Any nonrigid corkscrew-shaped bacterium that moves by means of flexions of the cell, produced by a series of rotatory axial fibrils underlying the cell’s outer sheath. Most spirochaetes are Gram-negative (see gram’s stain), anaerobic, and feed on dead organic matter. They are particularly common in sewage-polluted waters. Some, however, cause disease in humans and other animals; Treponema pallidum, the agent of syphilis, is an example.