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Treponema pallidum is a gram-negative spirochaete bacterium.
There are at least four known subspecies:
There is some variation as to which are considered subspecies, and which are species. For example, one source enumerates only three subspecies.[2]
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.[3]
This bacterium is too thin to be visualized with a standard Gram stain so two techniques to visualize it with a light microscope are dark field microscopy and immunofluorescence.
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.
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