1. Department of Mathematics, University of Illinois at Urbana-Champaign, Urbana, Illinois, USA
2. Department of Microbiology, University of Illinois at Urbana-Champaign, Urbana, Illinois, USA
3. Department of Microbiology and Immunology, Geisel School of Medicine at Dartmouth, Hanover, New Hampshire, USA
4. Carl R. Woese Institute for Genomic Biology, University of Illinois at Urbana-Champaign, Urbana, Illinois, USA