Mallard ducks - a waterfowl species with high risk of distributing Escherichia coli pathogenic for humans.

Mallard ducks may pose a so far underestimated risk to human and animal health by transmitting pathogenic Escherichia coli via their faecal deposits to various environmental sources. We processed Mallard duck faecal samples for E. coli and unique clones, as defined by pulsed-field gel electrophoresis (PFGE), were subsequently investigated for their virulence genotype and phylogenetic background. Multilocus sequence typing and in vivo experiments were performed for selected clones. Of 60 clones identified among 142 E. coli isolated from 175 samples, 15 (25%) were recovered from multiple individuals (up to 23 per clone). None of the clones harboured stx1 and stx2 genes and other intestinal pathogenic E. coli virulence-associated genes were only occasionally identified in the collection. In contrast, the clones possessed considerable numbers of VAGs (up to 30) linked with extraintestinal pathogenic E. coli (ExPEC). Their pathogenic potential was confirmed in chicken infection experiments. Moreover, selected clones were assigned to sequence types (STs) being most prominent for human ExPEC strains, including ST95 and ST73. One clone exhibited a multi-resistant phenotype against several antibiotics including beta-lactams, tetracyclines and sulfonamides. Mallard ducks have therefore to be considered as an important reservoir for zoonotic E. coli strains, thus serving as a substantial non-point source especially of strains capable of causing extraintestinal diseases.