, motility and chemotaxis regulators, outer membrane proteins, iron acquisition systems, and antibiotic resistance genes. A pathogenicity assay revealed that V. alginolyticus CG-1 induced dose-dependent mortality in Pacific oyster larvae, with an LD₅₀ of 5.04 × 10⁶ CFU/mL. The clinical symptoms were comparable to those caused by major pathogens such as V. coralliilyticus, although the overall mortality rates were comparatively lower. Overall, this study provides valuable insights into the genome-based mechanisms and pathogenicity of V. alginolyticus in oyster larvae, underscoring its role as an emerging marine pathogen with the potential to impact aquaculture health and sustainability.