1. Howard Hughes Medical Institute, Departments of Medicine, Biochemistry and Biophysics, and Microbiology, University of California, San Francisco, California 94143
2. Department of Pediatrics, Washington University, St. Louis, Missouri 63110
3. Department of Gastrointestinal Sciences, Christian Medical College, Vellore, India
4. Departments of Molecular Microbiology and Pathology and Immunology, Washington University, St. Louis, Missouri 63110