Impedance microbiology

Principle of operation

When a pair of electrodes are immersed in the growth medium, the system composed of electrodes and electrolyte can be modeled with the electrical circuit of Fig. 1, where R_m and C_m are the resistance and capacitance of the bulk medium, while R_i and C_i are the resistance and capacitance of the electrode-electrolyte interface. However, when frequency of the sinusoidal test signal applied to the electrodes is relatively low (lower than 1 MHz), the bulk capacitance C_m can be neglected, and the system can be modeled with a simpler circuit consisting only of a resistance R_s and a capacitance C_s in series. The resistance R_s accounts for the electrical conductivity of the bulk medium, while the capacitance C_s is due to the capacitive double-layer at the electrode-electrolyte interface. During the growth phase, bacterial metabolism transforms uncharged or weakly charged compounds of the bulk medium into highly charged compounds that change the electrical properties of the medium. This results in a decrease of resistance R_s and an increase of capacitance C_s.

In impedance microbiology, the technique works this way. The sample with the initial unknown bacterial concentration (C₀) is placed at a temperature favoring bacterial growth (in the range of 37 to 42 °C if a mesophilic microbial population is the target), and the electrical parameters R_s and C_s are measured at regular time intervals of a few minutes by means of a couple of electrodes in direct contact with the sample.

Until the bacterial concentration is lower than a critical threshold, C_TH the electrical parameters R_s and C_s remain essentially constant (at their baseline values). C_TH depends on various parameters such as electrode geometry, bacterial strain, chemical composition of the growth medium, etc., but it is always in the range of 10⁶ to 10⁷ cfu/ml.

When the bacterial concentration increases over C_TH, the electrical parameters deviate from their baseline values (generally in the case of bacteria there is a decrease of R_s and an increase of C_s; the opposite happens in the case of yeasts).

The time needed for the electrical parameters R_s and C_s to deviate from their baseline value is referred to as Detect Time (DT) and is the parameter used to estimate the initial unknown bacterial concentration C₀.

In Fig. 2 a typical curve for R_s as well as the corresponding bacterial concentrations are plotted vs. time. Fig. 3 shows typical R_s curves vs time for samples characterized by different bacterial concentration. Since DT is the time needed for the bacterial concentration to grow from the initial value C₀ to C_TH, highly contaminated samples are characterized by lower values of DT than samples with low bacterial concentration. Given C₁, C₂ and C₃ the bacterial concentrations of three samples with C₁ > C₂ > C₃, it is DT₁ < DT₂ < DT₃. Data from literature show how DT is a linear function of the logarithm of C₀:

${\displaystyle DT=A\cdot {\log }_{10}(C_0)+B}$

where the parameters A and B are dependent on the particular type of samples under test, the bacterial strains, the type of enriching medium used, and so on. These parameters can be calculated by calibrating the system using a set of samples whose bacterial concentration is known and calculating the linear regression line that will be used to estimate the bacterial concentration from the measured DT.

Impedance microbiology has different advantages over the standard plate count technique to measure bacterial concentration. It is characterized by faster response time. In the case of mesophilic bacteria, the response times range from 2 – 3 hours for highly contaminated samples (10⁵ - 10⁶ cfu/ml) to over 10 hours for samples with very low bacterial concentration (less than 10 cfu/ml). As a comparison, for the same bacterial strains, the Plate Count technique is characterized by response times from 48 to 72 hours.

Impedance microbiology is a method that can be easily automated and implemented as part of an industrial machine or realized as an embedded portable sensor, while plate count is a manual method that needs to be carried out in a laboratory by trained personnel.