Torque Generated in the Puller Mode Is Higher Than That in the Pusher Mode in Monoflagellated Bacteria

A helical flagellar filament in Caulobacter crescentus spins CW to push a cell forward and CCW to pull it backward.  Here, we studied cells that stuck to glass surfaces and pivoted about an axis normal to the surface.  The rotation of the cell was driven by hydrodynamic interactions between the filament and the glass.  We inferred the sign and relative magnitudes of the torque generated by the flagellar motor from the direction and speed of rotation of the cell body.  From these measurements, we determined that the filament spins twice as fast in the CCW direction.  In other words, the flagellar motor generates a higher torque when C. crescentus cells swim backward.  Yet, the backward and forward swimming speeds are identical, possibly due to the contribution of body-precession to the thrust when swimming.  The biological significance of higher motor torque in the puller state is that it aids Caulobacter crescentus cells in pulling away from the stalked mother cells during cell-division.