Evaluating the role of microplastics and wastewater in shaping Vibrio spp. and antibiotic resistance gene abundance in urban freshwaters.

Urban rivers in Sub-Saharan Africa are increasingly affected by microbial pathogens, antibiotic resistance genes (ARGs), and microplastic (MP) pollution. This study assessed the abundance of Vibrio spp. and their tetracycline gene (tetB) relative to the total bacterial community in water and MP biofilms collected from ten sites along the Swartkops and Kat rivers, Eastern Cape, South Africa. The aim is to explore whether MPs disproportionately associate with Vibrio spp. or tetB, relative to the surrounding water, by comparing gene abundance normalised to total bacterial 16 S rRNA. The physicochemical water quality was assessed, and quantitative PCR targeting the Vibrio-specific 16 S rRNA gene and tetB gene was performed. Mean relative abundance of Vibrio-specific 16 S rRNA gene was 0.18 copies per 16 S rRNA in water and 0.13 in MP biofilms, while tetB abundance was 1.78 × 10⁻⁵ and 5.32 × 10⁻⁵ copies per 16 S rRNA in water and MP biofilms. There was no statistically significant difference between water and MP biofilms. Vibrio 16 S rRNA and tetB gene abundance did not significantly correlate with MP concentrations. Higher tetB values downstream of wastewater treatment works (WWTWs) and in industrial zones suggest site-specific environmental influences, but overall patterns pointed to WWTW discharge, not MP, as the dominant driver. tetB abundance correlated with dissolved oxygen and turbidity, suggesting conditions favouring ARG persistence. A strong correlation between Vibrio and tetB suggests shared sources or co-occurrence. MPs may not be active reservoirs or persistence enhancers for Vibrio spp. and tetB in this setting. This study underscores the importance of improving wastewater infrastructure to mitigate ARG-associated health risks.