IMPEDANCE BACTERIOMETRY - MEDIUM AND INTERFACE CONTRIBUTIONS DURING BACTERIAL-GROWTH

We measured impedance 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 at different frequencies (from 18 Hz to 18 kHz) with a constant current impedance bridge as growth proceeded. It was found that: 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 18-18 000 Hz range. At low frequency, the resistive and capacitive reactive components, or R(b), and X(b), respectively, were directly measured, where R(b), = (2 . R(i) + R(m)) and X(b) = 2 . X(i) and, at high frequency (above 5 kHz), R(m) was obtained (for Z(i) is negligible). Thus, R(i) was easily discriminated from R(m) by simple arithmetic: R(i) = [R(b) (low f) - R(b) (high f)]/2. 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.