Impedance, like microcalorimetry, measures the changes in the growth media due to the metabolic activities of the contained microorganisms. In particular, the breakdown of large weakly charged molecules, such as proteins, results in the formation of many strongly charged amino acid molecules. This shift in ionic strength can be indirectly measured by the resistance in the growth media to the passage of an electric current. The relationship is defined by the equation:
Z = impedance R = resistance c = capacitance f = frequency
In application the electrical signal of a culture is continuously monitored, and at a certain level, when the results of the organisms metabolism allows the conductance of the electricity to be measured — the so-called detection threshold — the presence of this growth is detected. The technique has been commercialized and is extensively used in the food industry where microbial levels and limits can be considerably higher than in the pharma industry.
The technique has a number of drawbacks. The media used is very specific and is not that featured in the major pharmacopoeia. The high limit of detection demands preincubation for most pharmaceuticals, hence the system can only be used for detection, not enumeration. Similarly, the time for very low levels of microorganisms to reach the detection threshold in what could be a hostile environment, can be extensive. Finally, a number of organisms (notably non-fermentative Gram-negative bacteria) do not produce significant changes in the electrical characteristics of the growth medium, again leading to long incubation periods. Despite these drawbacks, some success has been achieved in using impedance for preservative efficacy screening, although the authors are not aware that regulatory approval has been gained for this application.
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