1. Departmentof Microbiology and Biomedical Genomics Center, University ofMinnesota, St. Paul, Minnesota 55108
2. National Animal DiseaseCenter, U.S. Department of Agriculture, Ames, Iowa50010
3. Food Animal Health ResearchProgram, Ohio Agricultural Research and Development Center,and Department of Veterinary Preventive Medicine, The OhioState University, Wooster, Ohio44691