The authors thank S. Lockett and K. Peifley at the Optical Microscopy and Analysis Laboratory, National Cancer Institute at Frederick (Fort Detrick, Frederick, MD, USA) for use of their confocal microscope. The authors thank L. Bollinger and J. Wada (NIH/NIAID Integrated Research Facility at Fort Detrick, Frederick, MD, USA) for critically editing the manuscript and figure preparation, respectively. The authors thank W. Discher and J. Braun at United States Army Medical Research Institute of Infectious Diseases (USAMRIID, Fort Detrick, Frederick, MD, USA) for the eye diagram and M. Kortepeter, A. Cardile, C. Shaia and A. Anderson for their critical reading of the manuscript. Work at USAMRIID was funded by The Joint Science and Technology Office for Chemical and Biological Defense (JSTO-CBD) of the Defense Threat Reduction Agency (DTRA) and the Medical Countermeasure Systems (MCS) of the Joint Program Executive Office for Chemical and Biological Defense (JPEO-CBD). This work was supported in part through Battelle Memorial Institute's prime contract with the US National Institute of Allergy and Infectious Diseases (NIAID) under contract no. HHSN272200700016I. A subcontractor to Battelle Memorial Institute who performed this work is: J.H.K., an employee of Tunnell Government Services, Inc. The views and conclusions contained in this document are those of the authors and should not be interpreted as necessarily representing the official policies, either expressed or implied, of the US Department of the Army, the US Department of Defense, the US Department of Health and Human Services, or of the institutions and companies affiliated with the authors. In no event shall any of these entities have any responsibility or liability for any use, misuse, inability to use, or reliance upon the information contained herein. The US departments do not endorse any products or commercial services mentioned in this publication.