Exposure to PET micro- and nanoplastics induces size-dependent interactions and microbial biotransformation in a...

Dietary exposure to micro- and nanoplastics (MNPs) is increasingly recognized, yet their gastrointestinal fate and size-dependent interactions with the gut microbiota remain poorly understood. Poly(ethylene terephthalate) (PET), widely present in food systems and human stool, represents a relevant exposure model. Here, we investigated microbiota responses to a broad spectrum of well-characterized PET particles under dietary-relevant conditions. PET microplastics (160 μm), micro-nanoplastics (50-2000 nm), and nanoplastics (89.9 nm) were subjected to in vitro gastrointestinal digestion (∼3.2 mg/intake), followed by colonic fermentation using fecal microbiota from three healthy donors. Structural changes were assessed by FESEM and micro-Raman spectroscopy. Microbial composition (qPCR, 16S rRNA gene sequencing), functional responses, and fermentative metabolites (SCFAs and ammonium) were analysed. A targeted UHPLC-MS/MS workflow was optimized to detect PET-derived intermediates in complex fecal matrices, enabling the characterization of microbial transformation processes in the gut. Gastrointestinal digestion modified PET surface properties, promoting microbiota-particle interactions. Particle size and microbiota variability jointly shaped microbial responses, including increases in pathobiont-related genera and reductions in beneficial taxa, together with alterations in SCFA and ammonium production, with nanoplastics showing distinct functional responses. Furthermore, functional predictions combined with the detection of key degradation intermediates, including mono(2-hydroxyethyl) terephthalate (MHET) and terephthalic acid (TPA), provided convergent evidence of early PET biotransformation, particularly for smaller-sized particles. Overall, PET particles interact with the gut microbiota both as modulators of microbial composition and function, and as substrates for microbial biotransformation, highlighting particle size and microbiota-driven processes as key determinants of the biological impact of MNP exposure.