Treponema pallidum is a slender, helically coiled spirochete bacterium that causes syphilis. This microaerophilic organism has an outer membrane lacking lipopolysaccharide and a periplasmic flagellar bundle that provides corkscrew motility. Because of its fragile envelope and dependence on host lipids, it cannot be cultured in routine media and is maintained in rabbit testes for research.
Biology and Pathogenicity
Treponema pallidum has a small genome with limited metabolic capacity and relies on the host for many nutrients. It penetrates mucosal surfaces or abraded skin and disseminates via the bloodstream and lymphatics. The bacterium expresses a low density of surface antigens and undergoes antigenic variation, which helps it evade immune responses and establish chronic infection. Infection progresses through distinct stages: the primary stage is marked by a painless chancre at the entry site; the secondary stage includes systemic spread with fever, rash and lymphadenopathy; and the tertiary stage, years later, manifests as destructive lesions in skin, cardiovascular tissue and the nervous system. The organism can also cross the placenta, leading to congenital infection. Diagnosis relies on serologic tests such as non‑treponemal and treponemal assays. Penicillin remains the treatment of choice, and no vaccine is yet available.
Notable Features and Clinical Forms
T. pallidum has several subspecies. Treponema pallidum subsp. pallidum causes venereal syphilis, while T. pallidum subsp. endemicum causes endemic syphilis (bejel), and T. pallidum subsp. pertenue causes yaws. These pathogens share similar morphology but differ in transmission and geographic distribution. Primary syphilis produces a solitary chancre; secondary syphilis can involve mucous patches and condylomata lata; tertiary syphilis leads to gummatous lesions, neurosyphilis or cardiovascular involvement. Congenital syphilis can cause stillbirth, bone abnormalities and hearing loss. Historical episodes, such as the Tuskegee study, highlight ethical issues surrounding syphilis research.
A proper understanding of Treponema pallidum biology and the stages of syphilis is essential for early diagnosis and effective treatment. Continued public health surveillance and access to antibiotics help reduce disease burden.
Related Terms: syphilis, spirochete, congenital syphilis, Borrelia, Leptospira