On the application of optical tweezers and electro-rotation to study rotational dynamics of flagellar motors

The bacterial flagellar motor is a rotary molecular engine driven by an electrochemical gradient; which turns a helical filament generating the thrust enabling the bacteria to swim. The mechanism of torque generation has been described by different models, each of them predicting a different relationship between speed and torque. In order to rule out the less effective models in favor of more plausible ones, Berg and Berry focused on one particular aspect of the rotational dynamics of flagellar motors, i.e. whether or not there is a barrier to the backwards rotation of the motor. Such a barrier, if present, should manifest itself in the form of a higher value of torque when the motor is driven against its natural direction of rotation beyond some limiting speed (Berry and Berg, Torque generated by the Flagellar motor of Escherichia coli while driven backward ). This was indeed the observation made in 1993 by Berg and Turner when they used the technique of electro-rotation (to be described further below), to study the process. However, in 1996 and 1999, Berg and Berry demonstrated that the observed doubling of torque for backwards rotation was not intrinsic to the motor, but merely a consequence of the particular experimental technique they were using. This claim was strengthened further when they employed a different technique i.e. optical tweezers (Berry and Berg)to show that the motor generated the same torque regardless of its direction of rotation. In this paper, we give a critical review of their approach towards concluding that there is no barrier to backwards rotation of the motor, and what implications their study has in arriving at particular functional models. . Major physical models predicting mechanistic details of torque generation are reviewed and the best possible model based on predictions made by a simple three step kinetic model is selected.