Antibiotic-driven shifts in bacterial dynamics of the polyethylene terephthalate and low density polyethylene plastisphere in wastewater treatment systems

Microplastics (MPs) are increasingly recognized as vectors for microorganisms in wastewater treatment plants, although their role in shaping microbial risks under antibiotic stress remains unclear. This study investigated the colonization dynamics of bacterial communities from activated sludge on polyethylene terephthalate (PET) and low-density polyethylene (LD-PE) MPs during a 5-day sludge retention time using 16S rDNA gene high-throughput sequencing and qPCR to study the microbial communities. Microcosm experiments (n = 24) were conducted under control conditions and exposure to ciprofloxacin (CPR, 100 μg·L⁻¹), trimethoprim (TMP, 100 μg·L⁻¹), and in combination (TMPCPR, 50 μg·L⁻¹ each). All MPs were rapidly colonized within 5 days, with antibiotics accelerating early biofilm formation. LD-PE supported faster colonization, reaching 10⁹-10 ¹ ⁰ 16S rDNA gene copies g⁻¹ MP by day 5, consistently one order of magnitude higher than PET. In contrast, PET plastispheres facilitated the enrichment of potential pathogens (Chryseobacterium, Flavobacterium, Clostridium, Candidatus Microthrix), showing a 10-100 × increase in predicted pathogenic functions (1.75-9.51 %) compared to activated sludge (0.09-0.16 %). The TMPCPR mitigated pathogen enrichment relative to single-antibiotic exposures. These findings highlight polymer-specific ecological risks as PET is more prone to pathogen colonization, accumulates in sludge, whereas buoyant LD-PE disperses through effluents carrying dense but less pathogen-enriched biofilms.