(240f) Dielectrophoretic Fractionation of a Mixture of Bacteria and Yeast

Insulator-based dielectrophoresis (iDEP) is a novel technique used for the manipulation of particles by employing insulating structures to create an electric field gradient¹. In this work, trapping, separation and fractionation of S. cerevisiae (ATCC 24858) and E. coli (Top10) cells, employing direct current (DC) electric fields, through an array of cylindrical insulating posts is presented.

Dielectrophoresis (DEP) is defined as the motion of particles resulting from polarization effects induced by non-uniform electric fields^2,3. This technique has a great potential for miniaturization, which allows for faster results, less reagents consumption and therefore less waste generation for the analysis of samples. DEP has also been proven as an effective way of manipulating microbes. However, majority of the research studies reported on dielectrophoretic manipulation of microorganisms were performed employing electrode-based DEP and alternating current electric fields.

In this work we report the fractionation of a mixture of E. coli and S. cerevisiae cells employing DC-iDEP.  A glass microdevice containing 24 microchannels that were 10.2-mm long, 2-mm-wide and 7-μm-deep, with different arrays of insulating structures was employed. The microchannels used for these experiments contained an array of 32 cylindrical posts straddled by an inlet and outlet reservoirs (Figure 1). The insulating post array consists of four rows of eight posts each with a diameter 440-μm and arranged 520-μm center-to-center. A DC electric field was applied across the length of the microchannel to create the non-uniform electric field across the array of insulating posts. Different buffer solutions of controlled pH and conductivity were employed to evaluate the effect of suspending medium properties on the dielectrophoretic response of each cell type to determine the minimum voltage required for DEP trapping.

Figure 1. Schematic representation of experimental set-up. a) Top view of microchannel. b) Side view with reservoirs and electrodes.

Figure 2 shows that the separation between yeast and bacteria is possible since yeast cells require a lower electric field in order to be dielectrophoretically immobilized. This difference is mainly due to size, yeast cells have a diameter of 5 mm while E. coli cells have a diameter of 1.1 mm. Figure 2a shows DEP response of yeast cells employing a suspending medium with pH 6 and conductivity of 50-μS/cm and an electric field of 300-V/cm. It can be seen that under this applied field, yeast cells are moving mainly under the influence of electrokinetic force. In Figure 2b, employing same suspending medium conditions, but increasing the electric field to 800-V/cm, DEP overcomes electrokinetic force and yeast cells are trapped, showing negative dielectrophoretic trapping, forming a band of cells prior to the region higher electric field intensity (the narrow section between the posts). Yeast cells are being repulsed from this region.

E. coli cells also exhibited negative dielectrophoresis, Figure 2c shows the response of bacteria cells employing the same suspending medium and an electric field of 400-V/cm. Under these conditions, it is possible to observe that DEP begins to struggle with the electrokinetic force, but still cells are passing through the array. In Figure 2d an increase of electric field to 1200-V/cm allows DEP force to overcomes electrokinetic force and bacteria cells are trapped, forming a band prior to the regions of higher field intensity.

Figure 2. DEP performance employing pH 6 and conductivity of 50-μS/cm with a) yeast cells applying 300, b) yeast cells applying 800-V/cm, c) bacteria cells applying 400-V/cm and d) bacteria cells applying 1200-V/cm.

The results presented confirm that DC-iDEP has the potential to achieve the fractionation of complex mixtures of microorganisms. DEP can be employed as separation and concentration technique for microbes in microfluidic devices.

References.

1. Lapizco BH, Simmons BA, Cummings EB, Fintschenko Y. Insulator-Based Dielectrophoresis for the Selective Concentration and Separation of Live Bacteria in Water. Electrophoresis (2004), 25(10-11): 1695-1704.
2. Li H, Zheng Y, Demir A, Bashir R. Characterization and modeling of a microfluidic dielectrophoresis filter for biological species. Journal of Microelectromechanical Systems (2005), 14(1).
3. Castellarnau M, Errachid A, Madrid C, Juárez A, Smaitier J. Dielectrophoresis as a tool to characterize and differentiate isogenic mutants of Escherichia coli. Biophysical Journal (2006), 91: 3937-3945.