1. 1Division of Bacterial and Mycotic Diseases, National Center for Infectious Diseases, Centers for Disease Control and Prevention, Atlanta, Georgia 30333, USA
2. 2Food Safety and Inspection Service, U.S. Department of Agriculture, Washington, D.C., USA
3. 3Minnesota Department of Health, Minneapolis, Minnesota 55440, USA
4. 4Center for Food Safety and Applied Nutrition, U.S. Food and Drug Administration, Washington, D.C., USA