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Environ Microbiol Rep. 2018 Oct;10(5):576-582. doi: 10.1111/1758-2229.12681. Epub 2018 Sep 18.

Long-term carriage and rapid transmission of extended spectrum beta-lactamase-producing E. coli within a flock of Mallards in the absence of antibiotic selection.

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Department of Medical Biochemistry and Microbiology, Uppsala University, Uppsala, SE - 751 23, Sweden.
Centre for Ecology and Evolution in Microbial Model Systems, School of Natural Sciences, Linnaeus University, Kalmar, SE - 391 82, Sweden.
Department of Infectious Diseases, Kalmar County Hospital, Kalmar, SE - 391 85, Sweden.
Section for Infectious Diseases, Department of Medical Sciences, Uppsala University, Uppsala, SE - 751 85, Sweden.


Wild birds have been suggested as transmitters and reservoirs for antibiotic resistant bacteria. We performed an experimental study investigating carriage time and interindividual transmission of extended spectrum beta-lactamase- (ESBL-)producing Escherichia coli in Mallards (Anas platyrhynchos) to assess if the birds carry the bacteria long enough to transfer them geographically during migration. Mallards were inoculated intraoesophageally with four different strains of ESBL-producing E. coli and kept together in a flock. The ESBL-strains belonged to sequence types previously shown to spread between birds and humans. Culturing from faecal samples showed presence of ESBL-producing E. coli the entire 29 day experimental period. An extensive and rapid transmission of the different ESBL-strains between individuals (including non-inoculated controls) was observed. In necropsy samples, we detected ESBL-strains in the cecum even in faeces-negative birds, indicating that this part of the intestine could function as a reservoir of resistant bacteria. We demonstrate that birds can carry ESBL-producing E. coli for long enough times to travel far during migration and the extensive interindividual transmission suggests spread between individuals in a dense bird population as a mechanism that allow persistence of resistant bacteria.

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