A Comprehensive Screening of Escherichia coli Isolates from Scandinavia's Largest Sewage Treatment Plant Indicates No Selection for Antibiotic Resistance

There is concern that sewage treatment plants (STPs) serve as hotspots for emergence and selection of antibiotic resistant bacteria. However, field studies investigating resistance selection by comparing bacterial populations in influents and effluents have produced variable and sometimes contradictive results. Also, large taxonomic changes between influents and effluents make interpretation of studies measuring relative gene abundances ambiguous. The aim here was to investigate whether within-species selection occurs by conducting a comprehensive screening of Escherichia coli isolated from composite influent and effluent samples collected at Scandinavia's largest STP, accompanied by analyses of antibiotics residues. In total, 4028 isolates, collected on eight occasions during 18 months, were screened for resistance to seven antibiotics. Although differences in proportions of resistant E. coli between influent and effluent samples were detected for a few antibiotics on two occasions, aggregated data over time showed no such differences for any of the investigated antibiotics. Neither was there any enrichment of multiresistant or extended-spectrum beta-lactamase-producing isolates through the treatment process. Despite some antibiotics were detected at or close to concentrations predicted to provide some selective pressure, field observations of resistance profiles in E. coli do not provide support for systematic selection in the investigated STP.