4 - The biology of low-Reynolds-number locomotion

Summary

Structure and physiology

Bacteria (references: Berg, 1975a, b; Adler, 1975)

The bacterial flagellum is very different from the organelle that bears that name in eukaryotic organisms (e.g., protozoans, algae, and multicellular organisms). It is a rigid or semirigid helical filament made of a single polymeric protein called fiaggelin, which has no enzymatic activity. At the base of the filament is a complicated set of components whose structure is fairly well known by now (see Figure 4.1). The basal body serves to anchor the flagellum in the cell and probably is part of the driving motor. A number of theories of motion have been suggested, and one based on experiments of Berg, Anderson, and others now seems most probable. In this model, the flagellum is a rigid or semirigid helix that is turned at the base by a rotary motor interacting with the basal body. If true, this would apparently be the only known use of the wheel and axle for locomotion in all biology! The experiments of Berg et al. involved getting the flagellum to stick to a glass surface or to tiny latex beads. In the first case, when the flagellum was immobilized, the organism was seen to rotate; in the second case, the attached spheres could be seen to rotate. From such observations, the picture of a rigidly rotating helix has emerged.