AimAlthough feed medicated with antibiotics is widely used in animal production to prevent and treat bacterial infections, the effect of these drugs on nontarget anaerobic bacteria is unknown. We aimed to clarify whether a single exposure of sulphate-reducing bacteria (SRB) from a tilapia pond to oxytetracycline (OTC) concentrations relevant to aquaculture impacts their function, abundance and community structure. Methods and ResultsTo demonstrate changes in SO42- content, SRB abundance, dsrB copy number and SRB diversity, sediment mesocosms were spiked with 5, 25, 50 and 75mg OTCkg(-1) and examined for 30days by means of ion chromatography, qPCR, cultivation and fluorescent insitu hybridization (FISH). On day 3, we measured higher SO42- concentrations (ca. two-fold) and a reduction in dsrB copy numbers of approximately 50% in the treatments compared to the controls. After 30days, a subtle yet measurable enrichment of bacteria from the order Desulfovibrionales occurred in mesocosms receiving 50mg OTCkg(-1), notwithstanding that SRB counts decreased two orders of magnitude. OTC was dynamically and reversibly converted into 4-epioxytetracycline and other related compounds in a dose-dependent manner during the experiment. ConclusionsA single exposure to rather high OTC concentrations triggered functional and structural changes in a SRB community that manifested quickly and persisted for a month. Significance and Impact of the StudyThis study improves our limited knowledge on the ecotoxicology of antibiotics in anaerobic environments.