Schaudinn & Hoffmann, 1905
Treponema pallidum is a spirochaete bacterium with subspecies that cause treponemal diseases such as syphilis, bejel, pinta and yaws. The treponemes have a cytoplasmic and outer membrane. Using light microscopy treponemes are only visible using Dark field illumination.
|This section requires expansion. (December 2010)|
There are at least four known subspecies:
- Treponema pallidum pallidum, which causes syphilis
- T. pallidum endemicum, which causes bejel or endemic syphilis
- T. pallidum carateum, which causes pinta
- T. pallidum pertenue, which causes yaws
There is some variation as to which are considered subspecies, and which are species. The cause of pinta is sometimes described as Treponema carateum, rather than a subspecies of Treponema pallidum, even when the subspecies convention is used for the other agents.
Laboratory identification 
This bacterium can be detected with special stains, such as the Dieterle stain.
Treponema pallidum is also detected by serology, including nontreponemal VDRL, rapid plasma reagin (RPR) and treponemal antibody tests (FTA-ABS), Treponema pallidum immobilization reaction (TPI) and syphilis TPHA test).
Clinical significance 
T. pallidum pallidum is a motile spirochaete that is generally acquired by close sexual contact, entering the host via breaches in squamous or columnar epithelium. The organism can also be transmitted to a fetus by transplacental passage during the later stages of pregnancy, giving rise to congenital syphilis. The helical structure of T. pallidum pallidum allows it to move in a corkscrew motion through a viscous medium such as mucus. It gains access to host's blood and lymph systems through tissue and mucous membranes.
The subspecies causing yaws, pinta, and bejel are morphologically and serologically indistinguishable from T. pallidum pallidum (syphilis); however, their transmission is not venereal in nature and the course of each disease is significantly different.
In the 17 July 1998 issue of the journal Science, a group of biologists reported how they sequenced the genome of T. pallidum. The recent sequencing of the genomes of several spirochetes permits a thorough analysis of the similarities and differences within this bacterial phylum. Treponema pallidum subsp. pallidum has one of the smallest bacterial genomes at 1.14 million base pairs (Mb), and has limited metabolic capabilities, reflecting its adaptation through genome reduction to the rich environment of mammalian tissue. The shape of Treponema pallidum is flat and wavy, unlike the other spirochetes, which are helical.
There is no vaccine for syphilis. The outer membrane of T. pallidum has too few surface proteins for an antibody to be effective. Efforts to develop a safe and effective syphilis vaccine have been hindered by uncertainty about the relative importance of humoral and cellular mechanisms to protective immunity and the fact that T. pallidum outer membrane proteins have not been unambiguously identified.
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