ETIOLOGY

Treponema pallidum is a species of spirochaete bacterium with subspecies that cause treponemal infections such as syphilis. It was discovered by Fritz Richard Schaudinn (1871—1906) and Erich Hoffmann (1863—1959). It is not seen on a Gram stained smear due to poor ability to perceive dyes.

T. pallidum is a spirochete that cannot survive for long outside the human body. T. pallidum invades the body through the mucous membranes or skin, reaches the regional lymph nodes within hours, and rapidly spreads throughout the body.

Kingdom: Eubacteria.

Phylum: Spirochaetes.

Class: Spirochaetes.

Order: Spirochaetales.

Family: Spirochaetaceae.

Genus: Treponema.

Species: T. pallidum.

This microorganism has a spiral shape. Its length ranges between 6 pm and 20 pm and its width is between 0,1 pm and 0,18 pm. Its spiral curves are located at equal distances from one another and have an amplitude of 0,2–0,3 pm. The microorganism can be detected with dark field microscopy or phase contrast microscopy (fig. 1) as well as silver staining and electronic microscopy (fig. 2).

Fig. 1. Treponema pallidum (dark field microscopy from http://www.netzeitung.de)

Fig. 2. Treponema pallidum (electronic microscope from http://www.primer.ru/std/gallery_std)

Dark field microscopy is used for diagnosis. The method was introduced into practice by Karl Landsteiner, the famous Austrian immunologist and pathologist, over 100 years ago. This technique can easily reveal morphological peculiarities of Treponema pallidum and its characteristic movements. This distinguishes Treponema pallidum from non-venereal treponemas that inhabit the mucous lining of the oral cavity and genitals. Treponema pallidum can be detected with special stains, such as the Dieterle stain. Treponema pallidum is also detected by serology, including nontreponemal tests [Venereal Disease Research Laboratory (VDRL), Rapid plasma reagin (RPR)] and treponemal antibody tests [FTA-ABS, Treponema pallidum immobilization reaction (TPI) and Syphilis TPHA test].

Treponemas are distinguished by their characteristic movement: rotations around its own longitudinal axis; pendulum-like movements; wave-like movements, and contractile movements. They actively move in their environmental area. This is accompanied by bending of the bacteria at a right angle without losing its spiral shape. Other spirochete bacteria lack such bending movements so this is one of the classifying parameters of Treponema.