High Prevalence and Temporal Variation of Extended Spectrum β-Lactamase-Producing Bacteria in Urban Swedish Mallards.

Antibiotic resistant bacteria present a growing global healthcare challenge. Previous research demonstrates that wild birds harbor extended spectrum β-lactamase (ESBL)-producing Enterobacteriaceae and may contribute to their dissemination. We aimed to assess prevalence and temporal variation in the detection rate of ESBL-producing bacteria in urban wild birds and to evaluate methods regarding sample handling. Monthly fecal sampling was performed in 2013 at an urban pond in Sweden. ESBL-producing Escherichia coli and Klebsiella pneumoniae were analyzed by polymerase chain reaction targeting blaCTX-M. Subsets of samples were analyzed in multiple replicates and without previous freezing. Pond water samples were screened for 12 antibiotics. Out of 813 fecal samples, 47% grew ESBL-producing E. coli, a higher prevalence than in similar studies. Detection rate varied considerably between months, ranging from 4.2% in May to 84% in July, and was significantly higher during warm months. A majority of isolates harbored CTX-M-15 type ESBL. Detection rates were increased by duplicating samples and by avoiding freezing. No antibiotics were detected in pond water. This study demonstrates high prevalence and a previously undescribed temporal variation in detection rate of ESBL-producing Enterobacteriaceae in wild birds. The distribution of CTX-M genes corresponds well with Swedish human isolates, indicating communication between the genetic pools of ESBLs in humans and wild birds. Urban ponds may serve as important natural reservoirs for antimicrobial resistance.