Persistence of Antibiotic-resistant Genes in Chlorinated Drinking Water: Evidence from Mahaweli River Water Treatment Plants, Sri Lanka

Abstract

Background: The presence of antibiotic-resistance genes (ARGs) has been reported in river water sources. Water treatment plants (WTPs) primarily focus on removing coliforms before distributing chlorinated water for human consumption. However, data regarding the presence of ARGs in chlorinated water in Sri Lanka are scarce. Objective: To investigate the presence of ARGs in chlorinated drinking water treated by WTPs located along the Mahaweli river Methods: Chlorinated water samples (each 100 mL) were collected from 15 WTPs situated along the Mahaweli river between the Kotmale and Victoria reservoirs. The samples were filtered using 0.22 μm membrane filters to separate bacteria. The retained bacteria were cultured on nutrient agar plates. The resulting colonies were pooled and underwent DNA extraction. Conventional PCR was employed with gene-specific primers targeting resistance to five major classes of antibiotics: β-lactams (bla-TEM), aminoglycosides (aadA1), sulfonamides (sul1, sul2), tetracyclines (tetM), quinolones (qnrS) and trimethoprim (dfrA14). PCR amplified products were analyzed for the presence of the expected genes via agarose gel electrophoresis. Results: The most frequently detected gene was bla-TEM (45.2%), followed by aadA1 (38.1%). Sul-1 and qnrS were found in 28.6% of the samples. Sul-2 was present in 14.3% of the samples, while tet-M was detected in 23.8%. The gene dfrA14 was found in 16.7% of the samples. The tested ARGs were highly abundant at the WTPs located near heavily polluted catchments, specifically Paradeka (62%), Balagolla (57%), Ulapane (48%), and Greater Kandy (Katugastota) (48%). Except for the Thalawakelle WTP, where less contaminated raw water from a forested catchment was used, all other WTPs had at least one tested ARG in their chlorinated water. Conclusion: Our results revealed the presence of various ARGs in chlorinated drinking water from WTPs along the Mahaweli river, showing higher abundances associated with the elevated pollution status of the river catchment.