The Study On Bending Stiffness Of Flagellar Hooks Of Escherichia Coli Based On Bead Assay

Most bacteria are able to swim in fluids via rotation of flagella,each consists of a rotatory motor,a helical flagellar filament,and a flagellar hook that couples rotation of the motor and the filament.The swimming mode of the cell is closely related to the rotation direction of the motor.The motion state of monotrichous bacteria,such as Vibrio alginolyticus,alternates between forward and backward as the rotational direction of the motor changes.For peritrichous bacteria,such as Escherichia coli,when all motors rotate counterclockwise,their flagella are bundled together and promote the cell body to swim smoothly;while one or more motors rotate clockwise,the bacteria reorient the swimming direction.The hook is a kind of universal joint with good flexibility,and can effectively transmit the torque of the motor to the filaments.Previous studies have revealed that flagellar hooks have a strong effect on both the turning of monotrichous bacteria and the bundling of flagella in peritrichous bacteria.Therefore,the mechanical properties of the hook are critical for the normal function of the flagella.The bending stiffness of the flagellar hook was measured in vitro and in vivo,and the effect of counterclockwise torsion of the motor on the bending stiffness of the hook was also revealed.However,the effect of motor-generated clockwise torsion on the bending stiffness of the flagellar hook remains unclear.Here,we propose a geometric model based on elastic theory to accurately calculate the bending stiffness of flagellar hooks in living cells of Escherichia coli.We performed high spatiotemporal resolution microbead assay with variable external flow fields and measured the bending stiffness of the flagellar hooks of two mutants of Escherichia coli whose motors were driven exclusively in a counterclockwise or clockwise direction.We found that the bending stiffness of the hook under clockwise rotation is about twice as high as that under counterclockwise rotation,and this is not due to loading effects.Our findings contribute to the study of the asymmetric structure of the morphology of the flagellar hook,and the experimental approach can also be applied to other similar measurements.