Evaluation of the turbot (Scophthalmus maximus) microbiome and host antiviral response to viral hemorrhagic septicemia virus (VHSV) after treatment with oxytetracycline

Aquaculture intensification has led to widespread antibiotic use to control disease outbreaks, but antibiotic-mediated disruption of the intestinal microbiota can have unintended consequences for host immunity and disease susceptibility. The microbiota is increasingly recognized as a key regulator of systemic immune response, yet the impact of antibiotic-induced dysbiosis on viral infections in fish remains poorly understood. Here, we show that exposure to oxytetracycline (OTC), a commonly used antibiotic in aquaculture, significantly increases susceptibility to viral hemorrhagic septicemia virus (VHSV) infection in juvenile turbot ( Scophthalmus maximus). Fish challenged with VHSV shortly after OTC treatment exhibited a modest but significant reduction in survival (10–13% across two independent trials). By contrast, infection at 30 days post-treatment resulted in survival rates comparable to untreated controls. To explore the mechanisms underlying this transient increase in viral susceptibility, we investigated gut microbiota composition, predicted microbial functional potential, host transcriptomic responses, and intestinal architecture following OTC exposure. OTC treatment induced a marked but transient intestinal dysbiosis, characterized at 24 h post-treatment by a drastic reduction of two key commensal bacterial families, Vibrionaceae and Marinobacteraceae . While alpha diversity remained largely unchanged, beta diversity and co-occurrence network analyses revealed pronounced microbial community restructuring, with reduced correlation structure and properties shortly after treatment, followed by partial recovery at 30 days, which may account for the absence of survival differences at the later infection time point. Functionally, OTC-induced dysbiosis was associated with shifts in the predicted microbial metabolic potential, including pathways related to lipid metabolism and glycan biosynthesis. Transcriptomic analyses of head kidney and intestine revealed that OTC altered the expression of key antiviral, antioxidant, and inflammatory genes, particularly in the intestine, where a strong upregulation of DNA replication and cell cycle-related genes suggested a direct effect of OTC on intestinal architecture, supported by histological analyses. Together, these findings could suggest that OTC-induced microbiota disruption, combined with its potential toxic effect on the host, compromises intestinal immune homeostasis and transiently increases susceptibility to viral infection, highlighting the need for more judicious antibiotic use in aquaculture.