Antimicrobial resistance, conjugative plasmids and pathogenicity in wastewater and freshwater <i>Escherichia</i> spp. in Stockholm, Sweden

Abstract

To investigate antibiotic resistance genes (ARGs), conjugative plasmids, and virulence genes in Swedish waterborne Escherichia spp., water samples were collected from urban freshwater and Baltic Sea beaches and a primary wastewater treatment plant (WWTP) in Stockholm, Sweden. During the summer of 2022, 68 isolates were recovered using ESBL-selective and non-selective agar, including 40 from wastewater and 28 from fresh or brackish water. Isolates were characterised by phenotypic antibiotic susceptibility testing, conjugation assays, and whole-genome sequencing. Antibiotic residues were quantified, with higher concentrations detected at WWTP inlets and outlets than in natural water sources. Overall, 28 isolates (41.17%) were phenotypically multidrug-resistant (MDR), and 18 (26.47%) carried >= 3 ARGs. WWTP-derived isolates showed a significantly higher prevalence of extended-spectrum beta-lactamase (ESBL) genes than freshwater isolates (p < 0.0001). Isolates represented diverse multilocus sequence types (MLST), and most harboured >= 1 plasmid. Sixteen strains transferred conjugative plasmids encoding resistance to cefotaxime, tetracycline, streptomycin, and trimethoprim. ESBL genes, including blaCTX-M-15, were located on IncF, IncN, IncB/O/K/Z, and IncI plasmids, with IncF plasmids showing lower transfer frequencies than IncN and IncI1 conjugative plasmids. These findings identify WWTPs as a major source of MDR and pathogenic E. coli, highlighting their role in environmental dissemination.