Microplastics May Not Proliferate Antibiotic Resistance during Mainstream Anaerobic Treatment

Microplastics and antimicrobial resistance are pressing environmental challenges requiring immediate action. Biofilms on the surface of microplastics, known as the plastisphere, may play a role in the proliferation of antibiotic resistance genes (ARGs). Microplastics can adsorb contaminants, such as antibiotics via hydrophilic interactions, exposing nearby microorganisms to elevated selective pressures. This study aimed to compare ARG levels in distinct microbial environments found in anaerobic membrane bioreactors (AnMBRs) across increasing microplastic exposure (i.e., 1, 2, and 5 g/L) while treating synthetic domestic wastewater supplemented with 50 μg/L antibiotics (sulfamethoxazole, ampicillin, erythromycin, and colistin). Novel duplex-digital droplet PCR (ddPCR) assays were developed to quantify ARGs conferring resistance to the spiked antibiotics. Microbial colonization of the plastisphere was observed through scanning electron microscopy. Fourier transform infrared spectroscopy confirmed a decreased intensity of chemical spectra over time, indicating weathering of polyethylene. Across the biomass, membrane biofilm, plastisphere, and effluent, a pronounced increase in the number of ARGs was not observed relative to the control AnMBR without polyethylene. DNA- and RNA-based amplicon sequencing showed that biomass, membrane biofilm, and plastisphere microbial communities were similar and were largely unaffected by long-term exposure to polyethylene. However, the effluent microbial community was remarkably different and was significantly impacted by polyethylene addition.