|Spirochaetes. Numbered ticks are 11 µm apart. Gram-stained.|
|Treponema pallidum spirochaetes.|
|Phylum:||Spirochaetae Cavalier-Smith 2002|
|Class:||Spirochaetes Cavalier-Smith 2002|
|Order:||Spirochaetales Buchanan 1917|
|Families & Genera|
Spirochaetes (also spelled spirochetes) belong to a phylum of distinctive diderm (double-membrane) bacteria, most of which have long, helically coiled (spiral-shaped) cells. Spirochaetes are chemoheterotrophic in nature, with lengths between 5 and 250 µm and diameters around 0.1–0.6 µm.
Spirochaetes are distinguished from other bacterial phyla by the location of their flagella, sometimes called axial filaments, which run lengthwise between the bacterial inner membrane and outer membrane in periplasmic space. These cause a twisting motion which allows the spirochaete to move about. When reproducing, a spirochaete will undergo asexual transverse binary fission.
Most spirochaetes are free-living and anaerobic, but there are numerous exceptions.
The spirochaetes are divided into three families (Brachyspiraceae, Leptospiraceae, and Spirochaetaceae), all placed within a single order (Spirochaetales). Disease-causing members of this phylum include the following:
- Leptospira species, which causes leptospirosis
- Borrelia burgdorferi, B. garinii, and B. afzelii, which cause Lyme disease
- Borrelia recurrentis, which causes relapsing fever
- Treponema pallidum subspecies which cause treponematoses such as syphilis and yaws.
- Brachyspira pilosicoli and Brachyspira aalborgi, which cause intestinal spirochaetosis
The currently accepted taxonomy is based on the List of Prokaryotic names with Standing in Nomenclature (LPSN)  and National Center for Biotechnology Information (NCBI) and the phylogeny is based on 16S rRNA-based LTP release 111 by 'The All-Species Living Tree' Project 
♦ Type strain lost or not available
♪ Prokaryotes where no pure (axenic) cultures are isolated or available, i. e. not cultivated or can not be sustained in culture for more than a few serial passages
♠ Strains found at the National Center for Biotechnology Information (NCBI) but not listed in the List of Prokaryotic names with Standing in Nomenclature (LSPN)
- Lyme disease microbiology
- Treponema pallidum
- Introduction to the Spirochetes University of California Museum of Paleontology (UCMP)