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Treponema pallidum

Treponema pallidum is the causative agent of syphilis, a sexually transmitted disease (STD). Other bacteria in the same genus cause the diseases yaws and bejel, which are transmitted through direct skin-to-skin contact or sharing of drinking vessels. T. pallidum has a characteristic helical shape, and is a member of the Spirochete family of bacteria. However, not all helical bacteria are Spirochetes:

This video shows a clinical specimen of syphilitic liver showning T. palludum (purple helices). The helical bacterial cells in this fixed specimen (~0.5 µm diameter, 10-25µm long) are no longer motile:

Spirochetes are simple bacteria, with less than 1000 genes.  Each type has a characteristic helical shape.  Some are tightly coiled like a telephone cord, while others are more open.  Very tight coils are seen in Leptospira, medium coils in Treponema and open coils are seen in Borrelia.  Spirochetes are Gram-negative and with a flagellar bundle running through their periplasmic space.  The spirochete flagella run in the perplasmic space, causing the cells to move in a corkscrew fashion. 

Syphilis is spread by mostly by sexual contact, except for congenital syphilis, which is spread from mother to fetus. Transmission by sexual contact requires exposure to moist lesions of skin or mucous membranes. As with many STDs, sexual promiscuity has resulted in an increase in syphilis during the last few decades. The symptoms of syphilis occur in three stages called primary, secondary and late:

Syphilis remains infectious for a period of up to two years, possibly longer. Transmission of infection depends on the existence of infectious lesions (sores), which may or may not be visible. No natural immunity to syphilis develops and past infection offers no protection to future exposure. Syphilis is relatively easily treated with antibiotics such as penicillin, usually given as consecutive daily intramuscular injections. Patients who are allergic to penicillin may be treated with tetracycline. Fortunately, unlike other STDs such as gonorrhea, there is little evidence of antibiotic resistance developing in syphilis.

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