Revealing bacterial free energy dynamics during loss of viability

Proton Motive Force (PMF) is an electrochemical gradient of protons across a biological membrane that consists of two components: membrane potential and the difference in pH. It plays a crucial role in the energetics of life, powering the production of ATP, transport of different molecules across the membrane, and as in the case of Escherichia coli, motility. As a coarse grain model of E. coli energetics, the cell can be approximated by a simple electrical circuit, with a battery and internal resistance representing catabolism. In external environments of pH equal to that of E. coli’s cytoplasm (~7.5) membrane potential drop in the circuit is equal to PMF. This representation allows us to ‘probe’ different components of the circuit by measuring PMF directly. For example, we can estimate changes in the resistance of the membrane under different stress conditions, and from it, infer the characteristics of the stress applied. To measure PMF in individual E. coli cells we use the bacterial flagellar motor and back focal plane interferometry while exposing the cells to several different stresses: photodamage, butanol, and indole. From the measurements, and using the circuit analogy we offer explanations for the nature of the membrane damage caused by each stress and discuss the model predictions for the overall cell energy maintenance.