Aging of microplastics weakens chlorination efficiency and enhances persistence of antibiotic resistance determinants

The increasing prevalence of antibiotic-resistant bacteria (ARB) and resistance genes (ARGs) in wastewater poses a major public health challenge, particularly as conventional disinfection methods are often insufficient to fully eliminate resistant determinants. Simultaneously, microplastics (MPs) are emerging as persistent pollutants in aquatic systems, where they interact with microorganisms and may interfere with treatment processes. This study investigates the role of microplastic aging in modulating chlorination efficiency against ARB and ARGs in hospital wastewater. Polyethylene microplastics were subjected to controlled UV aging (0-15 days) and characterized using FTIR and AFM, revealing increased oxidation, surface roughness, and biofilm formation capacity after aging. Chlorination experiments showed that while pristine MPs only modestly reduced chlorine efficacy, aged MPs significantly impaired disinfection, resulted in more than 2 log10 higher bacterial survival following chlorination and 12-25-fold increases in ARG persistence. Moreover, biofilm assays demonstrated a nearly three-fold enhancement of microbial colonization on aged MPs compared with fresh ones. These findings highlight that environmental weathering transforms MPs into highly interactive substrates that shield bacteria, enhance gene persistence, and compromise conventional chlorination. The study underscores the importance of considering MP aging in water treatment risk assessments and suggests the need for improved strategies to mitigate the combined threats of microplastic pollution and antimicrobial resistance. Because elevated microplastic concentrations were employed to facilitate mechanistic evaluation under controlled laboratory conditions, the findings should be interpreted primarily as proof-of-concept evidence rather than a direct representation of environmental exposure scenarios.