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a bacterium, measuring 0.1–0.6 micrometer in diameter and 5–500 micrometers in length, in the form of an elongated coiled spiral. The majority of species have a slender axis filament around which the body of the cell is spirally coiled. Spirochetes lack flagella and are characterized by undulating movements, during which the cells rotate around their long axis. Reproduction is by transverse fission.
Spirochetes may be nonpathogenic or pathogenic. The former inhabit freshwaters, and the latter parasitize mollusks and cause syphilis in man (Treponema pallidum), relapsing fever (Borrelia recurrentis), and other spirochetoses. Under artificial conditions, nonpathogenic spirochetes grow on ordinary nutrient mediums and pathogenic spirochetes grow on mediums that contain serum and pieces of fresh tissue or the internal organs of animals. Some forms have not yet been grown in the laboratory.