1. Channing Laboratory, Department of Medicine, Brigham and Women's Hospital, Harvard Medical School, Boston, Massachusetts 02115-58041;
2. Department of Microbiology, Montana State University, Bozeman, Montana 597172;
3. Department of Microbiology and Immunology, Medical College of Virginia Campus of Virginia Commonwealth University, Richmond, Virginia 23298-06783; and
4. McGuire Veterans Affairs Medical Center, Richmond, Virginia 232494