Microplastics provide new hotspots for frequent transmission of antibiotic resistance genes during anaerobic digestion of sludge containing antibiotics

Abstract

Antibiotics and microplastics are emerging contaminants of widespread concern. Individual exposure to either antibiotics or microplastics may facilitate the dissemination of antibiotic resistance genes (ARGs). However, little is known about how the co-exposure of antibiotics and microplastics drives the dissemination of ARGs during sludge anaerobic digestion. This study established a series of long-term operational anaerobic digesters and found that 20 mg/L ofloxacin facilitated the dissemination of ARGs, whereas 200 mg/L ofloxacin led to a decrease in the absolute abundance of ARGs. However, the coexistence of polyamide and ofloxacin further increased the abundance of five ARGs by 4.4–359.1 % compare to singular ofloxacin exposure. The presence of polyamide not only complicated the microbial community structure but also increased the abundance of ARGs hosts by 12.2–108.7 %. Metagenome-assembled genomes indicated that 42 out of 53 ARGs hosts were functional microorganisms, and their enrichment under polyamide exposure facilitated the conversion of substrates into methane. Further mechanism exploration found that ofloxacin and polyamide promoted the formation of biofilm and enhanced the adhesion and communication among microorganisms. The upregulation of functional genes related to antioxidant, DNA repair, and type IV secretion systems provided evidence for the stimulated horizontal transfer of ARGs under the co-exposure of ofloxacin and polyamide. Partial least squares path modeling analysis identified that mobile genetic elements predominantly determined the variation of ARGs in this study. These findings emphasized that the coexistence of microplastics and antibiotics in sludge may further exacerbate the risk of antibiotic resistance transmission.