Microfabricated Device for Impedance-Based Detection of Bacterial Metabolism
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Microfabricated Device for Impedance-Based Detection of Bacterial Metabolism Rafael G´omez1 , Michael R. Ladisch2 , Arun K. Bhunia3 , and Rashid Bashir1,4, ∗ 1 School of Electrical and Computer Engineering, 2 Department of Agricultural and Biological Engineering, 3 Department of Food Science, 4 Department of Biomedical Engineering Purdue University, West Lafayette, Indiana 47907, U.S.A. ABSTRACT We present the use of a microfabricated device for impedance-based detection of a few live bacterial cells. Impedance-based detection relies on measuring changes in the AC impedance of two electrodes immersed in a liquid were the bacteria are cultured, caused by the release of ionic species by metabolizing bacterial cells. Rapid detection of a few cells (1 to 10) is possible if the cells are confined into a volume on the order of nanoliters. A microfluidic biochip prototype has been fabricated to test this miniaturized assay. The conductance of the bacterial suspensions is extracted from measuring their complex impedance in a 5.27 nl chamber in the biochip, at several frequencies between 100 Hz and 1 MHz. Measurements on suspensions of the bacteria Listeria innocua, Listeria monocytogenes, and Escherichia coli in a low conductivity buffer demonstrate that, under the current experimental conditions, the minimum detection level is between 50 and 200 live cells, after two hours of off-chip incubation. Work is in progress to develop techniques for selective capture of bacteria inside the chip, and to minimize background changes in impedance during on-chip incubation. INTRODUCTION Detection of a few live pathogenic bacterial cells in food, water, or clinical samples is an important technological problem. In the US food-borne pathogens cause an estimated 14 million illnesses, and 1800 deaths each year [1]. Current official food screening procedures established by regulatory agencies are very sensitive, but require up to 7 days to complete, with the work of highly skilled laboratory personnel [2, 3]. There is an urgent need for a food screening method able to detect between 100 and 1000 live bacterial cells per milliliter of sample, with a sample volume of a couple of milliliters, in no more than about four hours (total time, including sample collection and preparation). One common automated bacterial detection method is based on the changes in the AC impedance of two electrodes immersed in a medium where bacteria are cultured [4,5]. These changes are produced by the release of ionic metabolites by live cells. If the impedance changes beyond a certain threshold, a positive detection is indicated. The main process responsible for ion release by bacteria into their environment is the energy metabolism (catabolism). Both aerobic and anaerobic catabolism can produce carbonic acid (from CO2 ), and organic acids such as lactic, acetic, and formic. Unfortunately, the detection time of the conventional impedance-based method can be quite long when the number of bacterial cells present in the sample is very small, because ∗
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