Occurrence of E. coli and antibiotic-resistant E. coli in the southern watershed of Lake Biwa, including in wastewater treatment plant effluent and inflow rivers.

The emergence of antibiotic-resistant bacteria (ARB) and their antibiotic resistance genes (ARGs) poses a serious challenge to human, animal, and environmental health worldwide. ARB can spread into the environment via various sources and routes. In this study, we investigated the occurrence of antibiotic-resistant E. coli in the southern watershed of Lake Biwa. Two-year monitoring of antibiotic-resistant E. coli was carried out in the southern part of Lake Biwa and inflow rivers and at three WWTPs around the southern part of the lake. Concentrations of E. coli in waters that are resistant to ampicillin (AMP), cefotaxime (CTX), ceftazidime (CAZ), levofloxacin (LVFX), tetracycline (TC), and amikacin (AMK) were measured using the culture method. Of these antibiotic-resistant E. coli, AMP-resistant E. coli were found at the highest prevalence, followed by LVFX, CTX, CAZ, TC, and AMK-resistant in both the influent and effluent of WWTPs. These resistance patterns in wastewater are the same as those in clinical samples in Japan. The numbers of antibiotic-resistant E. coli decreased by around a factor of 1000 during the wastewater treatment processes, but the rates clearly increased, suggesting that selection for antibiotic resistance might occur during the wastewater treatment process. AMP-resistant and TC-resistant E. coli were also detected in Lake Biwa and inflow rivers, which suggests that antibiotic resistance might come from not only WWTPs but also livestock farms and small-scale wastewater treatment facilities located in the river catchment.