High-frequency, dielectric spectroscopy for the detection of electrophysiological/biophysical differences in different bacteria types and concentrations

This paper describes a novel technique to quantify and identify bacterial cultures of Bacillus Subtilis (2.10–1.30 × 10⁹ CFU mL⁻¹) and Escherichia Coli (1.60–1.00 × 10⁹ CFU mL⁻¹), in corn oil using dielectric spectroscopy at elevated frequencies of 0.0100–20.0 GHz. This technique is using the electrophysiological/biophysical differences (e.g. gram positive and gram negative) between various bacteria types, as a basis to distinguish between bacteria concentrations and bacteria types. A close-ended, coaxial probe of 20.0 mm long sample-holder was developed and used to calculate the dielectric constant from the measured S parameters of the bacterial cultures, using the Nicolson-Ross-Weir method. This technique shows a linear relationship (r² ≥ 0.999) between the dielectric constant and the cell concentration, at 16.0 GHz. The sensitivity of the technique is 0.177 × 10⁹ (CFU mL⁻¹)⁻¹ for B. Subtilis (with a size of 10.0 × 1.00 μm), 0.322 × 10⁹ (CFU mL⁻¹)⁻¹ for E. Coli (with a size of 2.00 × 0.500 μm) and 0.913 × 10⁹ (CFU mL⁻¹) ⁻¹ for their 1:1 mixture, while the response time is 60.0s. The dependency of dielectric constant on the bacterial cell concentration at a given frequency can be potentially exploited for measuring bacterial concentrations and biophysical differences.