Freeze-fracture electron microscopy of T. pallidum subsp. pallidum demonstrating TROMP aggregation. Concave and convex outer membrane fracture faces (OM). T. pallidum incubated in heat-inactivated normal rabbit serum for 16 hours (A), in immune rabbit serum for 2 hours (B), and in immune rabbit serum for 16 hours (C) and (D). Arrows show individual (A&B) and aggregated (C&D) TROMPs. Bar in each micrograph represents 0.1mm. Photograph reprinted with the permission of the Journal of Immunology, copyright 1990, The American Association of Immunologists.