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$\cdot \mu$m². 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.

细菌鞭毛的细丝是一种管状细胞器，由单一蛋白质（鞭毛蛋白）组成，并组装成多种多态形式。灯丝可以沿径向方向进一步离散为四个亚基域（D0、D1、D2 和 D3）。然而，尚不清楚哪个亚基结构域在调节细丝的刚性方面起着重要作用。在这篇文章中，我们讨论了两个外部亚基结构域（D2 和 D3）的缺失如何影响枯草芽孢杆菌的弯曲刚度'鞭毛丝。我们首先从细胞体上剪下鞭毛丝，将其末端之一固定在微流体通道的壁上，并将丝的伸长与驱动背景流相关联。然后应用数值模型来确定灯丝的弯曲刚度。我们发现当灯丝从正常形式转变为超伸展形式时，弯曲刚度不会发生剧烈变化，估计为 2-3 pN$\cdot \mu$米²。此外，B. subtilis的鞭毛丝具有与沙门氏菌相似的弯曲刚度，尽管前者的半径几乎是后者的一半，这表明刚性来自内部 D0 和 D1 亚基结构域。