Introduction
In recent years, the study of commensal microbiota has gained increasing relevance, as endogenous microbial communities are now recognized as key modulators of disease progression, including viral infections. These microbiota-host interactions play a complex, bidirectional role in the establishment and outcome of infections. In particular, the gut microbiota and its metabolites have been shown to influence the host’s immune response and antiviral defenses (Mizutani et al., 2022).
Turbot (Scophthalmus maximus), a commercially important flatfish species, holds significant economic value in Spain, one of the world’s leading producers alongside China. The intestinal microbiota of turbot comprises a diverse community of microorganisms, including both non-pathogenic and potentially pathogenic Vibrio spp., which under specific conditions can cause vibriosis—a major bacterial disease in aquaculture responsible for considerable economic losses.
Oxytetracycline (OTC), a broad-spectrum antibiotic, is widely used in aquaculture to treat vibriosis. It remains one of the most commonly administered antibiotics for both prophylactic and therapeutic purposes and is often readily available to aquaculture operations (Sanches-Fernandes et al., 2022). However, the overuse of OTC has been linked to the emergence of drug-resistant Vibrio strains (Ji et al., 2020), raising concerns about the long-term sustainability of current disease management practices. In addition to fostering resistance, antibiotics like OTC may have unintended consequences on the host by disrupting the composition and function of the gut microbiota.
Materials and methods
In this study, we aimed to investigate the impact of OTC treatment—administered under routine aquaculture conditions via immersion (0.1 g/L for 1 hour over four consecutive days)—on the gut microbiota of turbot. Additionally, we assessed whether the resulting alterations in microbiota composition influence the host’s immune response and survival following infection with viral hemorrhagic septicemia virus (VHSV). Viral infections were conducted either one day after the final OTC treatment or 30 days later, to determine whether the effects of the antibiotic were transient or sustained over time. The intestinal bacterial microbiota was also analyzed at the one-month time point to evaluate the recovery of the normal microbial community. Changes in gut bacterial composition were characterized using 16S rRNA amplicon sequencing. To further assess the systemic effects of OTC, transcriptomic profiles of intestine and head kidney samples from both uninfected and VHSV-infected fish (24 hours post-infection) were analyzed using RNA-Seq.
Results
The results revealed a significant reduction in Vibrio spp. abundance across all OTC-treated individuals, with no detection of drug-resistant strains. The ecological niche vacated by Vibrio spp. was subsequently occupied by bacteria from various other taxonomic classes. This antibiotic-induced disruption of the gut microbiota had a marked effect on turbot survival following VHSV challenge: fish infected immediately after OTC treatment exhibited significantly reduced survival compared to untreated controls. In contrast, when infection was performed 30 days post-treatment, survival rates were comparable between treated and untreated groups, which is supported by a recovery of the gut microbiota and its associated protective functions over time.
At the transcriptomic level, RNA-Seq analyses revealed notable alterations in the expression of key immune-related genes in OTC-treated fish, which most likely contributed to their reduced survival following VHSV infection. These findings underscore the critical importance of maintaining microbiota integrity in aquaculture species. Beyond promoting antimicrobial resistance, the misuse or overuse of antibiotics can impair host immune defenses and increase susceptibility to viral infections.
References
Ji, Q., Wang, S., Ma, J., Liu, Q., 2020. A review: Progress in the development of fish Vibrio spp. vaccines. Immunol. Lett. https://doi.org/10.1016/j.imlet.2020.07.002
Mizutani, T., Ishizaka, A., Koga, M., Tsutsumi, T., Yotsuyanagi, H., 2022. Role of Microbiota in Viral Infections and Pathological Progression. Viruses. https://doi.org/10.3390/v14050950
Sanches-Fernandes, G.M.M., Sá-Correia, I., Costa, R., 2022. Vibriosis Outbreaks in Aquaculture: Addressing Environmental and Public Health Concerns and Preventive Therapies Using Gilthead Seabream Farming as a Model System. Front. Microbiol. https://doi.org/10.3389/fmicb.2022.904815