Electrical breakdown, electropermeabilization and electrofusion

Hydrophobic and electrostatic interaction chromatography for estimating changes in cell surface charge of Escherichia coli cells treated with pulsed electric fields

Pulsed electric field (PEF) treatments, a nonthermal process, have been reported to injure and inactivate bacteria in liquid foods. However, the effect of this treatment on bacterial cell surface charge and hydrophobicity has not been investigated. Apple juice (pH 3.8) purchased from a wholesale distributor was inoculated with cocktail of Escherichia coli O157:H7 at 7.4 log CFU/mL, processed with a PEF at a field strength of 18.4 kV/cm and 32.2 kV/cm at 25°C, 35°C, and 45°C with a treatment time of 160 μs and a flow rate of 120 mL/min. Bacterial cell surface charge and hydrophobicity of untreated and PEF-treated E. coli O157:H7 were determined immediately and after storage at 5°C and 23°C using hydrophobic and electrostatic interaction chromatography. Similarly, the populations surviving the PEF treatments including injured cells were determined by plating 0.1 mL of the sample on sorbitol MacConkey agar and tryptic soy agar (TSA) plates. The surviving populations of E. coli cells after PEF treatment varied depending on field strength and treatment temperature used. Percent injury in the surviving populations was high immediately after PEF treatment and varied among treatment temperatures. Cell surface charge of E. coli bacteria before PEF treatment averaged 32.10±8.12. PEF treatments at 25°C, 35°C, and 45°C reduced the above surface charge to 26.34±1.24, 14.24±3.30, and 6.72±2.82, respectively. Similarly, the surface hydrophobicity of untreated E. coli cells at 0.194±0.034 was increased to an average of 0.268±0.022, 0.320±0.124, and 0.586±0.123 after PEF treatments at 25°C, 35°C, and 45°C, respectively. The results of this study indicate that PEF treatment affects the outer cell envelope of E. coli bacteria as evidenced by the changes in surface hydrophobicity and cell surface charge leading to injury and subsequent inactivation of the cells.

Pulsed electric field-assisted vinification of aglianico and piedirosso grapes

Pulsed electric field (PEF) treatments were applied to increase the polyphenolic content of fresh red wines made from Aglianico and Piedirosso grapes. Prior to the fermentation/maceration step, the grape skins were treated at different PEF intensities (field strengths from 0.5 to 1.5 kV/cm and energy inputs from 1 to 50 kJ/kg), with their permeabilization being characterized by electrical impedance measurements. Furthermore, the release kinetics of the total polyphenols and anthocyanins were characterized during the maceration stage by spectroscopic and Folin-Ciocalteu colorimetric methods, respectively. Finally, the fresh wine, obtained after pressing, was characterized for total acidity, pH, reducing sugar, color intensity, total polyphenols, anthocyanins content, antioxidant activity, and volatile compound composition. PEF treatment on Aglianico grapes induced a significantly higher release of polyphenols (+20%) and anthocyanins (+75%), thus improving the color intensity (+20%) and the antioxidant activity of the wine (+20%) while preserving the other organoleptic characteristics. In contrast, there was only a minor impact on the polyphenolic release kinetics of Piedirosso grapes, despite the significant degree of cell membrane permeabilization.

Axon repair: surgical application at a subcellular scale

Injury to the nervous system is a common occurrence after trauma. Severe cases of injury exact a tremendous personal cost and place a significant healthcare burden on society. Unlike some tissues in the body that exhibit self healing, nerve cells that are injured, particularly those in the brain and spinal cord, are incapable of regenerating circuits by themselves to restore neurological function. In recent years, researchers have begun to explore whether micro/nanoscale tools and materials can be used to address this major challenge in neuromedicine. Efforts in this area have proceeded along two lines. One is the development of new nanoscale tissue scaffold materials to act as conduits and stimulate axon regeneration. The other is the use of novel cellular-scale surgical micro/nanodevices designed to perform surgical microsplicing and the functional repair of severed axons. We discuss results generated by these two approaches and hurdles confronting both strategies.

