Development, validation, and applications of a new laboratory-scale indirect impedancemeter for rapid microbial control

Abstract.

We introduce a new laboratory-scale impedance-meter which is specially intended for indirect technique. It consists of a software system enabling data acquisition via a connected bus which is wired to the measuring cells. These measuring cells are individual impedance-meters that can be activated independently of one another. In the current configuration, the device is slightly affected by temperature, but it can register as little as 10.9 µmol of CO₂. With Bacillus subtilis, Escherichia coli, and Saccharomyces cerevisiae cultures, the conductance responses were highly replicable and repeatable for inocula concentrations of 1–10⁸ colony-forming units (CFU) ml⁻¹. The main use for such devices could be the detection of contamination in foodstuffs. Several of these foodstuffs, when incubated at 37 °C, spontaneously release quite large amounts of CO₂. Our impedancemeter, however, was able to detect an E. coli presence in canned French beans at 2.35×10⁻² CFU ml⁻¹ and a S. cerevisiae contamination of apple purée in glass jars at 6.1×10⁻³ CFU ml⁻¹. The conductance response and the detection time (the time needed for a significant change in conductance) were correlated to the concentration of ampicillin (an antibiotic added to E. coli cultures). The device is thus able to detect the presence of inhibitory compounds in milk or other foodstuffs. Some industrial assays are in process to complement these laboratory tests. Compared with other available techniques for CO₂ measurement (manometry, infrared, radioactive labeling), the technique put forward here appears to be the best compromise between sensitivity, technical constraints, and cost. A commercial version of the impedancemeter would enable routine measurements in the quality control of foodstuffs, pharmaceuticals, cosmetics and in R&D laboratories.