1. B. J. Hinnebusch and T. G. Schwan, Laboratory of Microbial Structure and Function, Rocky Mountain Laboratories, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Hamilton, MT 59840, USA.
2. R. D. Perry, Department of Microbiology and Immunology, University of Kentucky, Lexington, KY 40536-0084, USA.