Impedancimetric bacterial detection: theoretical and experimental aspects

By means of the bipolar impedance technique, we detected bacterial growth in an inoculated broth as its time course absolute impedance. From it, the impedance change relative to sterile medium was obtained, calculating also its time derivative. The repeatability of the derivative curves (they overlapped within a band better than 3.3%) permitted the identification of a double-hump pattern which, in principle, could be accepted as an indicator of the type of bacteria (Escherichia coli). After six experimental series, the growth curves appeared as sensitive to the initial concentration of bacteria and to the culture time preceding inoculation; they were also dependent on the temperature and on the average basal impedance. Temperature showed a greater effect (one order of magnitude) on the lag-phase of the growth curve than on the stationary-phase. This effect occurs because the impedance growth curves tend to get away from the reference offered by the sterile medium. The best working conditions were obtained for an average basal impedance of 510 ohms under well controlled temperature conditions (variations smaller than or equal to 0.20 degrees C) with wire stainless steel electrodes vertically immersed in the culture broth. This impedance technique appears as inexpensive and easy to automatizing for large number of samples.