1. Departments of Microbiology-Immunology, Feinberg School of Medicine, Northwestern University, Chicago, IL 60611
2. Department of Microbiology and Molecular Genetics, Harvard Medical School, Boston, MA 02115
3. Department of Veterinary Pathology, Division of Laboratory Animal Resources, Duke University Medical Center, Durham, NC 27708
4. Pathology, Feinberg School of Medicine, Northwestern University, Chicago, IL 60611
5. Unité de Pathogénie Microbienne Moléculaire, Institut Pasteur, 75724 Paris Cedex 15, France