Biomechanics Bacterial flagellar switching under load

Flagellated bacteria swim up gradients of chemical attractants by modulating the direction of rotation of their flagellar motors, but how this switching mechanism works is not understood. Here we show that the probability of the motor rotating in the clockwise direction increases under high load, when the motor spins slowly (at less than 50 Hz). We suggest that either the switch is responding to small changes in torque - the torque increases only fractionally between 50 Hz and stall - or it senses motor speed, perhaps by means of proton flux.