Treponema pallidum

Treponema pallidum is a species of spirochaete bacterium with subspecies that cause treponemal diseases such as syphilis, bejel, pinta and yaws. Since the organism has a waxy coat around it won’t accept the gram stain. The Dieterle stain is used to detect the bacterium.

It is detected by serology, including nontreponemal, Rapid plasma regain, and treponemal antibody tests. It is a motile spirochaete that is generally acquired by close sexual contact.

It can also be transmitted to a fetus by transplacental passage during the later stages of pregnancy. It moves 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; however, their transmission is not venereal in nature and the course of each disease is significantly different.

A group of biologists sequenced the genome of T. pallidum in 1998. It has one of the smallest bacterial genomes at 1.14 million base pairs and has limited metabolic capabilities, reflecting its adaption through genome reduction to the rich environment of mammalian tissue. It has a flat and wavy shape unlike other spirochetes which are helical.

There is no vaccine due to the outer membrane of T. pallidum having too few surface proteins for an antibody to be effective.