Seasonal Impacts on Antibiotic Resistance in Oregon Wastewater Treatment Plants

With the overuse and disposal of antibiotics in hospitals, agriculture, and residentials, wastewater treatment plants are considered one of the main sources and reservoirs of antimicrobial resistance (AMR). To understand proliferation and persistence trends of antibiotic resistance in wastewater treatment plants, this study investigated the prevalence of eight different phenotypes of AMR and multi-drug resistance E. coli in 17 wastewater treatment plants across Oregon in winter and summer of 2019 and 2020. Overall, 1187 E. coli colonies were isolated from influent, secondary effluent, final effluent, and biosolids. 31.8% of all collected isolates exhibited resistance to at least one antibiotic, and 8.7% carried multi-drug resistance phenotypes, of which 16.2% harbored resistance to more than five classes of antibiotics. From a total 246 sampling incidences, 23 demonstrated multiple antibiotic resistance index above 0.2, suggesting high risk of antibiotic contamination. Significantly higher multiple antibiotic resistance index and prevalence of ciprofloxacin-resistant E. coli was observed in biosolids. Highest AMR rates amongst collected E. coli isolates were observed for ampicillin (18.4%) followed by streptomycin (13.8%) and tetracycline (13.6%). Additionally, resistance to trimethoprim-sulfamethoxazole showed significant associations with the occurrence of ampicillin, tetracycline, and streptomycin phenotypes (p Ë‚ 0.01). When comparing different phenotypes of AMR between winter and summer in the influent, significantly higher prevalence of ciprofloxacin-resistant E. coli was observed in summer (p Ë‚ 0.01), suggesting the seasonality of different phenotypes. Findings suggest that the removal efficiency of AMR E. coli in wastewater treatment plants is likely influenced by seasonal variations.