1. Department of Microbiology; Roy J. and Lucille A. Carver College of Medicine; University of Iowa; Iowa City IA 52242 USA
2. Department of Microbiology; New York University School of Medicine; New York NY 10016 USA
3. Department of Pathology & Microbiology; University of Nebraska Medical Center; Omaha NE USA
4. Department of Microbiology and Immunology; University of Rochester; Rochester NY USA