The Investigation of Antibiotic Residues, Antibiotic Resistance Genes and Antibiotic-resistant Organisms in a Drinking Water Reservoir System in Germany

Between August 2018 and June 2019, a river system in Germany that supplies a drinking water reservoir and is subject to the discharge from two sewage treatment plants was monitored for antibiotic residues via liquid chromatography-tandem mass spectrometry, antibiotic resistance genes (including bla, bla, bla, bla, bla, bla, bla, bla, bla, bla, bla, bla, bla, bla, mcr) via qualitative real-time PCR and antibiotic-resistant bacteria [belonging to the ESKAPE-group (Enterococcus faecium, Staphyhlococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa and Enterobacter ssp.; with resistance against Carbapenemases, Cephalosporines and Colistin) and Escherichia coli] based on cultivation methods followed by a characterization via MALDI-TOF MS and susceptibility testing applying microdilution. Residues of macrolide antibiotics such as clarithromycin (up to 0.60 μg/L) and residues of sulfamethoxazole (up to 0.40 μg/L) and trimethoprim (up to 0.39 μg/L) were detected downstream of the sewage treatment plants. In addition, no antibiotic residues were detected upstream the respective sewage treatment plants, except for anhydroerythromycin (n = 1, <LOQ). In total, bla was found in 6, and mcr in one water sample throughout the whole sample period. No MRSA, extensively drug-resistant or pan-resistant bacteria, according to Magiorakos et al. (2011), could be isolated. In contrast, multidrug-resistant bacteria, especially E. coli and Enterococcus faecium were found. In conclusion, sewage treatment plant effluents are point sources for antibiotic residues (p = 0.0000 and p = 0.0001) and antibiotic-resistant bacteria (p = 0.0021 and p = 0.0060) since their concentrations increased significantly after the discharge of treated wastewater into the investigated rivers. In this specific catchment area, which lacked clinical, hospitals, and intensive livestock farming, no significant bacterial or analyzed chemical (antibiotic residues) influences of the sewage treatment plants were observed in the river downstream of the drinking water reservoir during the study. Furthermore, no increased selection pressure could be expected as no measured antibiotic residues exceeded the predicted no effect concentration for antibiotic resistance selection, according to Bengtsson-Palme and Larsson (2016).