The elastic basis for the shape of Borrelia burgdorferi.

The mechanisms that determine bacterial shape are in many ways poorly understood. A prime example is the Lyme disease spirochete, Borrelia burgdorferi (B. burgdorferi), which mechanically couples its motility organelles, helical flagella, to its rod-shaped cell body, producing a striking flat-wave morphology. A mathematical model is developed here that accounts for the elastic coupling of the flagella to the cell cylinder and shows that the flat-wave morphology is in fact a natural consequence of the geometrical and material properties of the components. Observations of purified periplasmic flagella show two flagellar conformations. The mathematical model suggests that the larger waveform flagellum is the more relevant for determining the shape of B. burgdorferi. Optical trapping experiments were used to measure directly the mechanical properties of these spirochetes. These results imply relative stiffnesses of the two components, which confirm the predictions of the model and show that the morphology of B. burgdorferi is completely determined by the elastic properties of the flagella and cell body. This approach is applicable to a variety of other structures in which the shape of the composite system is markedly different from that of the individual components, such as coiled-coil domains in proteins and the eukaryotic axoneme.

Asymmetrical flagellar rotation in Borrelia burgdorferi nonchemotactic mutants.

The Lyme disease spirochete Borrelia burgdorferi has bundles of periplasmic flagella subpolarly located at each cell end. These bundles rotate in opposite directions during translational motility. When not translating, they rotate in the same direction, and the cells flex. Here, we present evidence that asymmetrical rotation of the bundles during translation does not depend upon the chemotaxis signal transduction system. The histidine kinase CheA is known to be an essential component in the signaling pathway for bacterial chemotaxis. Mutants of cheA in flagellated bacteria continually rotate their flagella in one direction. B. burgdorferi has two copies of cheA designated cheA1 and cheA2. Both genes were found to be expressed in growing cells. We reasoned that if chemotaxis were essential for asymmetrical rotation of the flagellar bundles, and if the flagellar motors at both cell ends were identical, inactivation of the two cheA genes should result in cells that constantly flex. To test this hypothesis, the signaling pathway was completely blocked by constructing the double mutant cheA1kan cheA2ermC. This double mutant was deficient in chemotaxis. Rather than flexing, it failed to reverse, and it continually translated only in one direction. Video microscopy of mutant cells indicated that both bundles actively rotated. The results indicate that asymmetrical rotation of the flagellar bundles of spirochetes does not depend upon the chemotaxis system but rather upon differences between the two flagellar bundles. We propose that certain factors within the spirochete localize at the flagellar motors at one end of the cell to effect this asymmetry.