Metagenomic Insights into Host-Associated Enrichment of Antibiotic Resistance Genes Under Oxygen-Limited Conditions Induced by PET Microplastics

Antibiotic resistance genes (ARGs) are increasingly recognized as emerging contaminants in wastewater treatment systems; however, their responses to dissolved oxygen (DO)-limited conditions caused by insufficient aeration, particularly in the presence of microplastics, remain poorly understood. In this study, three sequencing batch reactors (SBRs) were operated for 31 days under progressively oxygen-limited conditions with different concentrations of polyethylene terephthalate (PET) microplastics to investigate their combined effects on treatment performance, microbial communities, ARGs, mobile genetic elements (MGEs), and PET degradation-related genes using metagenomic analysis. Prolonged oxygen limitation maintained relatively stable organic matter removal but progressively deteriorated ammonium removal and sludge settleability, while PET addition significantly aggravated these effects. PET exposure markedly increased the absolute abundance of ARGs without substantially altering resistome composition or dominant resistance mechanisms, suggesting an amplification rather than restructuring of the resistome. Correlation analyses indicated that ARGs enrichment was primarily host-associated and driven by the proliferation of a limited number of microbial taxa. Several potential ARG hosts were also strongly associated with PET degradation-related genes, indicating shared microbial populations linking PET-associated functions and antibiotic resistance. In addition, strong positive correlations between ARGs and MGEs suggested an important role of gene mobility in resistome dynamics under oxygen-limited conditions. Overall, these results demonstrate that oxygen limitation combined with PET microplastics promotes host-associated ARG enrichment in wastewater systems, highlighting potential environmental and public health risks and emphasizing the importance of maintaining operational stability to mitigate antibiotic resistance dissemination.