Low-frequency dielectric dispersion of bacterial cell suspensions

•Dielectric properties of Escherichia coli cells were studied by dielectric spectroscopy.•Low-frequency (LF) dielectric dispersion was found around 1 kHz.•The LF dispersion was not related to whether cells are alive or dead.•Counterion polarization provided a reasonable explanation for the LF dispersion.

Dielectric spectra of Escherichia coli cells suspended in 0.1–10 mM NaCl were measured over a frequency range of 10 Hz to 10 MHz. Low-frequency dielectric dispersion, so-called the α-dispersion, was found below 10 kHz in addition to the β-dispersion, due to interfacial polarization, appearing above 100 kHz. When the cells were killed by heating at 60 °C for 30 min, the β-dispersion disappeared completely, whereas the α-dispersion was little influenced. This suggests that the plasma (or inner) membranes of the dead cells are no longer the permeability barrier to small ions, and that the α-dispersion is not related to the membrane potential due to selective membrane permeability of ions. The intensity of the α-dispersion depended on both of the pH and ionic strength of the external medium, supporting the model that the α-dispersion results from the deformation of the ion clouds formed outside and inside the cell wall containing charged residues.

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