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Treponema pallidum spirochetes.

Spirochaetes (or Spirochetes) belong to a phylum of double-membrane bacteria. They have long, helically coiled (spiral-shaped) cells. Spirochetes are between 5 and 250 µm long, with diameters about 0.1–0.6 µm.

Spirochaetes are distinguished from other bacterial phyla by their flagella, which run lengthwise between the cell wall and outer membrane. 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. Of the parasitic species, Treponema pallidum causes syphilis.