Antimicrobial Resistance in Chennai's Wastewater Treatment Plants: A Multi-Scale Assessment of Antibiotic-Resistant Bacteria and Genes Dissemination

Antimicrobial resistance (AMR) is an emerging threat to public and environmental health, with wastewater increasingly recognized as a significant pathway for the spread of resistance genes. While international AMR surveillance efforts are expanding, there are still limited targeted evaluations of antibiotic resistance genes (ARGs) within India’s complex urban wastewater treatment plants (WWTPs). This study monitored ten major ARGs in both influent and effluent samples from five WWTPs in Chennai: two municipal (WWTP A and B), a small community (WWTP C), a pharmaceutical (WWTP D), and a hospital plant (WWTP E). The integrated monitoring approach encompassed water quality monitoring, microbial enumeration, antibiotic susceptibility testing, biochemical characterization, and qPCR. Advanced WWTPs (WWTP C, E) achieved high organics removal efficiency (up to 87.9% (COD) and 79% (TOC)), yet bacterial loads and ARGs persisted in effluents. Pharmaceutical and hospital influents showed the highest bacterial counts and ARG concentrations in log copies/mL: int1 (8.5), sul2 (8.2), and blaNDM (4.6), with incomplete reductions post-treatment. Notably, WWTP E showed negative removal for bla _CTX-M (− 8 %). Multivariate analyses indicated no significant correlation between pollutant removal and ARG reduction, suggesting distinct removal mechanisms. Antibiotic susceptibility profiling of 98 E. oli isolates revealed high multidrug resistance, particularly from pharmaceutical, and hospital sources. Findings of this study highlight the the limitations of conventional WWTPs in mitigating AMR dissemination and highlight the need for advanced treatment interventions. Integration of AMR surveillance into environmental monitoring frameworks is essential to support One Health-based strategies and safeguard ecosystem and public health.