About the Authors
Department of Microbiology and Molecular Genetics, Harvard Medical School, Boston, Massachusetts, United States of America
The authors have declared that no competing interests exist.
Conceived and designed the experiments: GT SPJW. Performed the experiments: GT. Analyzed the data: GT AAR SPJW. Contributed reagents/materials/analysis tools: GT AAR. Wrote the paper: GT AAR SPJW.