Single-cell impedance spectroscopy of bacteria

Abstract

Bacteria are the causative agents of many diseases, and understanding biophysical changes in bacteria following physical or chemical treatment is of considerable interest. In this thesis, different bacteria were measured using single-cell impedance cytometry in order to understand changes in electrical properties following exposure to different perturbations. Traditionally, alternating current (AC) electrokinetic methods are used to measure the electrical properties of cells. However, the throughput of these techniques is low. To overcome this limitation single-cell impedance cytometry measures the impedance of individual cells at hundreds per second.
In this study, the electrical properties of single cells were measured across a broad range of frequencies at the rate of tens of thousands of cells in a few minutes. The complex impedance spectrum was modelled using Maxwell’s mixture equation together with the multi-shell model to extract dielectric parameters for each bacterium.
Two model organisms were used for experiments: Escherichia coli and Staphylococcus aureus. Impedance cytometry was used to characterise the effect of heat treatment (including pasteurisation and autoclaving) on bacteria and results showed that the dielectric properties of the cells were in agreement with AC electrokinetic methods. Bacteria were also exposed to three classes of antibiotics, namely β-lactam, polymyxin and aminoglycoside. Dielectric characterisation demonstrated that bacteria respond differently following exposure to different antibiotics. Their phenotypic response changes their dielectric parameters in a way that is consistent with the mode of action of antibiotics.
Single-cell impedance cytometry was also used to follow bacterial-phage interaction. Experiments showed that the phenotypic response of bacteria can be followed during the phage infection cycle, and that the technique can rapidly identify bacterial susceptibility to phages. The data also helps explain the complex bacterial defence mechanisms against phages from a dielectric perspective.
In summary, this thesis describes an electrical method for probing bacteria, linking measured biophysical changes to biological responses, furthering our understanding of the action of antibiotics and phage therapies.

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Identifiers

ORCID for Xiang Wang: orcid.org/0009-0002-2495-6034

ORCID for Hywel Morgan: orcid.org/0000-0003-4850-5676

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