Microbiological implications of electric field effects VII. Stimulation of plasmid transformation ofBacillus cereus protoplasts by electric field pulses

Factors and Conditions That Impact Electroporation of Clostridioides difficile Strains

Understanding the underlying biology of pathogens is essential to develop novel treatment options. To drive this understanding, genetic tools are essential. In recent years, the genetic toolbox available to Clostridioides difficile researchers has expanded significantly but still requires the conjugal transfer of DNA from a donor strain into C. difficile. Here we describe an electroporation-based transformation protocol that was effective at introducing existing genetic tools into different C. difficile strains.

An important risk factor for acquiring Clostridioides difficile infection is antibiotic use. Therefore, a detailed knowledge of the physiology and the virulence factors can help drive the development of new diagnostic tools and nonantibiotic therapeutic agents to combat these organisms. Several genetic systems are available to study C. difficile in the laboratory environment, and all rely on stably replicating or segregationally unstable plasmids. Currently, the transfer of plasmids into C. difficile can only be performed by conjugation using Escherichia coli or Bacillus subtilis as conjugal donors. Here we report a method to introduce plasmid DNA into C. difficile using electroporation and test factors that might contribute to higher transformation efficiencies: osmolyte used to stabilize weakened cells, DNA concentration, and recovery time postelectroporation. Depending on the C. difficile strain and plasmid used, this transformation protocol achieves between 20 and 200 colonies per microgram of DNA and is mostly influenced by the recovery time postelectroporation. Based on our findings, we recommend that each strain be tested for the optimum recovery time in each lab.

IMPORTANCE Understanding the underlying biology of pathogens is essential to develop novel treatment options. To drive this understanding, genetic tools are essential. In recent years, the genetic toolbox available to Clostridioides difficile researchers has expanded significantly but still requires the conjugal transfer of DNA from a donor strain into C. difficile. Here we describe an electroporation-based transformation protocol that was effective at introducing existing genetic tools into different C. difficile strains.

Abiotic Gene Transfer: Rare or Rampant?

Phylogenetic studies reveal that horizontal gene transfer (HGT) plays a prominent role in evolution and genetic variability of life. Five biotic mechanisms of HGT among prokaryotic organisms have been extensively characterized: conjugation, competence, transduction, gene transfer agent particles, and transitory fusion with recombination, but it is not known whether they can account for all natural HGT. It is even less clear how HGT could have occurred before any of these mechanisms had developed. Here, we consider contemporary conditions and experiments on microorganisms to estimate possible roles of abiotic HGT-currently and throughout evolution. Candidate mechanisms include freeze-and-thaw, microbeads-agitation, and electroporation-based transformation, and we posit that these laboratory techniques have analogues in nature acting as mechanisms of abiotic HGT: freeze-and-thaw cycles in polar waters, agitation by sand at foreshores and riverbeds, and lightning-triggered electroporation in near-surface aqueous habitats. We derive conservative order-of-magnitude estimates for rates of microorganisms subjected to freeze-and-thaw cycles, sand agitation, and lightning-triggered electroporation, at 10²⁴, 10¹⁹, and 10¹⁷ per year, respectively. Considering the yield of viable transformants, which is by far the highest in electroporation, we argue this may still favor lightning-triggered transformation over the other two mechanisms. Electroporation-based gene transfer also appears to be the most general of these abiotic candidates, and perhaps even of all known HGT mechanisms. Future studies should provide improved estimates of gene transfer rates and cell viability, currently and in the past, but to assess the importance of abiotic HGT in nature will likely require substantial progress-also in knowledge of biotic HGT.

Improved method for high-efficiency electrotransformation of Escherichia coli with the large BAC plasmids

High transformation competency of Escherichia coli is one of the critical factors in the bacterial artificial chromosome (BAC)-based DNA library construction. Many electroporation protocols have been published until now, but the majority of them was optimized for transformation of small plasmids. Large plasmids with a size above 50 kbp display reduced transformation efficiency and thereby require specific conditions in the preparation and electroporation of electrocompetent cells. In the present work, we have optimized the parameters critical to the application of BAC DNA electrotransformation into E. coli. Systematic evaluation of electroporation variables has revealed several key factors like temperature of growth, media supplements, washing buffer, and cell concentration. Improvements made in the transformation protocol have led to electrocompetent cells with transformation efficiency up to 7 × 10(8) transformants per microgram of 120 kbp BAC plasmid DNA. We have successfully used in-house prepared competent cells, the quality of which is comparable with those produced by different companies, in the construction of metagenomic libraries from the soil. Our protocol can also be beneficial for other application with limited DNA source.

