Enrofloxacin drives the ultra-selective enrichment of Ca. Kuenenia stuttgartiensis under microaerobic conditions

The acquisition of anaerobic ammonium oxidation (anammox) bacteria with a high relative abundance can serve as the basis for exploring their ecophysiological characteristics. Antibiotics are widely used in the selective enrichment of microorganisms, however, their feasibility for anammox bacteria and their metabolic response mechanisms in anammox bacteria remain to be elucidated. In this study, the anammox consortia (UB_ENR) was exposed to gradient concentrations (0.2–1 mg·L⁻¹) of enrofloxacin (ENR), with parallel operation of an antibiotic-free control system (UA_CK). Results showed that the UB_ENR system demonstrated 14 % enhanced nitrogen removal efficiency at 1 mg·L⁻¹ ENR exposure compared to the control. Furthermore, the consortium achieved 13–37 % ENR removal through combined mechanisms of sludge adsorption and biodegradation process. Ca. Kuenenia was selectively enriched with a higher relative abundance of 91.93 % in UB_ENR than that of UA_CK (75.03 %). Metagenomic analysis retrieved a unique anammox strain of Ca. Kuenenia stuttgartiensis, which exhibited upregulation of genes encoding cytochrome c peroxidase (ccp) potentially associated with ENR biotransformation and the ROS-SOS response system. These findings provide a novel insight for high-efficiency anammox bacterial enrichment while expanding the application potential of anammox processes in antibiotic-containing wastewater treatment systems.