Consumption of florfenicol-medicated feed alters the composition of the channel catfish intestinal microbiota including enriching the relative abundance of opportunistic pathogens

Antibiotics are frequently used in feed to control bacterial diseases in aquaculture settings. In addition to the intended purpose of controlling diseases, applications of medicated feed may significantly change the composition of the gut microbiota, which in turn may have impact on host-pathogen interactions. However, nothing is known concerning the effects of antibiotics on microbiota in the gut channel catfish (Ictalurus punctatus), the most important aquaculture species in the United States. In this study, we determined the impact of medicated feed containing florfenicol on microbiota in the gut of catfish at various times post-feeding. Through analysis of bacterial 16S rRNA gene amplicons, we found that the medicated feed had a dramatic impact on the composition of the gut microbiota, with significant enrichment of Plesiomonas spp., accounting for 66% of all gut bacteria. Along with other related species, Proteobacteria taxa accounted for 93% of all microbiota by day 10; in contrast, the gut microbiota of fish receiving non-medicated feed harbored a bacterial assemblage that had greater ribotype richness and a more even distribution (P <.05), with <10% Proteobacteria relative abundance. Moreover, florfenicol-medicated feed resulted in an increased relative abundance of potential opportunistic pathogens including Plesiomonas and Aeromonas species, which may have detrimental impacts on fish health. This study provided insights into the specific bacterial taxa within the channel catfish intestinal microbiome that were impacted by florfenicol-medicated feed and suggests that the abuse of this and other antibiotics by the aquaculture industry may induce the relative abundance of potentially pathogenic gut microorganisms.