Dielectrophoretic properties of yeast cells dividing by budding and by transversal fission

Non-invasive, electro-orientation-based viability assay using optically transparent electrodes for individual fission yeast cells

A non-invasive assay of cylindrical yeast cell viability based on electro-orientation (EO) in an alternating electric field was developed, in which cell viability can be determined by each cell's EO direction without the need for reagents. A cell suspension of a few microliters was sandwiched between a pair of optically transparent indium-tin-oxide (ITO) plate electrodes. Observation under a light microscope enabled easy identification of EO based on cell shape, e.g., cells were standing upright and appeared perfectly circular when oriented parallel to the electric field direction (standing position), and they were lying flat and had an elongated shape when oriented perpendicular to the field (lain-down position). The alternative EO positions of living or dead cells were dependent on the applied frequency: opposite EO positions were obtained by applying an AC voltage of 1.5V at 10MHz; at which point, only living cells rapidly attained a standing position, whereas dead cells were lain-down within 10s. All the cell's EO positions agreed well with a viability assay by florescence staining. Therefore, at the single-cell level and fluorescently label-free, it was possible to simply and accurately determine whether individual cells were alive or dead based on their shape.

Orientation of Schizosaccharomyces POMBE Nonliving Cells under Alternating Uniform and Nonuniform Electric Fields

When nonliving cells of Schizosaccharomyces pombe were subjected to the action of alternating uniform and nonuniform electric fields, two types of orientation were produced. The first one, with its longest axis parallel to the field lines, is similar to that obtained with living cells. The second, perpendicular to the direction of the field, is produced for relatively high frequencies and low conductivities; this probably takes place when the conductivities of the external and internal media (cell cytoplasm) become equal. A mixed cell population is produced in a discrete interval of the parameters used. Our results provide direct evidence that cell alignment does not depend on the physiological state of the cells.

Dielectric spectroscopy of Schizosaccharomyces pombe using electrorotation and electroorientation

Two complementary AC electrokinetic techniques electrorotation (ER) and electroorientation (EO) enabled the dielectric characterization of the rod-shaped fission yeast Schizosaccharomyces pombe. The use of microstructured electrodes allowed both ER and EO measurements to be performed over wide ranges of field frequency and medium conductivity. Due to their layered structure, living S. pombe cells exhibited up to three well resolved peaks in their ER spectra and also two distinct orientations, i.e., parallel or perpendicular to the imposed linear field. Heat treatment and enzymatic protoplast isolation led to dramatic changes in the electrokinetic behavior of fission yeast. Application of the theoretical models linking the ER and EO spectra yielded the dielectric parameters of the major structural units of S. pombe cells (cell wall, plasma membrane and cytosol). The dielectric characterization of yeasts has an enormous impact in biotechnology and biomedicine, because electric field pulse techniques (electrofusion and electropermeabilization) are widely used for production of transgenic yeast strains of economic importance. The present study also showed that combined ER and EO measurements can be employed as a powerful diagnostic tool for analyzing changes in yeast structure and physiology upon exposure to various stress conditions.

Automatic cell electrorotation measurements: studies of the biological effects of low-frequency magnetic fields and of heat shock

A computer-aided automatic imaging technique has been developed for measuring the electrorotation spectra of up to 256 particles at the same time. This offers advantages over the conventional manual method, especially when rapidly acquired statistical data are necessary in investigations of the response of cells or test beads to chemical exposure, for example. We have applied this technique to investigate the biological effects of heat shock and low-frequency EM fields reported by others for yeast cells. Although heat shock effects were observed, no changes of the electrorotational behaviour could be detected after exposing the cells to 50 Hz, 8 and 80 microT fields. Although this does not rule out the possibility that the cells were influenced by the magnetic fields, it does limit the number of possible physicochemical changes that might have occurred to their cell walls and membranes.

Effect of biocide concentration on electrorotation spectra of yeast cells

The effect of the biocide Cosmocil (polyhexanide) at different concentrations on the electrorotation spectra of yeast cells is investigated over the frequency range from 1 kHz to 10 MHz. The dielectric properties of the yeast, before and after biocide treatment, were deduced from the electrorotation spectra using two-shell ellipsoid modelling methods that have been well tested for other heterogeneous biological systems. The results show a gradual increase in the cytoplasmic membrane conductivity with increasing biocide concentration, rather than an "all-or-nothing' breakdown of the membrane. The technique gives a quantitative analysis of the toxic damage by chemicals to cells and can be exploited in the development of new pharmacological agents.

Dielectric energy of orientation in dead and living cells of Schizosaccharomyces pombe. Fitting of experimental results to a theoretical model

Using the experimental data obtained with killed cells of Schizosaccharomyces pombe (1), we have formulated a theoretical model that is able to predict cell orientation for microorganisms with ellipsoidal or cylindrical shapes as a function of the frequency of the electric field and of the conductivity of the external medium. In this model, comparison of the difference in potential energy for both orientations parallel-perpendicular with the thermal agitation energy allows one to interpret the intervals where these orientations occur. The model implies that the conductivity of the cytoplasm is slightly higher than that of the external medium. This assumption is easy to understand taking into account that not all the intracytoplasmic material is released to the exterior during cell death.

Dielectrophoretic behavior of yeast cells: effect of growth sources and cell wall and a comparison with fungal spores

Saccharomyces cerevisiae showed different dielectrophoretic behavior depending on the source of carbon for growth. Growth on fermentable carbon sources produced a dielectrophoretic response that decreased according to the amount of sugar present in the culture medium. Growth on nonfermentable carbon sources produced a constant dielectrophoretic yield, independent of the amount and source of carbon present in the medium. The dielectrophoretic yield, however, was independent of the nitrogen source. The yield spectrum for S. cerevisiae protoplasts was similar to that for the cells, although a decrease in the absolute value was observed. This decrease could be explained by the reduction in cell size and by assuming that the cell wall contributes a negative net charge to the yield. Fungal spores responded to the nonuniform electric field in the same range of frequencies as assayed for yeast cells.