About the Authors
Department of Microbiology, The University of Chicago, Chicago, Illinois, United States of America
The authors have declared that no competing interests exist.
Conceived and designed the experiments: KEC KLB NSH JDC RY GR. Performed the experiments: KEC KLB NSH JDC RY. Analyzed the data: KEC KLB NSH JDC RY GR. Wrote the paper: KEC GR.