Bacterial shape usually is dictated by the peptidoglycan layer of the cell wall. In this paper, we show that the morphology of the Lyme disease spirochete Borrelia burgdorferi is the result of a complex interaction between the cell cylinder and the internal periplasmic flagella. B. burgdorferi has a bundle of 7-11 helically shaped periplasmic flagella attached at each end of the cell cylinder and has a flat-wave cell morphology. Backward moving, propagating waves enable these bacteria to swim in both low viscosity media and highly viscous gel-like media. Using targeted mutagenesis, we inactivated the gene encoding the major periplasmic flagellar filament protein FlaB. The resulting flaB mutants not only were nonmotile, but were rod-shaped. Western blot analysis indicated that FlaB was no longer synthesized, and electron microscopy revealed that the mutants were completely deficient in periplasmic flagella. Wild-type cells poisoned with the protonophore carbonyl cyanide-m-chlorophenylhydrazone retained their flat-wave morphology, indicating that the periplasmic flagella do not need to be energized for the cell to maintain this shape. Our results indicate that the periplasmic flagella of B. burgdorferi have a skeletal function. These organelles dynamically interact with the rodshaped cell cylinder to enable the cell to swim, and to confer in part its flat-wave morphology.spirochete ͉ Lyme disease ͉ allelic exchange ͉ morphology S pirochetes have a unique position among the bacteria. These motile bacteria are one of the few bacterial phyla that can be identified by both 16S ribosomal RNA sequence analysis and morphology (1). Outermost is a membrane sheath, and within this sheath are the cell cylinder and periplasmic flagella. A given periplasmic flagellum is attached subterminally at only one end of the cell cylinder, and it resides within the periplasmic space (2). Depending on the species, the cell morphology is either a helix, a flat wave, or an irregularly shaped helix (2-4). The size of the spirochete, the number of periplasmic flagella attached at each end, and whether the filaments overlap at the center of the cell varies from species to species (2, 5). This phylum contains many medically important bacteria including Treponema pallidum (syphilis), several Borrelia species (relapsing fever), Borrelia burgdorferi (Lyme disease), Leptospira interrogans (leptospirosis), Brachyspira sp. (human diarrheal disease, swine dysentery), and oral treponemes associated with periodontal disease (5-8).The periplasmic flagella of spirochetes have been characterized in detail. Genetic evidence, including targeted mutagenesis studies in Treponema denticola and Brachyspira hyodysenteriae, have shown that these organelles are directly involved in motility (9, 10) (C. Li and N.W.C., unpublished observations). By analyzing protruding periplasmic flagella from certain motility mutants of T. phagedenis, and from stationary-phase cells of several spirochete species, these organelles have been shown to rotate in a manner similar to th...

Spirochete motility is enigmatic: It differs from the motility of most other bacteria in that the entire bacterium is involved in translocation in the absence of external appendages. Using the Lyme disease spirochete Borrelia burgdorferi (Bb) as a model system, we explore the current research on spirochete motility and chemotaxis. Bb has periplasmic flagella (PFs) subterminally attached to each end of the protoplasmic cell cylinder, and surrounding the cell is an outer membrane. These internal helically shaped PFs allow the spirochete to swim by generating backward-moving waves by rotation. Exciting advances using cryoelectron microscopy tomography are presented with respect to in situ analysis of cell, PF, and motor structure. In addition, advances in the dynamics of motility, chemotaxis, gene regulation, and the role of motility and chemotaxis in the life cycle of Bb are summarized. The results indicate that the motility paradigms of flagellated bacteria do not apply to these unique bacteria.