The influence of a pulsating electric field on selenium accumulation inKluyveromyces marxianus cells

K. marxianus was cultivated under conditions of PEF (pulse electric field) action and a selenium source. The culture duration after which cells were treated with PEF, the field exposure time and the selenium concentration in the medium were all optimized. Optimization of culture duration caused a 33% increase in selenium accumulation in cells as compared to the control with no PEF treatment. The highest selenium accumulation--about 167 microg/g dry mass (DM)--was recorded after 3-minute PEF treatment of 16-hour culture. A roughly two-fold increase in selenium content was achieved after optimization of culture duration and PEF treatment time. Finding the optimum selenium concentration in the medium brought about a 13-fold increase of selenium accumulation in the cells of K. marxianus.

Electrical forces for microscale cell manipulation

Electrical forces for manipulating cells at the microscale include electrophoresis and dielectrophoresis. Electrophoretic forces arise from the interaction of a cell's charge and an electric field, whereas dielectrophoresis arises from a cell's polarizability. Both forces can be used to create microsystems that separate cell mixtures into its component cell types or act as electrical "handles" to transport cells or place them in specific locations. This review explores the use of these two forces for microscale cell manipulation. We first examine the forces and electrodes used to create them, then address potential impacts on cell health, followed by examples of devices for both separating cells and handling them.

Electric pulses induce cylindrical deformations on giant vesicles in salt solutions

In this article, we report for the first time unusual shape changes of vesicles subjected to strong electric pulses in salt solutions of low concentration. The electric field is created by two parallel electrodes between which the vesicle solution is located. Surprisingly, the vesicles assume cylindrical shapes during the pulse. These deformations are short-lived (their lifetime is approximately 1 ms) and occur only in the presence of salt outside the vesicles, irrespective of their content. When the solution conductivities inside and outside are the same, vesicles with square cross section are observed. Using a fast digital camera, we were able to record these deformations and study the vesicle shape dynamics. The aims of this article are to report the new vesicle morphologies and their dynamics and to provoke theoretical work in this direction.

A practical guide to giant vesicles. Probing the membrane nanoregime via optical microscopy

Research on giant vesicles is becoming increasingly popular. Giant vesicles provide model biomembrane systems for systematic measurements of mechanical and rheological properties of bilayers as a function of membrane composition and temperature, as well as hydrodynamic interactions. Membrane response to external factors (for example electric fields, ions and amphiphilic molecules) can be directly visualized under the microscope. In this paper we review our current understanding of lipid bilayers as obtained from studies on giant unilamellar vesicles. Because research on giant vesicles increasingly attracts the interest of scientists from various backgrounds, we also try to provide a concise introduction for newcomers in the field. Finally, we summarize some recent developments on curvature effects induced by polymers, domain formation in membranes and shape transitions induced by electric fields.

Inactivation of foodborne viruses of significance by high pressure and other processes

The overall safety of a food product is an important component in the mix of considerations for processing, distribution, and sale. With constant commercial demand for superior food products to sustain consumer interest, nonthermal processing technologies have drawn considerable attention for their ability to assist development of new products with improved quality attributes for the marketplace. This review focuses primarily on the nonthermal processing technology high-pressure processing (HPP) and examines current status of its use in the control and elimination of pathogenic human viruses in food products. There is particular emphasis on noroviruses and hepatitis A virus with regard to the consumption of raw oysters, because noroviruses and hepatitis A virus are the two predominant types of viruses that cause foodborne illness. Also, application of HPP to whole-shell oysters carries multiple benefits that increase the popularity of HPP usage for these foods. Viruses have demonstrated a wide range of sensitivities in response to high hydrostatic pressure. Viral inactivation by pressure has not always been predictable based on nomenclature and morphology of the virus. Studies have been complicated in part from the inherent difficulties of working with human infectious viruses. Consequently, continued study of viral inactivation by HPP is warranted.

The early stages of Saccharomyces cerevisiae yeast suspensions damage in moderate pulsed electric fields

The objectives of this study were to investigate the effects of pulsed electric fields (PEF) application to colloidal suspension of Saccharomyces cerevisiae. The electrical conductivity measurements during the PEF-treatment of S. cerevisiae suspensions were used to monitor the extent of cell damages in the intervals of electric field strength E = 3-15 kV/cm and time of PEF treatment t(PEF) = 10(-4) to 1s. At relatively small fields (E < 7.5 kV/cm) the early stages of yeast cells damages were observed. At such treatment conditions, the damage was incomplete and developed at long time of PEF treatment, below the value of E = 7.5 kV/cm which is commonly referred in literature as a threshold for this culture. Data obtained for the disintegration in conductivity experiments were found in good correlation with direct counting of yeast lethality using light microscopy. The PEF-induced lethality of the yeast cells and size flocs increased with the mixing of suspensions and the increase of temperature.

