1. Food and Feed Safety Research Unit, U.S. Department of Agriculture/Agricultural Research Service (USDA/ARS), College Station, Texas.
2. Livestock Issues Research Unit, USDA/ARS, Lubbock, Texas.
3. Department of Animal and Food Sciences, Texas Tech University, Lubbock, Texas.
4. Present address: Iowa State University, Des Moines, Iowa.
5. Microbial Food Safety Research Unit, USDA/ARS, Wyndmoor, Pennsylvania.
6. National Pork Board, Des Moines, Iowa.