Periplasmic flagella are the organelles of spirochete mobility, and are structurally different from the flagella of other motile bacteria. They reside inside the cell within the periplasmic space, and confer mobility in viscous gel-like media such connective tissue [1]. The flagella are composed of an outer sheath of FlaA proteins and a core filament of FlaB proteins. Each species usually has several FlaA protein species [2].

1: Expression in Escherichia coli of the 37-kilodalton endoflagellar sheath protein of Treponema pallidum by use of the polymerase chain reaction and a T7 expression system. Isaacs RD, Radolf JD; Infect Immun 1990;58:2025-2034. PMID:2194955

2: An unexpected flaA homolog is present and expressed in Borrelia burgdorferi. Ge Y, Charon NW; J Bacteriol 1997;179:552-556. PMID:8990312