Voltage-Dependent Dielectrophoretic Response of Bacterial Cells: Implications for Reliable Cell Manipulation

Rapid bacterial detection is important for environmental monitoring, particularly for identifying microbial contamination in water resources. Dielectrophoresis (DEP) provides a label-free approach for bacterial detection by exploiting differences in cellular dielectric properties. Our previous studies established bacterial movement velocity, force, and crossover frequency as quantitative parameters toward the development of a DEP-based detection platform. However, optimization of the operating conditions is necessary to ensure reliable measurement. During voltage optimization, unexpected positive DEP (pDEP) was observed at low frequencies where negative DEP (nDEP) was expected, particularly at 20 Vpp. This study therefore investigated whether applied voltage affects bacterial viability and membrane integrity during DEP experiment. Escherichia coli and Staphylococcus aureus (pH 7), were exposed to 3–20 Vpp at 50 kHz, followed by pDEP quantification, colony-forming unit (CFU) analysis, Live/Dead fluorescence staining, and transmission electron microscopy (TEM). Despite increasing pDEP occurrence at higher voltages, CFU analysis showed no significant differences in culturability (p > 0.05). In contrast, fluorescence and TEM analyses revealed voltage-dependent changes in membrane integrity and structure. These findings indicate that DEP exposure can induce sublethal membrane perturbation sufficient to alter bacterial dielectric response without measurable loss of culturability. Appropriate voltage selection is therefore critical for preserving measurement reliability in DEP-based bacterial detection and its future application to environmental monitoring.