Lightning-triggered electroporation and electrofusion as possible contributors to natural horizontal gene transfer

Phylogenetic studies show that horizontal gene transfer (HGT) is a significant contributor to genetic variability of prokaryotes, and was perhaps even more abundant during the early evolution. Hitherto, research of natural HGT has mainly focused on three mechanisms of DNA transfer: conjugation, natural competence, and viral transduction. This paper discusses the feasibility of a fourth such mechanism--cell electroporation and/or electrofusion triggered by atmospheric electrostatic discharges (lightnings). A description of electroporation as a phenomenon is followed by a review of experimental evidence that electroporation of prokaryotes in aqueous environments can result in release of non-denatured DNA, as well as uptake of DNA from the surroundings and transformation. Similarly, a description of electrofusion is followed by a review of experiments showing that prokaryotes devoid of cell wall can electrofuse into hybrids expressing the genes of their both precursors. Under sufficiently fine-tuned conditions, electroporation and electrofusion are efficient tools for artificial transformation and hybridization, respectively, but the quantitative analysis developed here shows that conditions for electroporation-based DNA release, DNA uptake and transformation, as well as for electrofusion are also present in many natural aqueous environments exposed to lightnings. Electroporation is thus a plausible contributor to natural HGT among prokaryotes, and could have been particularly important during the early evolution, when the other mechanisms might have been scarcer or nonexistent. In modern prokaryotes, natural absence of the cell wall is rare, but it is reasonable to assume that the wall has formed during a certain stage of evolution, and at least prior to this, electrofusion could also have contributed to natural HGT. The concluding section outlines several guidelines for assessment of the feasibility of lightning-triggered HGT.

Plasmid uptake by bacteria: a comparison of methods and efficiencies

The ability to introduce individual molecules of plasmid DNA into cells by transformation has been of central importance to the recent rapid advancement of plasmid biology and to the development of DNA cloning methods. Molecular genetic manipulation of bacteria requires the development of plasmid-mediated transformation systems that include (1) chemical transformation, (2) electro-transformation, (3) biolistic transformation, and (4) sonic transformation, leading to the introduction of exogenous plasmid DNA into bacterial cells. In this review, the manipulation properties and transformation efficiencies of these techniques are described. In addition to these methods, a conceptually novel transformation technique, namely the hydrogel exposure method, was developed. The hydrogel exposure method, based on the Yoshida effect, provides a significant advance over chemical means for transforming many strains of Escherichia coli and a variety of other bacterial species. The new term "tribos transformation" has been proposed for this novel technique. We also determined that, compared to conventional methods, the hydrogel exposure method is a novel and convenient method by which to transform bacteria.

Electroporation: an arsenal of application

Electroporation is a way to induce nanometersized membrane pore for exogenous substances delivery into cytoplasm using an artificial electric field. Now it was widely used for molecules transfer especially in molecular experiments and genetic aspects. In recent years, modern electroporation on the embryo was developed, whose most important point is that it adopts low energy and rectangular pulse that could obtain high transfection efficiency and low damage to the embryo. This paper reviewed on the pool of application: from lab works to human clinical treatments.

Improvement of transformation and electroduction in avermectin high-producer, Streptomyces avermitilis

Factors affecting the PEG-mediated transformation and electrotransformation of Streptomyces avermitilis protoplasts, an industrial avermectin high-producer, were evaluated. The maximum protoplast transformation efficiency under optimum conditions with PEG was 3 x 106 transformants per microg plasmid pIJ702 DNA. The efficiency of electrotransformation with the same plasmid the intact cells grown in medium with 0.5 mmol/L CaCl2, suspended in buffer with 0.5 mol/L sucrose +1 mmol/L MgCl2, and pulsed at an electric field strength of 10 kV/cm, 800 ohms, 25 microF, was of 2 x 10(3) transformants per microg DNA. When the cells were electroporated after mild lysozyme-treatment, the efficiency was up to 10(4) transformants per microg DNA. Electroporation of protoplasts and germlings had a lower efficiency (10(2) transformants per microg DNA). We report that electroporation under optimum conditions can be used for direct transfer of nonconjugative plasmid pIJ699 between two different Streptomyces species, S. avermitilis and S. lividans.

