1. Research Centre for Infectious Diseases, School of Biological Sciences, The University of Adelaide, Adelaide, South Australia 5005, Australia
2. Australian Centre for Antimicrobial Resistance Ecology, The University of Adelaide, Adelaide, South Australia 5005, Australia
3. Department of Microbiology and Immunology, School of Medicine, University of North Carolina, Chapel Hill, NC 27599-7290, U.S.A.