Treponema pallidum

Lore & Background

*Treponema pallidum* is a helically coiled, microaerophilic, gram-negative spirochaete bacterium, typically measuring 6–15 μm in length and 0.1–0.2 μm in width. It was first identified microscopically in syphilitic chancres by Fritz Schaudinn and Erich Hoffmann in 1905. The bacterium is an obligate chemoorganoheterotroph with minimal metabolic activity, lacking a tricarboxylic acid cycle and oxidative phosphorylation; it relies on glycolysis to generate ATP from glucose obtained from its host. Its cellular structure includes a protoplasmic cylinder, a peptidoglycan layer, and both cytoplasmic and outer membranes, with an endoflagellum located in the periplasmic space that wraps around the cylinder and aids motility. The outer membrane is notable for containing very few transmembrane proteins—approximately 100-fold fewer than in other gram-negative bacteria—which limits antigenicity and contributes to its reputation as a stealth pathogen. This low protein density, combined with minimal surface protein expression, hinders vaccine development. The bacterium can enter hosts through mucosal membranes or skin lesions and is primarily transmitted sexually among humans and baboons. Three subspecies exist—*T. p. pallidum* (syphilis), *T. p. endemicum* (bejel), and *T. p. pertenue* (yaws)—which are morphologically and serologically indistinguishable but can be differentiated by genetic techniques such as restriction fragment length polymorphism. *T. p. pallidum* is the most invasive subspecies. Historically, laboratory identification relied on dark-field microscopy; it can also be detected using special stains like the Dieterle stain and through serological tests including VDRL, rapid plasma reagin, and FTA-ABS. The bacterium is effectively treated with antibiotics that inhibit cell wall synthesis, such as penicillin G.

Reader's Guide

Treponema pallidum is a microaerophilic, gram-negative spirochaete bacterium, formerly called *Spirochaeta pallida*, that is an obligate parasite dependent on its host for glucose, which it uses for energy via glycolysis due to its lack of a tricarboxylic acid cycle and oxidative phosphorylation. The organism is helically coiled, typically 6–15 μm long and 0.1–0.2 μm wide, and is visible only under dark-field microscopy. It is transmitted exclusively among humans and baboons, entering through mucosal membranes or skin lesions, primarily via sexual contact. The three subspecies—*T. p. pallidum*, *T. p. endemicum*, and *T. p. pertenue*—cause syphilis, bejel, and yaws, respectively, and are morphologically and serologically indistinguishable, differing genetically by only 0.2%. They are differentiated using restriction fragment length polymorphism analysis of specific genes. *T. p. pallidum* is the most invasive subspecies, while *T. carateum*, the cause of pinta, remains a separate species. The bacterium’s outer membrane has very few transmembrane proteins, roughly 100-fold fewer than other gram-negative bacteria, which limits antigenicity and enables stealth pathogenicity, allowing it to evade host immune defenses. This unique outer membrane structure, combined with minimal surface protein expression, has hindered vaccine development. The organism is effectively treated with penicillin-G, an antibiotic that inhibits bacterial cell wall synthesis.

Did You Know?

• Treponema pallidum lacks a tricarboxylic acid cycle and processes for oxidative phosphorylation, resulting in minimal metabolic activity.
• The three subspecies causing yaws, bejel, and syphilis are morphologically and serologically indistinguishable but can be differentiated by restriction fragment length polymorphism.
• TprK, a protein in the Treponema repeat family, constantly mutates to avoid host immune response.