Abstract

Simple SummaryThe greater amberjack (Seriola dumerili) is an important farmed fish that often becomes sick from harmful bacteria, causing serious losses in aquaculture. In this study, we examined a disease-causing bacterium taken from sick fish and confirmed that it can trigger illness in healthy fish. We also studied how the fish reacts after infection by looking at changes in gene activity in its immune tissues. The results show that many genes related to defense and inflammation become more active. These findings help us better understand how the fish fights infection and may support improved disease control in aquaculture.The greater amberjack (Seriola dumerili) 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 astoxR,toxS,vhpA,vhpB,vhhA,vhhB,luxR, andpap6. 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 ofV. harveyipathogenesis and immune responses ofS. dumeriliand provide new insights into the prevention and control ofV. harveyiinfections in marine fish.