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Curved, spiral and filamentous bacteria Some bacteria have unique and often characteristic shapes. For example, bacteria with a spiral shape, known as spirochetes, form a distinct group called *Spirochetes*. This group includes bacteria responsible for serious diseases, such as *Borrelia burgdorferi*, which causes Lyme disease, and *Treponema pallidum*, the causative agent of syphilis. The comma-shaped *Vibrio* form is exemplified by *Vibrio cholerae*, the bacterium that causes cholera. The spiral-shaped *Helicobacter pylori* lives in the human stomach and is associated with gastric ulcers. Meanwhile, scientists have discovered filamentous bacteria of the *Candidatus* species in the stomachs of rodents.