Gut dysbiosis after successive administration of florfenicol-medicated feed alters intestinal inflammatory response and growth patterns in healthy Atlantic salmon (Salmo salar)

Successive treatments with florfenicol-medicated feed are the primary strategy for controlling bacterial diseases in salmonid production in Chile. However, it is well established that antibiotic administration in the diet can cause dysbiosis in the intestinal microbiota, which in turn promotes inflammatory processes with potential negative effects on nutrient absorption. In this study, we evaluated the impact of repeated use of florfenicol-medicated diets on the composition and diversity of the intestinal microbiota, its influence on the inflammatory response of the distal intestine, and growth performance in healthy Atlantic salmon. Our findings revealed an increase in the abundance of bacterial genera considered opportunistic pathogens in this species. At the immunological level, although florfenicol initially induced an anti-inflammatory state in the intestine, a subsequent increase in the expression of pro-inflammatory cytokines was recorded. Additionally, goblet cells reduced mucin production, and significant morphological lesions were observed in the distal intestinal mucosa after repeated antibiotic exposure. While florfenicol administration at doses of 15 and 30 mg/kg slightly improved weight gain rate, specific growth rate, feed conversion efficiency, and condition factor during the treatment period, these effects were not sufficient to consider its use as a growth-promoting strategy. Our results suggest that, although healthy Atlantic salmon may experience a temporary improvement in growth and well-being while consuming florfenicol-medicated diets, repeated exposure leads to microbiota alterations and severe intestinal tissue damage. This may increase susceptibility to infections by opportunistic pathogens, facilitating superinfection processes and promoting greater antibiotic use, ultimately compromising the sustainability of the industry.