Treponema pallidum subsp. pallidum, the causative agent of syphilis, is a highly invasive spirochete pathogen that uses the vasculature to disseminate throughout the body. Identification of bacterial factors promoting dissemination is crucial for syphilis vaccine development. An important step in dissemination is bacterial adhesion to blood vessel surfaces, a process mediated by bacterial proteins that can withstand forces imposed on adhesive bonds by blood flow (vascular adhesins). The study of T. pallidum vascular adhesins is hindered by the uncultivable nature of this pathogen. We overcame these limitations by expressing T. pallidum adhesin Tp0751 (pallilysin) in an adhesion-attenuated strain of the cultivable spirochete Borrelia burgdorferi. Under fluid shear stress representative of conditions in postcapillary venules, Tp0751 restored bacterial-vascular interactions to levels similar to those observed for infectious B. burgdorferi and a gain-of-function strain expressing B. burgdorferi vascular adhesin BBK32. The strength and stability of Tp0751-and BBK32-dependent endothelial interactions under physiological shear stress were similar, although the mechanisms stabilizing these interactions were distinct. Tp0751 expression also permitted bacteria to interact with postcapillary venules in live mice as effectively as BBK32-expressing strains. These results demonstrate that Tp0751 can function as a vascular adhesin.Treponema pallidum subsp. pallidum is a bacterium from the spirochete family and the causative agent of syphilis. Syphilis is a sexually transmitted disease, which is often considered a disease of the past. However, it still represents a global health problem with 36 million cases and 11 million new infections annually 1 . Moreover, syphilis incidence has increased dramatically over the past two decades in Canada 2 , the United States 3 , China 4 , and in a number of European countries [5][6][7] . Given its high prevalence, syphilis has a substantial impact on global health. Syphilis infection in pregnancy has up to 80% morbidity and can result in stillbirth, neonatal death, and complications associated with congenital syphilis in newborns 8 . Active syphilis infection also significantly increases infectivity of HIV-positive patients and risk of HIV acquisition 9 , and modeling studies suggest that controlling syphilis will contribute to HIV prevention 10 . Since elimination of syphilis has not been achieved by antibiotic treatment, novel preventative approaches are needed to fight this disease.To date, the molecular mechanisms of T. pallidum pathogenesis are still poorly understood. This is mainly because the organism is an obligate human pathogen that cannot be cultured continuously in vitro 11 . The inability to culture T. pallidum has limited the use of many experimental approaches, including genetic manipulation to