Lyme Disease Bacteria, Borrelia burgdorferi. The Lyme disease spirochete, Borrelia burgdorferi, is an obligate parasite that cycles between ticks and vertebrate hosts. B. burgdorferi alters the proteins expressed on its outer surface, depending on the state of each host. Here, we used immunofluorescent antibodies to identify spirochetes that express outer surface protein D (yellow and red) and merged the image with an image of all the spirochetes labeled with an anti-B. burgdorferi antibody (green).