Prevalence of antibiotic-resistant Escherichia coli and resistance genes in wastewater-impacted Sites of the Mahaweli River in Kandy, Sri Lanka

Rivers receiving untreated or poorly treated wastewater act as reservoirs for antibiotic-resistant bacteria (ARB) and antibiotic resistance genes (ARGs), posing serious environmental and public health risks. This study investigated bacterial loads, antibiotic-resistant Escherichia coli, and ARGs in the Mahaweli River in Kandy district, Sri Lanka, at two locations: a) at Gatambe, where the river received discharges from the canal Meda Ela (ME) and Kandy city wastewater treatment plant (WWTP), and b) at Haragama, where the river received the discharge of the Ntional Livestock Development Board (NLDB) farm. Water samples were collected from the river upstream, at discharge points, downstream, and from a buffalo pond inside the farm during two visits in 2024. Bacterial isolation, total coliform counts, and fecal coliform counts were conducted using standard microbiological methods. Susceptibility tests were conducted with E. coli isolates against six antibiotics, i.e., amoxicillin (AMX), tetracycline (TET), co-trimoxazole (COT), ciprofloxacin (CIP), ceftazidime (CAZ), and streptomycin (S), using the Kirby-Bauer disk diffusion method. Polymerase Chain Reaction (PCR) was used to detect ARGs blaCTX-M, tetA, aadA1, dfrA5/14, and qnrB. The highest microbial contamination and prevalence of antibiotic-resistant E. coli were recorded at the ME discharge point, with resistance highest to AMX at 87%, followed by CAZ at 43% and COT at 35%. At WWTP effluent, resistance to AMX and TET was 76% and 67%, respectively. Amoxicillin (50%) and TET (44%) resistance persisted downstream, while CIP resistance declined to 0%. Escherichia coli from the National Livestock Development Board (NLDB) farm discharge exhibited lower resistance, while water from the buffalo pond of the farm showed 50% resistance to AMX and TET, with the tetA gene. Multidrug resistance was 35% at ME and 48% at WWTP discharge points. All tested genes were detected in ME and WWTP discharges, except qnrB at WWTP. Strengthening wastewater treatment and effluent monitoring is essential to curb environmental dissemination and horizontal transfer of ARGs through Mahaweli water, which is an important source of drinking water.