Microfabricated device for impedance-based electronic detection of bacterial metabolism

Rafael Gomez-Sjoberg, Purdue University

Abstract

The objective of this research project was to develop a microfabricated technology platform for studying the use of a microscale impedance-based method for the detection of bacterial metabolism. Listeria innocua and Listeria monocytogenes were chosen as model microorganisms for testing the technology. Impedance-based detection of bacterial metabolism 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. Two silicon-based microfluidic biochip prototypes were fabricated, containing a network of microfluidic channels and chambers fitted with interdigitated metallic electrodes. The first biochip was used to demonstrate that the metabolic activity of bacterial cells can be detected in a few hours, and to study methods for extracting metabolism-related information from the impedance data. Bacterial suspensions were injected into the chip and incubated in a nutrient broth in a 5.3 nl chamber, at a temperature that results in optimal bacterial growth. During incubation the complex impedance of the suspensions was measured over time at several frequencies between 100 Hz and 1 MHz, using electrodes in direct contact with the suspensions. A circuit model of the electrodes was fitted to the measured impedance curves, and some of the parameters extracted from the fit, together with the raw impedance, were used as detection signals. The shape and magnitude of the changes in these parameters over time indicated the presence or absence of metabolic activity inside the chip. Impedance measurements of suspensions of live L. innocua and L. monocytogenes cells showed a very clear metabolic signal, compared with measurements done on suspensions of dead cells or of sterile media. The second biochip was designed to concentrate and capture cells from dilute suspensions, using dielectrophoresis, into a 0.4 nl chamber where incubation and impedance measurements were performed after the cells were captured. This biochip was used to concentrate cells from dilute suspensions by factors of 104 to 105. The metabolic activity of concentrated L. monocytogenes cells could be detected in one hour or less, while that of non-concentrated cells was detected after more than 7 hours of incubation.

Degree

Ph.D.

Advisors

Bashir, Purdue University.

Subject Area

Electrical engineering|Biomedical research

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