Comparison of antimicrobial-resistant <i>E. coli</i> and other coliforms from the upstream to estuary sections in three rivers with different basin characteristics

The presence of antibiotic resistant bacteria in three rivers with different basin characteristics focusing in particular on ampicillin (AMP)-resistant and extended-spectrum β-lactamase (ESBL)-producing bacteria were investigated. Antimicrobial-resistant <i>Escherichia coli</i> and other coliform groups were isolated from the upstream, midstream, downstream, and estuary sections of each river. Analyses of bacterial counts as well as of antimicrobial resistance profiles and ESBL-related gene groups/types were performed and the results were compared among river basins. In the Kaeda River, where approximately 90% of the catchment area is covered by forests, AMP-resistant <i>E. coli</i> had a low prevalence of only 1-2% from the midstream to the estuary. However, the prevalence of other coliforms showing AMP resistance in the Kaeda River basin was between 10 and 19%, which was 2-5 times higher than the prevalence in the upstream and midstream sections of the Yae River, where the abundance of other coliforms was still significantly high. The main ESBL-producing coliforms were <i>Enterobacter bugandensis, Aeromonas hydrophila</i>, and <i>Stenotrophomonas maltophilia</i>. To protect public health, it is essential to monitor the sources and fates of coliform species throughout entire watersheds, from headwaters to downstream reaches including urban areas, as they are of comparable importance to <i>E. coli</i>.