About the Authors
Department of Microbiology and Immunology, University of California, San Francisco, San Francisco, California, United States of America
Howard Hughes Medical Institute, University of California, San Francisco, San Francisco, California, United States of America
The authors have declared that no competing interests exist.
The author(s) have made the following declarations about their contributions: Conceived and designed the experiments: SB MG MV AS. Performed the experiments: SB MG. Analyzed the data: SB MG MV. Contributed reagents/materials/analysis tools: SB MG MV. Wrote the paper: SB AS.