Therapeutic-Dose Enrofloxacin Immersion Induces Intestinal Immunostimulation and Microbiota Dysbiosis in Juvenile Black Seabream Acanthopagrus schlegelii.

Enrofloxacin (ENR) is a commonly identified veterinary pharmaceutical in global aquaculture products, and short-term therapeutic applications of ENR at recommended doses may exert adverse effects on farmed fish. The present investigation evaluated the toxicological impacts of ENR on intestinal detoxification capacity, immune response, and microbial community dynamics of commercially significant Acanthopagrus schlegelii. Short-term treatment of ENR at the clinically recommended dose (5 mg/L) showed little modulation on expression levels of key detoxification enzymes, while 10 mg/L ENR (over-therapeutic concentration) induced significantly elevated Phase I/II biotransformation enzymes in treated fish. Interestingly, 5 mg/L ENR (decontaminated for 72 h) triggered immunostimulation effects via significantly promoting inflammatory cytokines and lysozyme gene expression. In addition, persistent suppression of the diversity and richness of gut microbial communities was observed following immersion in 5 and 10 mg/L ENR. ENR exposure triggered restructuring of intestinal microbial composition of black seabream, characterized by a reduction in beneficial microbiota and an expansion of opportunistic and antibiotic-resistant bacteria, which may contribute to the observed immune perturbation and long-term health risks. In summary, our findings raise concerns about the safe use of ENR in aquaculture, suggesting that current practices may pose long-term health risks to marine farmed fish.