Co-occurrence of Antibiotics, Antibiotic Resistance Genes, and MultidrugResistant Bacteria in a Himalayan Foothill Urban River, India

Abstract

Urban tributaries are increasingly recognized as important conduits for the dissemination of antibiotic residues and antibiotic resistance determinants into downstream ecosystems. The Bindal River, which traverses Dehradun (Uttarakhand, India), receives untreated municipal wastewater, effluent from healthcare facilities, and diffuse urban runoff, creating conditions that favor the introduction and persistence of multiple antibiotic classes. The objective of this study was to conduct cross-sectional monitoring at six sites (B1–B6) along a downstream gradient to quantify representative antibiotics (ciprofloxacin, tetracycline, sulfamethoxazole, azithromycin, ampicillin) using validated SPE-UHPLC-MS/MS methods, to quantify antibioticresistance genes (ARGs; sul1, tetA, qnrS, intI1) via quantitative PCR, and to assess culturebased resistance patterns in heterotrophic bacterial isolates. Measured concentrations indicated a pronounced downstream increase for fluoroquinolones and tetracyclines, with ciprofloxacin reaching concentrations up to 1.57 µg L⁻¹ at the most impacted site (B6). ARG abundances and multidrug-resistant (MDR) isolate frequencies mirrored these chemical gradients. Multivariate ordination (RDA) linked antibiotic concentrations, conductivity, and nutrient loads to ARG profiles, with integron abundance (intI1) emerging as a key correlate of MDR prevalence. Our findings demonstrate the co-occurrence of antibiotics and resistance determinants in an urban Himalayan tributary and underscore the need for integrated wastewater management, hospital effluent control, and routine chemical–molecular surveillance to mitigate environmental AMR risks.