Bending stiffness characterization of Bacillus subtilis' flagellar filament

The filament of a bacterial flagellum is a tube-like organelle made of a single protein-flagellin-and assembled into multiple polymorphic forms. The filament can be further discretized into four subunit domains (D0, D1, D2, and D3) along the radial direction. However, it remains unclear which subunit domain plays an important role in regulating the rigidity of the filament. In this article, we address how the absence of two outer subunit domains (D2 and D3) affects the bending stiffness of the bacterium B. subtilis' flagellar filament. We first shear off flagellar filaments from the cell body, anchor one of its ends to the wall of a microfluidic channel, and correlate the elongation of the filament with the driving background flow. A numerical model is then applied to determine the bending stiffness of the filament. We find that the bending stiffness does not change drastically when the filament transforms from normal to hyperextended forms, which is estimated to be 2-3 pN⋅μm². Furthermore, B. subtilis' flagellar filament has similar bending stiffness to Salmonella's, although the radius of the former is almost half of that of the latter, suggesting that the rigidity comes from the inner D0 and D1 subunit domains.