High frequency electric polarizability of bacteria E. coli: Dependence on the medium ionic strength

There are two main kinds of electric polarizability of bacteria: surface charge dependent (SChD) and Maxwell–Wagner (MW) polarizability. The aim of this article is to distinguish SChD and MW components on the external bacteria surface. An electro-optic method (electric turbidity) was used to study the polarizability of E. coli fixed by formaldehyde at the frequency range 20 kHz to 20 MHz. According to the literature the SChD polarization disappears at such high frequencies and MW one gives the main contribution. However we found unexpected dependence on the outer medium electrolyte concentration, which cannot be explained by MW polarization. The results show that the polarizability decreases by ionic strength increasing in the same way as the double electric layer thickness does. Such behaviour is a characteristic for SChD polarizability, which allows us to conclude that this component has the main contribution on the external bacteria surface at the experimental frequencies mentioned.