Scanning electron micrograph of the spirochete Treponema pallidum attached to testicular …—ASM/Science Source

Any of an order (Spirochaetales) of spiral-shaped bacteria. Some are serious pathogens for humans, causing such diseases as syphilis, yaws, and relapsing fever. Spirochetes are gram-negative (see gram stain) and motile. They are unique in that their flagella, which number between two and more than 200 per organism, are contained within the cell. Most spirochetes are found in a liquid environment (e.g., mud and water, blood and lymph). Several species are borne by lice and ticks, which transmit them to humans.

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