Interaction of Treponema pallidum, the syphilis spirochete, with human platelets

Extracellular bacteria that spread via the vasculature employ invasive mechanisms that mirror those of metastatic tumor cells, including intravasation into the bloodstream and survival during hematogenous dissemination, arrestation despite blood flow, and extravasation into distant tissue sites. Several invasive bacteria have been shown to exploit normal platelet function during infection. Due to their inherent ability to interact with and influence other cell types, platelets play a critical role in alteration of endothelial barrier permeability, and their role in cancer metastasis has been well established. The highly invasive bacterium and causative agent of syphilis, Treponema pallidum subspecies pallidum, readily crosses the endothelial, blood-brain and placental barriers. However, the mechanisms underlying this unusual and important aspect of T. pallidum pathogenesis are incompletely understood. In this study we use darkfield microscopy in combination with flow cytometry to establish that T. pallidum interacts with platelets. We also investigate the dynamics of this interaction and show T. pallidum is able to activate platelets and preferentially interacts with activated platelets. Platelet-interacting treponemes consistently exhibit altered kinematic (movement) parameters compared to free treponemes, and T. pallidum-platelet interactions are reversible. This study provides insight into host cell interactions at play during T. pallidum infection and suggests that T. pallidum may exploit platelet function to aid in establishment of disseminated infection.