Environmental surveillance of antibiotic-resistant Escherichia coli in Sri Lankan inland waters: Detection of resistance genes and Shiga toxin-producing strains.

The global rise in antimicrobial resistance (AMR) among environmental bacteria poses an escalating threat to public health, particularly in developing regions where antibiotic use in agriculture, aquaculture, and clinical settings remains poorly regulated. This study characterized antibiotic resistance patterns, virulence traits, and antibiotic resistance genes (ARGs) of Escherichia coli isolated from 24 major inland water bodies across Sri Lanka. A total of 120 isolates were recovered and subjected to susceptibility testing against seven clinically and agriculturally relevant antibiotics: amoxicillin (AMX), cloxacillin (CLOX), tetracycline (TET), oxytetracycline (OTC), piperacillin (PIP), cefotaxime (CTX), and ceftazidime (CAZ). Isolates were confirmed using standard biochemical tests and Gram staining, and minimum inhibitory concentrations (MICs) were determined via the agar dilution method (60 to 360 µg/mL). Multiple antibiotic resistance (MAR) indices were calculated to assess prior antibiotic exposure at each sampling site. PCR assays detected resistance genes (tetA, tetM) and virulence genes (eae, stx1, stx2). Widespread resistance was observed against AMX (100%) and CLOX (100%), while most isolates remained susceptible to the third-generation cephalosporins CTX and CAZ, suggesting the absence of extended-spectrum β-lactamase activity. MAR indices exceeded 0.2 across all sites, indicating pervasive prior antibiotic exposure. Resistance genes tetA (8.3%) and tetM (6.0%) were detected at moderate prevalence, consistent with widespread TET use in agricultural and veterinary contexts. Virulence genes eae and stx2 were absent; nevertheless, stx1 was detected in 2.0% of isolates from Minneriya Canal and Mahakandarawa Tank, indicating the presence of Shiga toxin-producing E. coli (STEC) with pathogenic potential. These findings demonstrate that Sri Lankan inland water bodies serve as critical reservoirs for ARG dissemination and potentially pathogenic strains, posing direct risks to communities dependent on these waters for domestic use, irrigation, and food production. Furthermore, this study provides a replicable methodological framework for future AMR surveillance studies in environmental and freshwater contexts. Systematic AMR surveillance, enhanced wastewater management, and targeted public health interventions are urgently needed, in alignment with One Health priorities and the United Nations Sustainable Development Goals.