Cometabolic biotransformation mediates the detoxification of oxytetracycline in Flavobacterium-enriched microbiome

Oxytetracycline (OTC), a widely used antibiotic that is readily detectable in the environment, potentially has a number of negative ecological consequences. The present study revealed the key role of the cometabolic breakdown of OTC in activated sludge microbiomes, which led to a high removal efficiency and detoxification of both OTC and its derivatives. An AS bioreactor was established to treat environmentally relevant levels of OTC. The bioreactor achieved an overall OTC removal efficiency of 94–97% at the steady state, with cometabolic biotransformation as the primary contributor to this ahead of other removal routes such as direct metabolic biodegradation, biosorption, and phosphorolysis. A novel biotransformation pathway for OTC proposed in this study involved dehydration, hydroxylation, ring cleavage, and alcohol dehydrogenation, distinguishing it from previously reported pathways. Although transformation products with potentially higher toxicity than OTC formed during the biotransformation process, the AS microbiome eliminated the overall toxicity of both OTC and its derivatives, as illustrated by both molecular structure-based toxicity simulations and antimicrobial susceptibility testing. Analysis of microbiome sequence data revealed the bacterial populations potentially facilitating cometabolic (e.g., Tolumonas) and direct metabolic (e.g., Flavobacterium) biotransformation. Overall, our study provides new insights into the removal routes and kinetics for OTC in AS microbiomes and the key species that potentially promote OTC transformation and detoxification. •Cometabolic biotransformation of OTC outperforms other removal routes.•A new biotransformation pathway distinguishable from previous ones is proposed.•The transformation product, ecotoxicity, and enzymes in the pathway are discussed.•Microbes responsible for cometabolic and direct transformation are determined.•Flavobacterium-enriched microbiome mediates detoxification of OTC and derivatives.