Effect of anibiotics in Sea Bass intestinal microbiome

The use of antibiotics in open-water aquacultures is often unavoidable when faced with pathogens with high mortality rates. Those seasonal pathogen surges are becoming more common and more intensive over the years. Besides the apparent cost of antibiotic treatment, it has been observed that in aquaculture practice the surviving fish often display measurable growth impairment. This growth delay can be in the range of 10% of the commercial-sized fish weight. In order to understand better the role of gut microbiota on the observed growth impairment we follow an incidence of Photobacterium damselae surge in a Sea Bass commercial open-water aquaculture setting. Fish around 9 months of age were fed with food containing oxytetracycline (120 mg/kg/day) for twelve days, followed by a twelve-day withdrawal period, followed by another eighteen days of treatment. Fishes were sampled before the start of the treatment and a month after the end of the second treatment cycle. Sequencing of the 16S rRNA gene was used to measure the changes in the gut microbiome. Overall, the gut microbiota community even a month after treatment was highly dysbiotic and characterized by very low alpha diversity. High abundances of alkalophilic bacteria in the post-antibiotic-treated fishes indicate a rise in pH that was coupled with a significant rise of gut parasites. This study indicates that OTC treatment causes persistent dysbiosis even a month after withdrawal, providing a more suitable environment for the raise of parasites and highlighting the need for interventions to restore a healthy and beneficial gut microbiome.