**6. Acknowledgements**

The authors would like to thank the US National Veterinary Service Laboratories, Animal and Plant Health Inspection Services, US Department of Agriculture, Ames, Iowa, for serotyping. We also thank the *E. coli* reference center (University Park, PA) for the genomic subtyping of the isolates. The authors thank North Dakota State University-Veterinary Diagnostic Laboratory, North Dakota producers, graduate students and postdoctoral researchers for participating in the studies. Funding for these studies was provided by USDA-APHIS (Agrosecurity: Disease Surveillance and Public Health) and USDA-CSREES (Food Safety Risk Assessment) grants.
