Stochastic fluctuations in the rotation of the bacterial flagellar motor

The bacterial flagellar motor is a nanometer-sized rotary motor that generates the torque to drive the rotation of the flagellar filament. The output torque is an important property of the motor. The motor rotation was usually monitored by attaching a μm-sized bead to a shortened flagellar filament, and the torque was extracted by calculating the torque due to the viscous drag of the medium on the bead rotation. We sought for an independent extraction of the torque from thermal fluctuation in the motor rotation using the fluctuation theorem(FT). However, we identified an overwhelming fluctuation beyond the thermal noise that precluded the use of FT. We further characterized the timescale and the amplitude of this fluctuation, finding that it was probably due to the stepping of the motor. The amplitude of torque fluctuation we characterized here provided new information on the torque-generating interaction potential curve.