Inactivation of Escherichia coli, Listeria innocua and Saccharomyces cerevisiae in relation to membrane permeabilization and subsequent leakage of intracellular compounds due to pulsed electric field processing

Membrane permeabilization, caused by pulsed electric field (PEF) processing of microbial cells, was investigated by measurement of propidium iodide (PI) uptake with flow cytometry. Inactivation of Escherichia coli, Listeria innocua and Saccharomyces cerevisiae was determined by viable counts, and leakage of intracellular compounds, such as ATP and UV-absorbing substances, was measured in the extracellular environment. Electrical field strength and pulse duration influenced membrane permeabilization of all three tested organisms of which S. cerevisiae was the most PEF sensitive, followed by E. coli and L. innocua. It was shown by viable counts, PI uptake and leakage of intracellular compounds that L. innocua was the most resistant. Increased inactivation corresponded to greater numbers of permeabilized cells, which were reflected by increased PI uptake and larger amounts of intracellular compounds leaking from cells. For E. coli and L. innocua, a linear relationship was observed between the number of inactivated cells (determined as CFU) and cells with permeated membranes (determined by PI uptake), with higher number of inactivated cells than permeated cells. Increased leakage of intracellular compounds with increasing treatment severity provided further evidence that cells were permeabilized. For S. cerevisiae, there was higher PI uptake after PEF treatments, although very little or no inactivation was observed. Results suggest that E. coli and L. innocua cells, which took up PI, lost their ability to multiply, whereas cells of S. cerevisiae, which also took up PI, were not necessarily lethally permeabilized.

Growth of pulsed electric field exposed Escherichia coli in relation to inactivation and environmental factors

Pulsed electric fields (PEF) have been proven to inactivate microorganisms during nonthermal conditions and have the potential to replace thermal processing as a method for food preservation. However, there is a need to understand the recovery and growth of survivors and potentially injured microorganisms following PEF processing. The purpose of this investigation was to study the growth of Escherichia coli at 10 degrees C following exposure to electrical field strengths (15, 22.5 and 30 kV/cm) in relation to inactivation and the amount of potentially sublethally injured cells. One medium was used as both a treatment medium and an incubation medium, to study the influence of environmental factors on the inactivation and the growth of the surviving population. The pH (5.0, 6.0 and 7.0) and water activity (1.00, 0.985 and 0.97) of the medium was varied by adding HCl and glycerol, respectively. Growth was followed continuously by measuring the optical density. The time-to-detection (td) and the maximum specific growth rate (micromax) were calculated from these data. Results showed that the PEF process did not cause any obvious sublethal injury to the E. coli cells. The number of survivors was a consequence of the combination of electrical field strength and environmental factors, with pH being the most prominent. Interestingly, the micromax of subsequent growth was influenced by the applied electrical field strength during the process, with an increased micromax at more intense electrical field strengths. In addition, the micromax was also influenced by the pH and water activity. The td, which could theoretically be considered as an increase in shelf life, was found to depend on a complex correlation between electrical field strength, pH and water activity. That could be explained by the fact that the td is a combination of the number of survivors, the recovery of sublethal injured cells and the growth rate of the survivors.

Generation of bactericidal and mutagenic components by pulsed electric field treatment

Inactivation of stationary phase Escherichia coli, Salmonella Typhimurium and Listeria innocua (10(8) CFU/ml) by high intensity pulsed electric fields (PEF) was studied in water and different buffers at pH 7.0. The fraction of survivors after PEF treatment with 300 pulses (5 Hz) of 26.7 kV/cm and a pulse width of 2 micros varied between 0.050% and 55%, but was always lower in Tris-HCl buffer than in HEPES-KOH buffer and water. When cell suspensions were stored for 24 h at 25 degrees C after PEF treatment, the survivor fraction further decreased, except for E. coli in water and HEPES-KOH. By following the survival of untreated cells added to water or buffers that were previously PEF treated, this secondary inactivation could be ascribed to the formation of bactericidal components as a result of PEF treatment. Buffers and water containing 10 mM NaCl became bactericidal against all three bacteria upon PEF treatment, and the bactericidal effect could be neutralized by thiosulfate, suggesting that chlorine and/or hypochlorite had been formed. Also in the absence of Cl- ions, PEF treated water and buffers had bactericidal properties, but the specificity of the bactericidal effects against different bacteria differed depending on the buffer used. In the Ames mutagenicity test using His- S. Typhimurium mutant strains, PEF treated Tris buffers containing 10 mM Cl- ions, as well as PEF treated grape juice showed a mutagenic effect. The implications of these findings for the safety of PEF treated foods are discussed.

