The flagellar motor of Caulobacter crescentus generates more torque when a cell swims backward.

Caulobacter crescentus , a monotrichous bacterium, swims by rotating a single right-handed helical filament. CW motor rotation thrusts the cell forward ¹ , a mode of motility known as the pusher mode; CCW motor rotation pulls the cell backward, a mode of motility referred to as the puller mode ² . The situation is opposite in E. coli , a peritrichous bacterium, where CCW rotation of multiple left-handed filaments drives the cell forward. The flagellar motor in E. coli generates more torque in the CCW direction than the CW direction in swimming cells ^3,4 . However, monotrichous bacteria including C. crescentus swim forward and backward at similar speeds, prompting the assumption that motor torques in the two modes are the same ^5,6 . Here, we present evidence that motors in C. crescentus develop higher torques in the puller mode than in the pusher mode, and suggest that the anisotropy in torque-generation is similar in two species, despite the differences in filament handedness and motor bias (probability of CW rotation).