1. Department of Bacteriology, University of Wisconsin‐Madison, Madison, Wisconsin 53706, USA.
2. Departments of Stomatology and Microbiology and Immunology, 513 Parnassus Avenue, Box 0512, University of California, San Francisco, California 94143, USA.
3. Department of Biochemistry and Biophysics, University of California, San Francisco, California 94143, USA.