Isolation, identification, and pathogenicity of associated with oyster disease outbreaks in summer.

With the continuous expansion of oyster farming scale, disease has become one of the main obstacles to restricting the development of oyster farming. In the present study, 20 bacterial strains were identified from with pustulosis, among which emerged as the predominant strain, characterized by its rod-shaped morphology and possession of flagella. exhibited α-hemolytic activity at 28°C and displayed high susceptibility to all 20 chemotherapeutic agents tested. After infection, the oyster mortality rate increased. The gills were swollen and eroded, and the mantle was green with pustules after infection. The gill filaments exhibited swelling and necrotic cells, and the mantle showed a loose histological structure with cavities and disruption of epithelial cells. The extracellular products (ECPs) from had urease, protease, and amylase activities. The potential virulence proteins identified from ECPs were GroL, ClpB, and HtpG proteins. After injection with ECPs, there was an increase in the oyster mortality rate, and the observed symptoms in gill filaments and mantle were consistent with those observed after infection. In addition, the mRNA expressions of inflammation- and programmed cell death-related genes were significantly upregulated in gills and mantle. The relative abundances of , and also exhibited a significant increase in the gills and mantle. The results demonstrated that was the pathogenic bacterium for oysters, and its pathogenicity mechanism was systematically clarified, which provided valuable insights for the prevention and control of bacterial disease in oysters.IMPORTANCEDisease has currently emerged as one of the principal impediments to restricting the development of the oyster breeding industry. In the present study, was identified from with pustulosis. After infection, the oyster mortality rate increased. The gills were swollen and eroded, and the mantle was green with pustules. Extracellular products (ECPs) from had urease, protease, and amylase activities. The potential virulence proteins identified from ECPs were GroL, ClpB, and HtpG proteins. After injection with ECPs, the oyster mortality rate increased. The mRNA expressions of inflammation- and programmed cell death-related genes in gills and mantle increased significantly, and the relative abundances of , and exhibited a significant increase after infection. The results demonstrated that was the pathogenic bacterium for oysters, and its pathogenicity mechanism was systematically clarified, which provided valuable insights for the prevention and control of bacterial disease in oysters.