The basal body is a rotary motor, traversing the bacterial membrane, which generates torque by means of a transmembrane ion gradient ( Glagolev and Skulachev, 1978 Larsen et al., 1974 Manson et al., 1977), similar to ATP synthase ( Abrahams et al., 1994 Boyer, 1997 Ma et al., 2002). Each individual flagellum consists of three key segments/components: the flagellar basal body, the filament, and the hook that connects the two ( Fig. Rotating flagella propel motile bacteria, such as Escherichia coli and Salmonella typhimurium ( Berg and Anderson, 1973 Silverman and Simon, 1974). This value is orders of magnitude smaller than experimentally determined Young's moduli of the filament, hence in agreement with empirical evidence linking compliance in the flagellum mainly to the hook. These two ratios, along with previous experimental measurements, allowed us to propose a theoretical Young's modulus ( E) between 10 6 and 10 7 dyn/cm 2 for the hook. Both ratios were much <1.0, an average of 0.0440 for the filament and 0.0512 for the hook, indicating that within each structure bending is favored over twisting. A new computational method, the quantized elastic deformational model, was employed in this article to calculate the dimensionless twist/bend ratio ( EI/GJ) of the filament and hook, providing a quantitative means to compare their relative stiffness. The relative flexibility of two key components of the flagellum, filament and hook, is partially responsible for the mechanistic workings of this motor. Certain motile bacteria employ rotating flagella for propulsion.
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