Intestinal Microplastic Retention Reshapes Gut Microbial Ecology through Surface-Associated Colonization and Additive Leaching

Microplastics (MPs) have attracted increasing attention due to their potential impacts on the human gut microbiota, yet the mechanisms governing MP-microbiota interactions remain insufficiently resolved. Here, we employed the Simulator of the Human Intestinal Microbial Ecosystem, a dynamic host-free <i>in vitro</i> model, to investigate how intestinally retained poly(ethylene terephthalate) MPs influence gut microbial communities. We show that MP retention is associated with two separable processes: surface-mediated spatial redistribution of microbes and additive-associated perturbations. Compared with additive-eluted MPs and inert SiO₂ particles, MPs exhibited greater surface roughness and hydrophobicity, promoting selective colonization by hydrophobic, potentially pathogenic, and organic-degrading taxa. This surface-associated colonization coincided with the displacement of luminal keystone taxa and pronounced restructuring of microbial co-occurrence networks, reflected by reduced negative cohesion and altered community stability. In parallel, leachable MP-associated additives independently shifted microbial composition and predicted functional potential, including enrichment of pathways related to xenobiotic degradation. This work provides mechanistic insight into how retained MPs may condition gut microbial ecosystems and underscores the importance of considering MP-associated microbial perturbations in gut-relevant exposure assessments.