Authors Howard C. Berg & Nicholas C. Darnton
In many of the bacteria that swim by rotating helical flagella, the flagellum itself is not a simple, passive propeller. The flagellar filament (a homopolymer of tens of thousands of flagellin monomers, accounting for more than 99% of the flagellar length) can adopt several helical shapes of varying pitch, radius and handedness. Although a single polymorphic form dominates during forward swimming, other variants are commonly seen during tumbling.
In this paper, we use an optical trap to apply forces comparable to those a filament would experience during swimming. From force-extension curves obtained from individual reconstituted, fluorescently labeled Salmonella filaments, we identify the forces required to cause polymorphic transformations. Within a single polymorphic form, the filament acts as a simple elastic object with a stiffness of 3.5 pN-µm2 Under extension, the filament transforms to a different, longer form. The transformation occurs in successive micron-long sections, at progressively lower forces. The force required depends on strain rate as well as on strain, suggesting there is a rate associated with the transformation, consistent with an activation phenomenon. Since the filament is a uniform polymer of flagellin protein, whose structure is known, it provides a simple, macroscopically visible model of highly cooperative conformational changes in a biological polymer.