Deciphering the role of polyethylene microplastics on antibiotic resistance genes and mobile genetic elements fate in sludge thermophilic anaerobic digestion process

The retention of microplastics in sewage sludge, a reservoir of antibiotic resistance genes (ARGs) and mobile genetic elements (MGEs), has attracted increasing attention. However, the impact of polyethylene microplastics (PE-MP) on the fate of ARGs and MGEs is far from fully understood in sewage sludge's thermophilic anaerobic digestion process (AD). The present study demonstrated that PE-MP with the size of 40–48 µm at concentrations of 50, 100, and 200 particles/g-TS could enlarge total ARGs abundance. Meanwhile, a significant positive (r = 0.96, p < 0.05) correlation was observed between PE-MP concentration and total MGEs content. Microbial analysis revealed the enrichment of some functional genus under the stress of PE-MP, which might be the potential host of ARGs and MGEs. The relative abundance of acetogens Thermoanaerobacter increased from 27.60 % to 32.80 % and hydrolytic bacteria Caldicoprobacter enriched by 28.03 % when PE-MP content increased from zero to 200 particles/g-TS. The potential hosts were subsequently identified through co-occurrence networks, some of which were identified as pathogens. Strong positive correlations between PE-MP, ARGs, and MGEs were observed using redundancy analysis and correlation analysis. Additionally, the structural equation model indicated the direct and indirect influence of PE-MP on the abundance of ARGs and MGEs. MGEs could enhance the acquisition and dissemination of ARGs directly. Taking together, our findings deciphered the contribution of PE-MP to the spread of ARGs and MGEs in the sludge AD process, which might pose risks to human health and should be considered during sewage sludge disposal.