Occurrence of antibiotics and antibiotic-resistant bacteria (ARB) in the Nervión river

The main hydrometeorological, microbiological and physico-chemical characteristics of the Nervión river were monitored during a year, including 10 antibiotics and the presence of bacteria resistant to these antibiotics among faecal coliforms (FC) and total aerobic bacteria at 22 °C (TAB₂₂). The characteristics of the river water were variable without a clear seasonal component, strongly influenced by rainfall, with a good quality for drinking water production throughout the year according to the physico-chemical parameters. The antibiotic resistant bacteria isolated from the water of the Nervión river were especially resistant to β-lactams and macrolide antibiotics, highlighting the absence of resistance to derivatives of tetracyclines among strains of TAB₂₂. A third of the isolated strains were multi-resistant to antibiotics with a seasonal component in its presence, with multi-resistant FC more abundant during summer and multi-resistant TAB₂₂ more abundant during winter. The presence of antibiotics in the waters of the Nervión river was not very significant, with total absence of β-lactams, minocycline and ciprofloxacin. Erythromycin and clarithromycin can be considered ubiquitous with mean concentrations of 2.5 ± 2.3 ngL^-1 and 5.7 ± 4.6 ngL^-1 respectively, and the presence of sulfamethoxazole and trimethoprim was also noticeable with maximum concentrations of 78.3 ngL^-1 for sulfamethoxazole. Dilution due to the increase of rainfall was observed for several analysed antibiotics, but without significant seasonal differences.