Dynamics of the bacterial flagellar motor with multiple stators - Supporting Information

The measurement of the torque-speed curve is usually done by fixing the cell to a glass slide and tethering a polystyrene bead to the flagellar hook. An optical trap monitor the rotational speed of the bead and the motor torque is calculated from τ = (ξL + ξR)ω ≈ ξLω, where ξR is the drag coefficient due to the internal friction in the motor, ξL is the bead drag coefficient, and ω is the angular velocity. The torque-speed curve is then obtained changing ξL by varying the bed size [1] or the viscosity of the external medium. An alternative method is to tether a cell to a glass coverslip by one of its shortened flagellar filament and expose the cell to a rotating electric field [2]. Then the motor is broken spinning the cell backward. The difference between the cell body speeds before and after the broke of the motor, at the same value for the external applied torque, is proportional to the motor torque.