Department of Medicine, Division of Infectious Diseases, Washington University School of Medicine, St. Louis, Missouri, USA. Department of International Health, Johns Hopkins Bloomberg School of Public Health, Baltimore, Maryland, USA. Department of Medicine, Division of Gastroenterology, Johns Hopkins University School of Medicine, Baltimore, Maryland, USA. Center for Cancer Research, Chemical Biology Laboratory, National Cancer Institute, Fredrick, Maryland, USA. Department of Medicine, Division of Gastroenterology, Washington University School of Medicine, St. Louis, Missouri, USA. Enteric Disease Department, Infectious Disease Directorate, Naval Medical Research Center, Silver Spring, Maryland, USA. Molecular Microbiology and Microbial Pathogenesis Program, Division of Biology and Biomedical Sciences, Washington University School of Medicine, St. Louis, Missouri, USA. Medicine Service, Veterans Affairs Medical Center, St. Louis, Missouri, USA.