1. R. J. Geraghty and P. G. Spear, Department of Microbiology-Immunology, Northwestern University Medical School, Chicago, IL 60611, USA.
2. C. Krummenacher and G. H. Cohen, School of Dental Medicine and Center for Oral Health Research, University of Pennsylvania, Philadelphia, PA 19104, USA.
3. R. J. Eisenberg, Center for Oral Health Research and School of Veterinary Medicine, University of Pennsylvania, Philadelphia, PA 19104, USA.