Science 333, 345–348 (2011)

Credit: AAAS

Bacterial processes ranging from metabolism to flagellar motion are driven by changes in membrane potential (Vm), yet little is known about the electrophysiology of individual bacterial cells because they are too small to be probed by conventional techniques. To monitor Vm in individual live Escherichia coli cells, Kralj et al. developed a genetically encoded indicator based on a proteorhodopsin called PROPS that they engineered to change fluorescence with fluctuations in Vm. After calibrating PROPS with known Vm values, they went on to discover that many cells showed quasi-periodic blinks in fluorescence independently of one another. The blink durations varied from cell to cell, ranging from 1 to 40 seconds, as did the intensities of the blinks. The blinks did not correlate with intracellular pH changes; instead, they were linked to electrical fluctuations (probably depolarization), and blinking corresponded to a loss of proton motive force. Additionally, the cells had to be alive and undergoing aerobic respiration to blink, and blinking was correlated with rapid efflux of a membrane-permeable dye. These results suggest that blinking might arise from a stress response and that efflux may be electrically regulated.