Treponema pallidum

Treponema Pallidum is a spirochete bacteria that has two subspecies that cause human disease. These are T. pallidum subspecies pallidum, which causes venereal syphilis; and T. pallidum subspecies pertenue, which causes yaws. Both of these subspecies are members of the family, Spirochaetales, which also incudes the spirochetes Leptospira, the casue of the human disease, leptospirosis, and Borrelia, a genus of several species of spirochetes that cause the human diseases of relapsing fever and Lyme disease. Treponema pallidum is an unusual bacterium that is particularly parasitic. It lacks the enzymes of the Krebs cycle and oxidative phosphorylation, and transports many nutrients and structural substrates, such as amino acids and sugars, into its cytoplasm from its environment. This is one reason that the organism cannot live outside its host's body for any appreciable length of time and why infections must be transmitted through close contact.

Pathogenic subspecies of Treponema pallidum are very difficult to culture in the laboratory. Unlike most other bacteria that colonize and infect the human body, even special techniques for culture on artificial media are unsuccessful for the routine identification of these organisms. Instead, classically, dark field examination of a body fluid containing a large number of these spiral shaped motile bacteria was required for laboratory identification. More recently, PCR techniques are able to distinguish the organisms; this is especially helpful because both of the two subspecies of Treponema Pallidum, as well as other harmless species of Treponema look identical or similar under darkfield exam.

References

Sheila A. Lukehart. Chapter 153. "Syphilis" in Dennis L. Kasper, Eugene Braunwald, Anthony S. Fauci, Stephen L. Hauser, Dan L. Longo, J. Larry Jameson, and Kurt J. Isselbacher, Eds.Harrison's Principles of Internal Medicine, 16th Edition