Isolation and characterization of a highly pathogenic strain of Vibrio harveyi EFL-2201 from pearl gentian grouper

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In May 2022, a significant mortality event involving pearl gentian groupers ( Epinephelus fuscoguttatus ♀ × E. lanceolatus ♂) was reported at an aquaculture farm in Zhangzhou, China, under water conditions of approximately 25 °C, pH 8.0, and salinity 32 ‰. A highly pathogenic bacterial strain, EFL-2201, was isolated from the spleen of a diseased fish. The outbreak was traced back to Vibrio harveyi through a series of scientific methods including isolation, identification, and regression infection. In vitro assays indicated that the conditions during the outbreak were conducive to the proliferation, motility, biofilm formation, and chemotaxis of EFL-2201. Whole-genome sequencing of the strain revealed that mechanisms related to adhesion, secretion, toxins, and invasion significantly contributed to its pathogenicity. Temporal and spatial bacterial load of EFL-2201 in various tissues of the infected groupers indicated that bacterial loads peaked at 24 h post-infection (hpi). The spleen, which had the highest bacterial load among all tissues, was subjected to transcriptome sequencing at 24 hpi, revealing significant enrichment in pathways related to immunity, cell cycle, and red blood cell oxygen transport. In conclusion, this study unveiled the pathogenic mechanisms of V. harveyi and immune response mechanisms of pearl gentian grouper spleens to infection, laying a theoretical foundation for the prevention and control of V. harveyi -induced disease. • Vibrio harveyi was the causative agent of epidemic of pearl gentian groupers. • Conditions during epidemic were conducive to the proliferation, motility, biofilm formation, and chemotaxis of V. harveyi. • The spleen was identified as the primary site of bacterial infection in the pearl gentian groupers . • Cytokine-cytokine receptor interaction and P53 signaling pathways were most significantly enriched.