The potential role of Escherichia coli as an indicator of environmental antimicrobial resistance in an urban river in Japan

Coliforms, including Escherichia coli, are widely used as fecal indicator bacteria (FIB) in water quality assessments. However, the potential of these bacteria to act as reservoirs of antimicrobial resistance (AMR) in urban rivers under wet-weather conditions potentially influenced by combined sewer overflow (CSO) discharge remains insufficiently characterized. Using a One Health perspective, we evaluated temporal and weather-related variation in the concentrations of Escherichia coli and non-E. coli coliforms and in the proportions of colonies exhibiting ampicillin resistance or intermediate tetracycline resistance in the Hirano River, an urban river in Osaka, Japan, between October and November 2023. Sampling was conducted 10 times between October 17 and November 14, 2023, including sampling conducted 33 and 15 h after strong rainfall events classified as having the potential to trigger CSOs. Samples were analyzed using CHROMagar™ ECC medium for bacterial enumeration and to test resistance to ampicillin and tetracycline. Both non-E. coli coliform and E. coli concentrations increased by more than two orders of magnitude in the sample collected 15 h after a strong rainfall event classified as having the potential to trigger a CSO, suggesting possible contributions from CSO-related inputs and/or other fecal contamination sources. E. coli exhibited higher resistance ratios, with averages of 23.1% for ampicillin resistance and 12.3% for intermediate tetracycline resistance, compared with 3.7% and 0.8%, respectively, in the non-E. coli coliform fraction, supporting the potential utility of E. coli as an indicator of antibiotic resistance among culturable coliforms in urban river water. These findings support the potential utility of E. coli for both fecal contamination assessment and environmental AMR surveillance in urban river systems.