Figure 1.

(A) Electric circuit equivalent of an E. coli cell. Oxidative (or substrate-level) phosphorylation is shown as a battery V_c with an internal resistance R_i, the membrane with capacitance C and resistance R_e, and i₁ to i₃ are the currents. Bacterial flagellar motor (BFM) is shown as a “voltmeter” that measures membrane potential, V_m. (B) Shown is a schematic of the “bead-assay” and back focal-plane interferometry. A cell is attached to a cover glass with a truncated flagellar filament made “sticky” to polystyrene beads. The bead is brought into a heavily attenuated optical trap, and its position is measured with position sensitive detector. I₁ to I₄ indicate currents read by the position-sensitive detector at four different locations (see Materials and Methods). (C) An example of raw motor speed trace recorded with back focal-plane interferometry is shown. Positive frequencies correspond to counterclockwise and negative to the clockwise rotation of the flagellar motor (27). In the subsequent figures, we show absolute values of the rotational speeds. To see this figure in color, go online.