Impediometric Technique as a Rapid Estimation to Detect Pathogenic Bacteria in Quality Control

Abstract

It is of interest for drug- and food-producing industries to acquire information about the microbiological quality of their products as early as possible. Conventional mi- crobiological methods for the detection and identification of pathogens include microbiological culturing and isola- tion of the pathogens, followed by approbation with bio- chemical and serological tests. These methods are time- consuming and labor-intensive, not suitable for a rapid re- sponse for high-risk pathogens. As a result, several instru- ments have been developed for use much earlier, including nucleic acid-based tests, serological methods, and some automated instrumental diagnostic assays (1). The impedance measurement is used as a rapid method to detect microbial growth. It is based on a measurement of the changes in electrical conductance of the culture media that are produced by microbial metabolism. Impedance is the resistance to flow of an alternating current through a conducting material. The examined microorganisms are cultivated in impedance glass tubes (cells) in fluid growth medium, with highly uncharged molecules that are fitted with measuring electrodes. The culture medium is metabolized by micro- bial activity, and the impedance changes and decreases because of the production of small charged molecules by the microorganism's metabolites (2). The impedance measurement is a complex entity composed of a combi- nation of a conductive element and a capacitive element. Therefore, in monitoring microbial growth, conductance measurements can indicate changes taking place in the bulk solution or medium (3). The specific growth medium used in the impediometric technique supports the target bacteria, and must be formulated to provide optimal impedance signals to provide specificity to the impedance microbiological method (2). Impedance measurements are based on the bacte- rial growth and metabolic processes that produce electri- cally measurable changes in the growth medium. These changes are due to the metabolism of high-molecular- weight nutrients into smaller charged ionic molecules, which increase the electricity conductivity of the spe- cific medium. Therefore, the impedance technique dis- tinguishes viable and dead pathogens rapidly, within 24 hours (4). Variation in electrical conductivity is related to the change in bacterial number, and thus bacterial growth can be measured. Accordingly, impedance measurements are considered for quality control in the food, drug, and hygiene-product industries, especially for the identifica- tion and enumeration of indicator microorganisms (2, 5). In glass tubes, the electrical change is recorded using a pair of electrodes immersed in the growth medium. The impedance measurement can be done by two methods: di- rect or indirect. In the direct method, a pair of metal elec- trodes is submerged in the medium that is inoculated with the testing sample. The electrical change caused by the re- lease of ionic metabolites from live cells in the medium is monitored over time (6). In the indirect method, the electrodes are immersed in a separate solution (usually a potassium hydroxide solution) instead of the inoculated growth medium. The gases (mainly CO2) released from the metabolism of bacteria are absorbed by the potassium hy- droxide solution, which leads to decreased conductance of the alkaline solution. The indirect method is applicable to all microorganisms, including yeasts and molds (2, 6). In the impedance technique, by means of an elec- trode system in the glass tubes, impedance values can be recorded over time for a pre-selected incubation period (7). The concentration of ions generated by the bacteria reach a magnitude at which a measurable increase in conductiv- ity can be detected. The bacterial level associated with this change in conductance is called the bacterial threshold level, and the impedance detection time (IDT) is defined as the point in hours. Usually, the IDT is inversely propor-