1. Department of Microbiology and Immunology, University of British Columbia, 2350 Health Sciences Mall, Vancouver, British Columbia V6T 1Z3, Canada, Department of Chemical Engineering, University of California, 5531 Boelter Hall, 420 Westwood Plaza, Los Angeles, California 90095, Department of Microbiology, University of Iowa, Iowa City, Iowa 52242, and Department of Microbiology, University of Washington, Box 357242, Seattle, Washington 98195-7242