Inactivation of bacteria in seawater by low-amperage electric current

Seawater used in mariculture has been suspected of being a potential source of infection. In this study, the lethal effects of low-amperage electric treatment on microorganisms were examined in natural seawater and in seawater inoculated with Vibrio parahaemolyticus. In both cases, bacteria including V. parahaemolyticus in seawater were completely eliminated in 100 ms by a 0.5-A, 12-V direct current. Electron microscopic investigation of the electrically treated bacteria revealed substantial structural damage at the cellular level. In conclusion, our results indicate that low-amperage electric treatment is effective for rapid inactivation of microorganisms in seawater.

Comparison of sublethal injury induced in Salmonella enterica serovar Typhimurium by heat and by different nonthermal treatments

We have studied sublethal injury in Salmonella enterica serovar Typhimurium caused by mild heat and by different emerging nonthermal food preservation treatments, i.e., high-pressure homogenization, high hydrostatic pressure, pulsed white light, and pulsed electric field. Sublethal injury was determined by plating on different selective media, i.e., tryptic soy agar (TSA) plus 3% NaCl, TSA adjusted to pH 5.5, and violet red bile glucose agar. For each inactivation technique, at least five treatments using different doses were applied in order to cover an inactivation range of 0 to 5 log units. For all of the treatments performed with a technique, the logarithm of the viability reductions measured on each of the selective plating media was plotted against the logarithm of the viability reduction on TSA as a nonselective medium, and these points were fined by a straight line. Sublethal injury between different techniques was then compared by the slope and the y intercept of these regression lines. The highest levels of sublethal injury were observed for the heat and high hydrostatic pressure treatments. Sublethal injury after those treatments was observed on all selective plating media. For the heat treatment, but not for the high-pressure treatment, sublethal injury occurred at low doses, which were not yet lethal. The other nonthermal techniques resulted in sublethal injury on only some of the selective plating media, and the levels of injury were much lower. The different manifestations of sublethal injury were attributed to different inactivation mechanisms by each of the techniques, and a mechanistic model is proposed to explain these differences.

Highly efficient, large volume flow electroporation

Electroporation is widely used to transfect and load cells with various molecules. Traditional electroporation using a static mode is typically restricted to volumes less than 1 mL, which limits its use in clinical and industrial bioprocessing applications. Here we report efficient, large volume transfection results by using a scalable-volume electroporation system. Suspended (Jurkat) and adherent cells (10T1/2 and Huh-7) were tested. A large macromolecule, FITC-conjugated dextran (MW=500 kD) was used to measure cell uptake, while a plasmid carrying the gene coding for enhanced green fluorescence protein (eGFP) was used to quantitate the flow electrotransfection efficiency as determined by flow cytometry. The flow electroloading efficiency of FITC-dextran was >90%, while the cell viability was highly maintained (>90%). High flow electrotransfection efficiency (up to 75%) and cell viability (up to 90%) were obtained with processing volumes ranging from 1.5 to 50 mL. No significant difference of electrotransfection efficiency was observed between flow and static electrotransfection. When 50 mL of cell volume was processed and samples collected at different time points during electroporation, the transgene expression and cell viability results were identical. We also demonstrated that DNA plasmid containing EBNA1-OriP elements from Epstein-Barr virus were more efficient in transgene expression than standard plasmid without the elements (at least 500 too 1000-fold increase in expression level). Finally, to examine the feasibility of utilizing flow electrotransfected cells as a gene delivery vehicle, 10T1/2 cells were transfected with a DNA plasmid containing the gene coding for mIL12. mIL12 transfected cells were injected subcutaneously into mice, and produced functional mIL12, as demonstrated by anti-angiogenic activity. This is the first demonstration of efficient, large volume, flow electroporation and the in vivo efficacy of flow electrotransfected cells. This technology may be useful for clinical gene therapy and large-scale bioprocesses.