(A) Schematic of spring network model for bacterial flagellum. Green particles discretize the flagellum while blue particles represent the optical beads. Right inset: closeup of a region of the filament, showing that neighboring masses are connected by linear springs of stiffness which give rise to a stretching energy according to Equation 2, where is the stress-free length of the springs. In addition, each triplet of neighbors determines an angle which dictates the bending energy of the unit: (Equation 3). Left inset: closeup of the region where the bead connects to the flagellum. The bead particle is connected to a single particle of the flagellum by a very stiff spring of stress-free length corresponding to radius of the optical bead. In addition, there is a bending interaction , where is the angle determined by the bead, its connecting particle on the flagellum, and the latter’s left neighbor. (B) Force-strain curve from experiment (blue) and simulation (yellow) for a flagellum of length 4.1 . Errorbars are obtained by binning data spatially as well as averaging over multiple runs. The bending rigidity obtained from the fit is . The classical buckling force for a rod of the same length and bending rigidity with pinned boundary conditions is .