Pathogenic Characterization and Host Immune Response to Vibrio harveyi in Diseased Seriola dumerili

The greater amberjack (<i>Seriola dumerili</i>) is an economically important marine species that is prone to bacterial infections, resulting in high mortality rates and substantial economic losses. In this study, a virulent bacterial strain, Vh-2, was isolated from diseased greater amberjack and identified as Vibrio harveyi. Experimental infections caused high mortality and severe splenic damage characterized by tissue necrosis, abnormal pigment deposition, cellular disintegration, and extensive immune cell infiltration. A virulence gene analysis revealed that Vh-2 harbored multiple virulence-associated genes such as <i>toxR</i>, <i>toxS</i>, <i>vhpA</i>, <i>vhpB</i>, <i>vhhA</i>, <i>vhhB</i>, <i>luxR</i>, and <i>pap6</i>. Antibiotic susceptibility testing demonstrated ampicillin resistance but sensitivity to ceftriaxone, florfenicol, and meropenem. Transcriptomic profiling of infected spleens identified 396 differentially expressed genes (DEGs) compared to the control group, of which 293 were upregulated and 103 were downregulated. A functional enrichment analysis indicated that these genes were primarily involved in cell cycle regulation, DNA repair, metabolic processes, and immune-related pathways. These findings enhance our understanding of <i>V. harveyi</i> pathogenesis and immune responses of <i>S. dumerili</i> and provide new insights into the prevention and control of <i>V. harveyi</i> infections in marine fish.