Measurements of ion-motive force across the cell membrane

Cells need energy to survive. Ion-motive force (IMF) is one of the most important biological energy formats in bacterial cells. Essentially, the ion-motive force is the sum of electrical and chemical potential differences across the cell membrane. For bacteria, the ion-motive force is involved not only in ATP production but also in flagellar motility. The bacterial flagellar motor is driven either by proton or sodium ion. The ion-motive force measurement therefore requires the measurement of membrane potential, proton concentration, or sodium ion concentration. The bacterial flagellar motor is the most powerful molecular machine we have known so far. To understand the energetic condition of bacterial flagellar motors, together with single-motor torque measurement, methods for single-cell ion-motive force measurement have been developed. Here, we describe fluorescent approaches to measure the components of ion-motive force.