1. Department of Microbiology and Immunology, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina
2. Department of Epidemiology, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina
3. Cystic Fibrosis/Pulmonary Research and Treatment Center, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina
4. Institute for Research in Biomedicine, Bellinzona, Switzerland