T. pallidum subsp. pallidum strains used in this study.

1The Nichols Seattle strain was provided by James N. Miller, University of California, Los Angeles, CA. The Nichols Houston strain was provided by Steven J. Norris, University of Texas Health Science Center, Houston, TX. The Nichols Dallas strain was provided by Michael Norgard, University of Texas Southwestern Medical Center, Dallas, TX.2Year refers to the isolation of the parent Nichols strain by HJ Nichols and WH Hough [68].3 The Dal-1 strain [69] was provided by Rob George, CDC, Atlanta, GA4Strains provided by Paul Hardy and Ellen Nell, Johns Hopkins University, Baltimore, MD.5Strain isolated in Seattle by Sheila A. Lukehart, University of Washington, Seattle, WA.6Strain provided by Sandra A. Larsen, Center for Disease Control and Prevention, Atlanta, GA.T. pallidum subsp. pallidum strains used in this study.