PET Micro/Nanoplastic–Tetracycline Co-Exposure in Defibrinated Blood: Exploratory Spectroscopic, Redox, and Escherichia coli Responses

Previous studies have indicated that polyethylene terephthalate (PET) micro/nanoplastics (MNPs) may coexist with antibiotics in environmental and biological matrices; however, their combined behavior in blood remains insufficiently characterized. This study examined PET MNPs prepared from water bottles at 0.5, 2.5, and 7.5 mg/mL together with tetracycline (2–50 µg/mL) in defibrinated horse blood. After 24 h exposure and particle removal, UV–visible absorbance, intrinsic fluorescence, redox indicators, and subsequent Escherichia coli responses were evaluated. The blood biochemical results showed condition-dependent changes in hemoglobin-associated absorbance, tryptophan-dominated fluorescence, reactive oxygen species, reduced glutathione, superoxide dismutase, and lipid peroxidation. When treated blood supernatants were applied to Escherichia coli, tetracycline alone reduced bacterial OD600, whereas selected PET MNP–tetracycline co-exposures partially restored bacterial proliferation and modified oxidative-stress responses. ATR–FTIR analysis of Escherichia coli pellets showed dose-dependent modulation of phosphate-, lipid-, and protein-associated bands, indicating changes in bacterial biochemical fingerprints under specific exposure combinations. Overall, the findings suggest that PET MNPs can modify tetracycline-associated spectral, redox, and bacterial response patterns in a blood matrix. Future studies incorporating adsorption assays, free tetracycline quantification, protein-corona profiling, time-course exposure designs, and antibiotic susceptibility testing would further clarify the mechanistic basis and biological relevance of these matrix-dependent interaction effects.