Elastic properties of bacterial flagellar filaments. II. Determination of the modulus of rigidity.

Elongation of a helical bacterial flagellar filament subjected to fluid flow was calculated on the assumption that one end of the filament is firmly attached to a substratum. It was found that the quantity [E(d/2 pi r)2 + 2 mu] could be determined by measuring the elongation at various flow rates, where E is Young's modulus, mu the modulus of rigidity, r the radius of the helix, and d the helical pitch. Experiments were carried out to determine the above quantity for Salmonella flagellar filaments assuming a close-coil form. Because the above quantity is almost equal to 2 mu for a helical form with a large radius/pitch ratio, we were able to determine the modulus of rigidity for this kind of flagellar filament from plots of elongation vs. flow rates. The modulus of rigidity was determined to be about 1 X 10(11) dyn/cm2, i.e., 2 orders of magnitude larger than the previously estimated value.