1. 1Division of Infection and Immunity, The Roslin Institute and Royal (Dick) School of Veterinary Studies, University of Edinburgh, Easter Bush EH25 9RG, UK
2. 2Gastrointestinal Bacterial Reference Unit, 61 Colindale Avenue, Public Health England, London NW9 5EQ, UK
3. 3U.S. Meat Animal Research Center, Agricultural Research Service, U.S. Department of Agriculture, Clay Center, NE 68933-0166, USA