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 _i, X _i) and the culture medium component R _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 _i and X _i showed a similar temporal pattern within the frequency range of 18-100 Hz. Dispersion was not observed in R _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 _i and X _b, respectively, were directly measured, where R _b=(2*R _i+R _m) and X _b=2*X _i and, above 5 kHz, R _m was obtained (for Z _i is negligible). Thus, R _i was easily discriminated from R _m by simple arithmetic. In four experiments, the maximum spread of X _i, R _i, and R _m was smaller than 5% indicating good repeatability. There is potential new information in dissecting out the growth curve in three separate component curves.