The MicroTester developed by our company is based on the detection of the change in redox-potential caused by microbial activity. The evaluation system of the instrument is similar to the impedimetric method’s, but with wider field of application.
The energy source of the microbial growth is the biological oxidation which results in a reduction in the environment. This is due to the oxygen depletion and the production of reducing compounds in the nutrient medium.
The redox potential is one of the most complex indicators of the physiological state of microbial cultures and its measurement could be a useful tool for the qualitative and quantitative determination of the microbial contamination.
MicroTester can be used favourably for evaluating the classical membrane-filtration and surface swabbing methods. During inspection of the microbial count of natural water sources, drinking water, mineral water and other beverages usually consecutive members of the dilution series are membrane-filtered and placing the filters on agar
surface, the samples are incubated. The reason for using different dilutions in the classical method is that the samples can be evaluated reliably only in a specific interval (between 30-300 cells). Because MicroTester – with determining the calibration curves – is able to carry out measurements and reliably determines the cell concentration in a wide interval (between 1-107 cells), there is no need for dilution series; it is enough to make only one membrane filtration from a sample. Similarly, in case of the microbiological control of surfaces by swabbing, the swab could be put directly into the measuring cell, without any washing and dilution.
During the laboratory and industrial validation of MicroTester the typical characteristics were considered:
- Selectivity: it depended on the media used for identification. MicroTester is able to work with any commercial nutrient medium.
- Linearity: defining the time required to reach a significant change in redox potential, a strict linear correlation could be established between the ‘Time to Detection’ and the logarithm of the initial concentration of microorganisms (above concentration of 1 cfu/test flask).
- Sensitivity: 1 logarithm unit increase in the initial cell concentration decreases the TTD values with 50-130 minutes, depending on the microorganisms tested.
- Detection limit: at minimum 1 cell/test flask. MicroTester is suitable for the absence/presence control of the microbes in the sample.
- Quantitation limit: the theoretical quantitation limit is 10 cell/inoculum (1 log unit), which is in agreement with the obtained calibration curves.
- Range: on the base of the calibration curves the range lasted from 1 to 7 log unit. Below 10 cells the Poisson-distribution causes problems in quantitation (but in detection not!), over 107 cells the TTD is too short comparing to the transient processes (temperature-, redox- equilibrium, lag-period of the growth).
- Accuracy/Trueness: As the redox-potential measuring method is based on the regression equation representing the connection between the logarithm of the real cell concentration and detection time, the accuracy of the method depends on the reliability of the calibration curves. Each combination of microorganisms and culture broth require special calibration curve.
- Precision (repeatability, reproducibility): during the validation of the system the repeatability and reproducibility were determined.
- Robustness: the most important parameter is the temperature, which has a double effect on the results – the growth rate of the microorganisms and the measured redox-potential are temperature depending. Performing the measurements at the temperature optimum of microorganisms, the growth rate in a ±0.5 °C interval does not change. The influence of the temperature on the measured redox-potential was determined experimentally. The results showed that the effect of the temperature variation is negligible.
- O. Erdősi, K. Szakmár, O. Reichart, P. Székely-Körmöczy, P. Laczay (2012): Application of the redox potential measurementbased rapid method in the microbial hygienic control; Acta Alimentaria, Vol. 41(1), pp. 45-55, DOI: 10.1556/AAlim.2011.0005
- O. Reichart, K. Szakmár, Á. Jozwiak, J. Felföldi, L. Baranyai (2007): Redox potential measurement as a rapid method for microbiological testing and its validation for coliform determination; International Journal of Food Microbiology, Vol. 114(2), pp. 143-148, DOI: 10.1016/j.ijfoodmicro.2006.08.016