Spirochetes, like T. pallidum and Borrelia burgdorferi, the causitive agent of Lyme disease, have flagella that extend from one end of the bacterium to the other inside the periplasmic space. This causes spirochetes to move in a corkscrew motion and allows them to move through very viscous mediums, like mucus. T. pallidum invades host tissue at mucus membranes where it causes infections and gains access to the blood and lymph systems. Once the pathogen has entered the host it is successful at evading defense mechanisms by not being immunogenic. The outer membrane of T. pallidum has very few surface proteins and thus few antibodies can be generated against this organism, which is why no vaccine exists for syphilis.
The paucity of membrane associated proteins allows T. pallidum to evade host defense mechanisms, makes serological diagnostic tests ambiguous and is the reason why there is no vaccine for syphilis. T. pallidum has 22 lipoproteins on its surface compares to 105 in B. burgdorferi. Both pathogens have the ability to alter the fatty acids that are associated with these proteins and there is increasing evidence that this might help them avoid the host immune response. T. pallidum's recently sequenced genome revealed 12 duplicated genes that encode putative membrane proteins that function as porins and adhesins. This hypothesis comes from sequence homology with major outer sheath protein (MSP) in Treponema denticola. It has been speculated that the 12 paralogs tpr(A-L) may provide a means of producing antigenic variation in T. pallidum , which may contribute to syphilis' ability to relapse. However, this is very speculative as only four of the paralogs seem to have cleavable signal sequences. T. pallidum also evades the immune system by covering itself in serum proteins, like a2-macroglobulin, albumin, IgG heavy chain, transferrin and MHC I molecules. It also forms a strong attachment to fibronectin, which aids its ability to invade host tissue and especially strong attachments might generate antibodies against the antigen-fibronectin complex causing autoimmunity. Upon attaching to fibronectin the bacteria causes vascular endothelial cells to express ICAM on their surface. ICAM then interacts with integrins on the surface of neutrophils to produce temporary gaps between endothelial vells allowing T.pallidum into the lymph system, which is how it travels to different tissues. It lacks lipopolysaccharides (endotoxin) and the necessary protein secretory machinery for exotoxins, but has 5 genes that encode for hemolysins that might contribute to its virulence.