Date Published: January 18, 2019
Publisher: Public Library of Science
Author(s): Brigette Church, Erika Wall, John R. Webb, Caroline E. Cameron, Christoph E. Hagemeyer.
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.
Syphilis, caused by the spirochete Treponema pallidum subsp. pallidum (hereafter T. pallidum), is a chronic, sexually transmitted infection affecting an estimated 36 million people worldwide, with 11 million new cases occurring annually [1,2]. In recent years rates of primary and secondary syphilis have risen sharply in particular populations, most prominently amongst men who have sex with men, while a general increase in infectious syphilis cases in both heterosexual men and women has been observed in cities across North America, Europe, and Asia [1,3,4]. Paralleling the rise in syphilis cases amongst women, congenital syphilis cases have increased in middle and high income countries and continue to be a leading cause of stillbirth in low income nations, affecting an estimated 1.36 million pregnant women each year and resulting in over 500,000 adverse outcomes from maternal syphilis .
In this study we have established, for the first time, that live T. pallidum interacts with human platelets. By using a modified darkfield microscopy method, wherein the slide volume is reduced to enhance resolution, we were able to compare physical and kinematic parameters between platelet-interacting and non-interacting treponemes. Platelet-interacting treponemes increased their axial rotation and translational velocity and decreased their wavelength compared with non-interacting treponemes, to an extent that achieved statistical significance. A trend towards increased amplitude and decreased length was also observed with platelet-interacting treponemes compared to non-interacting treponemes, although these parameters did not achieve statistical significance. Interestingly, B. burgdorferi has demonstrated decreased cell length following co-incubation with soluble fibronectin, present in plasma at 300–400 μg/mL [52,53].