Treponema pallidum is a spirochaete bacterium with various subspecies that cause the diseases syphilis, bejel, and yaws. It is transmitted only amongst humans. It is a helically coiled microorganism usually 6–15 µm long and 0.1–0.2 µm wide. The treponemes have a cytoplasmic and an outer membrane. Using light microscopy, treponemes are visible only by using dark field illumination.
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- Treponema pallidum pallidum, which causes syphilis
- T. p. endemicum, which causes bejel or endemic syphilis
- T. p. pertenue, which causes yaws
These bacteria were originally classified as members of separate species, but DNA hybridization analysis indicates they are members of the same species. Treponema carateum, the cause of pinta, remains a separate species because no isolate is available for DNA analysis.
T. pallidum was first microscopically identified in syphilitic chancres by Fritz Schaudinn and Erich Hoffmann at the Charité in Berlin in 1905. This bacterium can be detected with special stains, such as the Dieterle stain. T. pallidum is also detected by serology, including nontreponemal VDRL, rapid plasma reagin, treponemal antibody tests (FTA-ABS), T. pallidum immobilization reaction, and syphilis TPHA test. Successful long-term cultivation of T. pallidum in a tissue culture system has been reported in 2018.
T. p. 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. p. pallidum allows it to move in a corkscrew motion through mucous membranes or enter minuscule breaks in the skin. In women the initial lesion is usually on the labia, the walls of the vagina, or the cervix; in men it is on the shaft or glans of the penis. It gains access to the host's blood and lymph systems through tissue and mucous membranes. In more severe cases, it may gain access to the host by infecting the skeletal bones and central nervous system of the body.
The three subspecies causing yaws, pinta, and bejel, which often cause lesions on the skin and mucous membranes as well as on soft tissue and bone, are morphologically and serologically indistinguishable from T. p. pallidum (syphilis); however, their transmission is not venereal and the course of each disease is significantly different.
The incubation period for a T. p. pallidum infection is usually around 21 days, but can range from 10 days to 90 days.
The genome of T. pallidum was sequenced in 1998. The recent sequencing of the genomes of several spirochetes permits a thorough analysis of the similarities and differences within this bacterial phylum and within the species. T. p. pallidum has one of the smallest bacterial genomes at 1.14 million base pairs, and has limited metabolic capabilities, reflecting its adaptation through genome reduction to the rich environment of mammalian tissue. The shape of T. pallidum is flat and wavy, unlike the other spirochetes, which are helical.
No vaccine for syphilis is available as of 2017. 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 because T. pallidum outer membrane proteins have not been unambiguously identified. In contrast, some of the known antigens are intracellular and antibody against them are ineffective to clear the infection.
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