The periplasmic flagellum of spirochetes

Although spirochete periplasmic flagella have many features similar to typical bacterial flagella, they are unique in their structure and internal periplasmic location. This location provides advantages for pathogenic spirochetes to enter and to adapt in the appropriate host, and to penetrate through matrices that inhibit the motility of most other bacteria. These flagella are complex, and they dynamically interact with the spirochete cell cylinder in novel ways. Electron microscopy, tomography and three-dimensional reconstructions have provided new insights into flagellar structure and its relationship to the spirochetal cell cylinder. Recent advances in genetic methods have begun to shed light on the composition of the spirochete flagellum, and on the regulation of its synthesis. Because spirochetes have a high length to width ratio, their cells provide an opportunity to study two important features. These include the polarity or distribution of flagellar synthesis as well as the mechanisms required for coordination of the movement of the cell ends, to enable it to move in the forward or reverse direction.