1 Division of Biology, California Institute of Technology, Pasadena, California, 91125, USA.
2 Department of Biological Sciences, Stanford University, Stanford, California, 94305, USA.
3 Departments of Microbiology and Neurology, University of Southern California School of Medicine, Los Angeles, California, 90033, USA. jfrelin@arizona.edu.
4 Department of Microbiology, University of Geneva Medical School, Geneva, Switzerland.