Impedance bacteriometry: medium and interface contributions during bacterial growth

Impedance was measured in a cell containing culture broth inoculated with E. coli, before and during bacterial growth. The electrode interface impedance components (R/sub i/, X/sub i/) and the culture medium component R/sub m/ were separated by making use of the Warburg's model frequency dependent properties. Measurements were carried out from 18 Hz to 18 kHz with a constant current impedance bridge as growth proceeded. Growth curves for R/sub i/ and X/sub i/ showed a similar temporal pattern within the frequency range of 18-100 Hz. Dispersion was not observed in R/sub m/, meaning that the same growth response was obtained within the 18-18000-Hz range. At low frequency, the resistive and capacitive reactive components, or R/sub i/ and X/sub b/, respectively, were directly measured, where R/sub b/=(2*R/sub i/+R/sub m/) and X/sub b/=2*X/sub i/ and, above 5 kHz, R/sub m/ was obtained (for Z/sub i/ is negligible). Thus, R/sub i/ was easily discriminated from R/sub m/ by simple arithmetic. In four experiments, the maximum spread of X/sub i/, R/sub i/, and R/sub m/ was smaller than 5% indicating good repeatability. There is potential new information in dissecting out the growth curve in three separate component curves.<
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