Reusable immobilised quaternary ammonium particles reduce microbial and resistome burdens without promoting resistance selection during wastewater post-treatment

Abstract Antibiotic residues, antibiotic-resistant bacteria and antimicrobial resistance genes (ARGs) converge in wastewater treatment plants, yet conventional processes are not designed to mitigate resistance dissemination from their effluents. While chemical disinfectants are effective, they can generate subinhibitory exposure gradients that promote resistance selection and co-selection after release into waters. Here, we evaluate a contact-restricted alternative: benzyldimethyldodecyl ammonium chloride (BDMDAC) immobilised microparticles as a reusable post-treatment strategy. Across single-strain assays, treated wastewater exposure and experimental community evolution, immobilised BDMDAC-functionalised particles achieved concentration-dependent antimicrobial activity without detectable leaching. Optimal exposure resulted in a ~5.5 log reduction in bacterial abundance and removal of clinically relevant ARGs. Antimicrobial efficacy was retained after reuse, supporting operational stability. Plasmid-borne QAC ARGs did not confer protection, and no enrichment of QAC-associated or non-QAC ARGs was observed. Conjugation assays demonstrated suppression of horizontal gene transfer even under suboptimal exposure, and mobility-associated markers remained stable or declined during long-term community incubation. Collectively, data support a contact-restricted mechanism in which antimicrobial pressure is confined to the particle interface, generating high local lethality while limiting diffuse subinhibitory exposure. This spatial confinement decouples antimicrobial efficacy from resistance selection and mobility amplification, providing a distinct and evolution-conscious framework for wastewater polishing technologies.