Surveillance of antimicrobial resistance among Escherichia coli in wastewater in Stockholm during 1 year: does it reflect the resistance trends in the society?

The resistance patterns of Escherichia coli in untreated (raw) urban wastewater (UW) was monitored by repeated sampling during 1 year. Comparison with data from wastewater samples collected from hospital wastewater (HW) in the same urban area was made. A total of 1326 E. coli isolates from 17 UW samples and 451 isolates from six HW samples were analysed by typing using the PhenePlate™ system, and their susceptibility towards 10 antibiotics was determined. Resistance to at least one antibiotic was observed in 34% of the UW isolates and 55% of the HW isolates. For UW isolates, phenotypic diversity was lower among antibiotic-susceptible than among antibiotic-resistant isolates, indicating a higher presence of clonal groups among susceptible isolates. Total antibiotic resistance measured as the Multiple Antibiotic Resistance (MAR) index was 0.08 for UW compared with 0.19 for HW, and increased over time for UW isolates, indicating increasing resistance among E. coli in the urban population during the studied time period. Resistance to all included β-lactam antibiotics was detected in 2.4% of UW isolates and 14.0% of HW isolates, and 73/75 (97%) analysed isolates were confirmed to be extended-spectrum β-lactamase (including plasmid-mediated AmpC)-producing E. coli. Thus, by cultivating samples from wastewater and analysing many independent isolates per sample, increasing frequencies of antibiotic resistance in UW were detected during 1 year that may reflect increasing faecal carriage of resistant bacteria in the society. Surveillance of antibiotic resistance in wastewater could be a valuable tool for screening of resistance trends on a population level.