1. From the Departments of Medicine and of Microbiology and Immunology, and the Digestive Disease Center, Stanford University, Stanford, Calif. (D.A.R., S.F.); the Department of Microbiology, Miami University, Oxford, Ohio (T.M.S.); and the Gastroenterology Division, Department of Medicine, University of Pennsylvania, Philadelphia (R.P.M.). Address reprint requests to Dr. Relman at the Palo Alto Department of Veterans Affairs Medical Center, 154T, 3801 Miranda Ave., Palo Alto, CA 94304.