Non-invasive determination of bacterial single cell properties by electrorotation

So far, electrorotation and its application to the determination of single cell properties have been limited to eukaryotes. Here an experimental system is described that allows the recording of electrorotation spectra of single bacterial cells. The small physical dimensions of the developed measuring chamber combined with a single frame video analysis made it possible to monitor the rotation of objects as small as bacteria by microscopical observation despite Brownian rotation and cellular movement. Thus physical properties of distinct organelles of E. coli could be simultaneously determined in vivo at frequencies between 1 kHz and 1 GHz. Experimental data were evaluated following a three-shell model of the cell. Electrical conductivities of cytoplasm and outer membrane were determined to 4.4 mS/cm and 25 μS/cm, respectively, that of the periplasmic space was found to increase with the square root of the medium ionic strength. Specific capacitances of inner and outer membrane amounted to 1.4 μF/cm2 and 0.26 μF/cm2, respectively, the thickness of the periplasm to about 50 nm. Heat treatment of the cells lead to a reduction of cytoplasmic conductivity to 0.9 mS/cm, probably caused by an efflux of ions through the permeabilized inner membrane.