The spirochaetes are a group of distinctive bacteria, which have long, helically coiled cells. They are distinguished by the presence of flagella running lengthwise between the cell membrane and cell wall, called axial filaments. These cause a twisting motion which allows the spirochaete to move about. Most spirochaetes are free-living and anaerobic, but there are numerous exceptions. A few are important pathogens, for instance Treponema pallidum, which causes syphilis.

It has been suggested by Lynn Margulis that eukaryotic flagella were derived from symbiotic spirochaetes, but few biologists accept this, as there is no close structural similarity between the two.