1. Department of Microbiology and Institute for Cellular and Molecular Biology, The University of Texas at Austin, Austin, Texas 787121;
2. Department of Microbiology and Immunology, The University of Texas Medical Branch at Galveston, Galveston, Texas 77555-10192; and
3. Department of Pathology and Laboratory Medicine, Texas A&M University Health Science Center, College Station, Texas 77843-11143