Selective detection of viable bacteria using dielectrophoretic impedance measurement method

This paper describes a selective detection technique of viable bacteria based on dielectrophoresis and electrical impedance measurements. The authors have previously proposed a detection technique of biological particles called dielectrophoretic impedance measurement (DEPIM) method using positive dielectrophoretic force to capture biological cells in suspension onto an interdigitated microelectrode array. By combining antigen-antibody reaction with the DEPIM, selective detection of a particular species of bacteria was demonstrated. In this present work, the authors demonstrated another selective DEPIM method utilizing cell viability dependency of dielectrophoretic force without introducing the antigen-antibody reaction. It was found that dielectrophoresis of heat-treated Escherichia coli showed strong dependency on viability when applied field frequency was as high as 1 MHz. As a result, viable bacteria could be exclusively collected by positive dielectrophoresis and selectively detected by the DEPIM technique from a suspension also containing heat-treated nonviable cells. On the other hand, nonviable bacteria obtained by UV irradiation showed little dielectrophoresis dependency on viability. According to a theoretical analysis of the dielectrophoretic force, it is suggested that heat treatment alters the dielectric properties of treated cells. In particular, a decrease in cytoplasmic conductivity, which might be caused by heat-induced perforation of cell membrane, was expected to considerably affect dielectrophoresis characteristics. Proposed selective DEPIM method was also applied to evaluation of heat sterilization effect on a real time basis. It was experimentally proved that DEPIM could evaluate viable cell number variation with heat treatment time in a considerably shorter time than conventional microbiological method based on cell incubation.