ACKNOWLEDGEMENTS
We thank Bob Gilmore, Kevin Brandt and Brittany Armstrong for thoughtful and critical reading of the manuscript. We thank Jean-Marc Lanchy, James Kovaks and Crystal Vander Zanden for useful discussions. We thank Robert Landick for the RL211 strain. We thank Irina Goodrich for excellent technical support. The findings and conclusions in this report are those of the authors and do not represent the official position of the Centers for Disease Control and Prevention.