Effect of cell size and tethering geometry on rotation rate and torque of E.coli cells

The bacterial flagellum has a rotary motor embedded in its membrane surrounded by stator-units. Torque is generated by electro-steric interactions between rotor and stator-units. Chemotactic signals entail the motor to switch its direction of rotation. However, other factors such as protonmotive force and torque are involved in switching. In this work, we used peritrichously flagellated E.coli that stochastically tethers on surfaces in random geometries and studied how the cell size and position of the rotational axis affect the output of the motor. We developed a Cell Tethering Analysis Program (CTAP) to measure the length of the cells, the axis of rotation, rotational frequency of tethered cells. A D/L ratio (diameter traced by the cell body to the length of the cell body) was used to quantify the location of the rotational axis wherein, a D/L ratio of 1 and 1.9 means the axis of rotation is at the center of the cell body and near the tip of the cell body respectively. We performed experiments in controlled conditions and quantified the effect of cell size and tethering geometry on the output of the flagellar motor. The estimated torque of the tethered cells was 951 pN.nm and 1390 pN.nm for a D/L ratio of [~]1.0 and [~]1.9 respectively. As the torque increased, the motors rotated exclusively in CCW direction. We conclude that quantifying the cell size and tethering geometry is significant to characterize the output of the flagellar motors in a tethered cell assay.