We thank Karen Fahrner for helpful discussions about flagella and bacterial swimming behavior, and Michael Bucaro and Wendong Wang for helpful experimental discussions. This work was par- tially funded by the Office of Naval Research under the award N00014-11-1- 0641 and by the BASF Advanced Research Initiative at Harvard University. R.S.F. is supported by the National Science Foundation (NSF) Graduate Research Fellowship Program. Part of this work was carried out through the use of the Massachusetts Institute of Technology’s Microsystems Technology Laboratories and the Center for Nanoscale Systems at Harvard University, a member of the National Nanotechnology Infrastructure Network, sup- ported by the NSF under Award ECS-0335765.