Monitoring Infection and Antibiotic Treatment in the Skin Microbiota of Farmed European Seabass (Dicentrarchus Labrax) Fingerlings.

The microbiota of fish skin, the primary barrier against disease, is highly dynamic and modulated by several factors. In fish aquaculture, disease outbreaks occur mainly during early-life stages, with associated high economic losses. Antibiotic treatments sometimes remain the best option to control bacterial diseases, despite many reported negative impacts of its use on fish and associated microbiota. Notwithstanding, studies monitoring the effects of disease and antibiotic treatment on the microbiota of fingerlings are scarce. We sequenced the bacterial 16S rRNA V4 gene region using a metabarcoding approach to assess the impact of a mixed infection with Photobacterium damselae ssp. piscicida and Vibrio harveyi and subsequent antibiotic treatment with flumequine, on the skin microbiota of farmed seabass (Dicentrarchus labrax) fingerlings. Both infection and antibiotic treatment led to a significant increase in bacterial diversity and core microbial communities and impacted microbiome structure. Dysbiosis was confirmed by changes in the abundance of potential pathogenic and opportunistic bacterial taxa. Skin bacterial metabolic function was also significantly affected by flumequine administration, suggesting a detriment to fish skin health. Our results add to an increasing body of literature, showing how fish microbiome response to infection and antibiotics cannot be easily predicted.