Impedance Microbiology: Applications in Food Hygiene

Abstract

Impedance microbiology is a rapid method that enables qualitative and quantitative tracing of microorganisms by measuring the change in the electrical conductivity. With direct impedance technology, the change in the conductivity of a liquid culture medium serves as a measuring parameter, whereas with indirect impediometry, the change in the electrical conductivity of a reaction solution, which occurs through the absorption of gases from the inoculated bacterial culture, is measured. Most investigations concerning the applicability of impediometry in food microbiology deal with the impedimetric detection or enumeration of Enterobacteriaceae, especially the detection of Salmonella. However, impediometry has been applied to other bacterial groups or species as well. Furthermore, a great number of published findings concern the impedimetric determination of the total bacterial count. The successful application of this fast method on further areas of food hygiene, such as tracing antibiotics and testing additives for their antimicrobiological effect, has also been described. In general the use of impediometry for the application areas stated has been judged positively. However, the time and expense required by the user to optimize the method, the deficits when testing slightly contaminated sample material or determining the bacterial count in those cases in which the microorganisms are sublethally damaged, and the necessity of performing individual calibration for each food category limit the applicability of impediometry.

Implications of Electrical Impedance-Based Microbiological Technology in Pork Meat Processing Industry for the Rapid Detection and Quantification of Salmonella Spp.

“…Only differences between results given by plate and impediometry appear for high levels of bacteria since the slope of the regression line becomes higher than 1. This fact may indicate a higher sensitivity of impediometry (112% ± 2%) against standard count plate (100%) which is in agreement with Donaghy and Madden [18], Bolton and Gibson [13], Quinn et al [21], Wawerla et al [22], Hoorfar [8] and Russell [14]. Moreover, higher errors are attained by standard plate count (horizontal error bars) compared to those of impedance-based method (vertical error bars) which reduces the reproducibility of the quantification assay when using classic microbiology [14,28].…”

“…Regarding the shape of the impedance growth curves, three regions are observed according to other authors [2,20,22,27,30]: (1) an initial region where the impedance change is hardly affected. Bacteria are adapting their metabolism to the nutrients present in the growth medium, so bacterial population is not high enough or their growth rates are not fast enough to produce detectable changes in impedance; (2) the second region where the impedance change sharply increases, since microorganisms have reached a concentration of ca.…”

“…This fact obliges to carry out a calibration for each strain with quantification purposes. As a result, since bacteria may be stressed by compounds present in either growth medium or matrix, a new calibration would be required for each matrix and growth medium to be tested [22].…”

“…These metabolites are responsible for the decrease in the impedance of the medium [2,[12][13][14]. After the important work of Torry Research Station [15] in 1978 and Eden and Eden [16] in 1984, and Owens et al [17] in 1989 reporting the principle of indirect impedance, several efforts have been made on the detection of Salmonella by impedance-based technology in food samples [9,12,13,[18][19][20][21][22][23].…”

Development of a novel conductance-based technology for environmental bacterial sensing

“…Use of impedance-based technology from classic impedance microbiology to novel on-chip impedance microbiology has been thoroughly reviewed somewhere [13,14]. However, most of these studies investigated the use of the technique for detecting and/or quantifying the food-borne bacteria based on the electrophysiological and impedance properties of biological cells.…”