Co-exposure to microplastics and antibiotics: Effect on susceptibility profiles and growth trends in E. coli and P. aeruginosa

Abstract The widespread presence of pharmaceutical residues and microplastics in the environment has raised concerns about their combined effects on ecosystems and human health. This study investigated the adsorption of three antibiotics, ciprofloxacin, oxytetracycline, and gentamicin onto polyvinyl chloride (PVC) microplastics, and how this interaction influences the growth, drug susceptibility, and biofilm formation of Escherichia coli and Pseudomonas aeruginosa . Sorption experiments were carried out under varying antibiotic concentrations (4–12 µg/mL), contact times (4–24 h), pH levels (4, 7, 10), and temperatures (10–40°C). Oxytetracycline and ciprofloxacin showed temperature- and pH-dependent sorption, with maximum adsorption at 40°C and pH 7. Gentamicin exhibited negligible sorption due to the absence of UV-active groups. FT-IR analysis indicated non-covalent binding, including hydrogen bonding and electrostatic interactions, between antibiotics and PVC surfaces. Biological assays revealed that PVC microplastics reduced antibiotic efficacy, notably increasing the minimum inhibitory concentration (MIC) of gentamicin under coexposure. Furthermore, microplastics correlated with higher optical density (OD600nm) and enhanced biofilm staining, particularly at sub-inhibitory antibiotic levels, suggesting that microplastics act as both physical anchors and chemical reservoirs for bacterial colonization. These findings underscore the need to incorporate co-contaminant interactions into environmental risk assessments and public health policies, as microplastics can compromise antibiotic activity and alter phenotypic susceptibility without directly addressing genetic resistance mechanisms.