Poulsen, A., Kobayasi, T., Secher, L., & Weismann, K. (1985). The ultrastructure of treponema pallidum isolated from human chancres. Morphologic variations from Nichols´ strain. Acta Dermato-Venereologica, 65(5), 367–373. https://doi.org/10.2340/0001555565367373