Decoupled sulfonamide biodegradation and resistance gene responses in leachate treatment sludge.

Municipal solid waste leachate is an important sink of sulfonamides and resistance genes, yet the coupling between antibiotic removal and resistance-associated risks across exposure levels in leachate sludge remained unclear. Here, sludge from the primary (PNS) and secondary (SNS) nitrification units of a full-scale leachate treatment plant were exposed to a mixture of sulfadiazine (SDZ), sulfamethoxazole (SMX), and trimethoprim (TMP) at 10 and 500 µg/l for each compound in batch experiments. At 10 µg/l, PNS and SNS removed 73.4 % of SDZ and 95.0 % of SMX, while TMP concentrations decreased below the quantification limit by day 5. Their removal efficiencies decreased at 500 µg/l. Metagenomic profiles indicated greater biodegradation-related potential under low-level exposure but reduced catabolic potential under high-level exposure. Meanwhile, 10 µg/l exposure increased the relative abundances of sulfonamide resistance genes and intI1, indicating the enrichment of resistance-associated genes and increased genetic mobility potential during the 14-day incubation. Additionally, SNS exhibited stronger short-term changes in resistance-associated genes and bacterial community composition than PNS. An exploratory cross-plant comparison suggested greater dissimilarity in resistance-related than biodegradation-related profiles. Apparently,antibiotic biodegradation potential and resistance-associated risks were not coupled in leachate treatment systems. These results will shed some lights into standardized antibiotic-associated risk assessments of leachate biological treatment systems.