The effects of medium-strength electric impulses on human blood

Leukocyte subsets, total leukocyte isolates or full blood samples were subjected to medium-strength square-wave electric impulses (100 V/cm field force, 5 ms duration). On the surface of the leukocytes, the expressions of several markers (CD3, CD4, CD8, CD11a, CD11b and ICAM-1) were determined in order to study the influence of pulsed ionic currents on different aspects of the cellular immune response. Large individual differences were observed among randomly chosen healthy donors, both in the initial expression rate and in the response patterns of different antigens. As a general conclusion, it can be stated that electric impulses with the above parameters activate the state of immune response alertness of human leukocytes. Changes in the activities of several enzymes in the serum in response to electric impulses were also tested in order to examine the feasibility of ex vivo electric treatment of human blood for the establishment of an antiviral and immune activated condition. Slightly elevated lactate dehydrogenase (LDH) levels point to a possibility of enhanced haemolysis, while the lack of an elevation in the membrane-bound peroxidase activity indicates the absence of haemolysis. Significant rises were detected in the serum superoxide dismutase (SOD) activities. Since most ex vivo blood manipulations are characterised by the appearance of superoxide radicals in the serum, a SOD activity enhancement is considered beneficial in these cases. A mild, but significant reduction in the blood clotting time indicates that electric treatment of human blood should be performed with special attention to thrombosis-prone conditions, and adequate precautions and countermeasures should be introduced. Although wider examinations are required before this method can be fully recommended, ex vivo blood treatment with medium-strength electric impulses seems to be a promising adjuvant course for the establishment of acute immune potentiation and an antiviral state in patients undergoing dialysis treatment.

Factors affecting the electroporation of Bacillus subtilis

Bacillus subtilis 168 trp- was found to be transformable with the tetracycline resistance plasmid pAB124 by electroporation of whole cells, inconsistently and at very low frequencies. Supplementation of the growth medium with glycine, or particularly DL-threonine, produced cells that could be electrotransformed much more efficiently at frequencies up to 2.5 x 10(3) transformants per microgram plasmid DNA. Transformation was optimal with cells grown in medium containing a racemic mixture of the D- and L-isomers of threonine, and no transformants were obtained when pure forms of the D- and L-threonine isomers were used. The cell walls of B. subtilis grown in the presence or absence of D-, L- and DL-threonine had a similar amino acid composition which did not include threonine. A more complex biochemical explanation of the enhancement of electroporation by growth in DL-threonine is likely, and this is discussed. Lysozyme treatments to weaken the cell wall and possibly mimic the effect of DL-threonine did not yield any transformants. The effects of buffer composition and culture incubation time were also determined and the electroporation protocol optimized accordingly. The response of a range of other B. subtilis strains to electroporation by the method produced was found to be variable. In all cases, transformation was verified by recovery of the plasmid DNA from putative transformants.

Use of microbial spores as a biocatalyst

Endospores of a bacterium Bacillus subtilis and ascospores of a yeast Saccharomyces cerevisiae contained almost all the activities for the same enzymes as vegetative cells. The biotechnological potential of spores was studied by selecting adenosine 5'-triphosphatase and alkaline phosphatase in bacterial and yeast spores, respectively, as model enzymes. The activity of both enzymes was efficiently expressed when the spores were treated by physical (sonication or electric field pulse) and chemical (organic solvents or detergents) methods. The yeast spores were immobilized in polyacrylamide gel without any appreciable loss of activity. The immobilized spores were packed in a column and used successfully for the continuous reactions of alkaline phosphatase and glyoxalase I. The microbial spores were confirmed to be promising as a biocatalyst for the production of useful chemicals in bioreactor systems.

Spectrofluorometric assay for the quantitation of cell-tissue electrofusion

Fluorometric assay for quantitating electrofusion, FAQE, was developed to measure the number of cells fused to intact tissue. The fluorescent vital dye hydroethidine is used in this method. The fluorescent intensity detected in the cell tissue hybrids is proportional to the number of individual cells fused. The number of cells fused was determined after fusion by lysing the epithelial layer with 0.2% sodium dodecyl sulfate and the supernatant fluids were measured in a spectrofluorometer and compared to established standard curves. The mean number of cells fused, in five separate experiments, was determined to be approximately 5000. All the experimental corneas had approximately the same number of fused cells with less than 10% variation. In addition, the technique was used to demonstrate an increase in the number of cells fused when multiple fusions were applied to the cell-tissue electrofusion system. These results demonstrate that FAQE can be utilized to quantitatively analyze the fusion yields.

Optimized conditions for electrotransformation of bacteria are related to the extent of electropermeabilization

Electropulsation is a simple and efficient way to introduce cloned genes into a variety of cell types, even with walled species. In the case of bacteria, we observed no direct correlation between survival rate and transformation yield. In the present work, we show that the yield of transformation is directly related to the level of the electric-field induced level of cell permeabilization. From experiments on Escherichia coli, it was confirmed that the extent of associated ATP leakage was a reliable assay. This approach was extended to other strains, such as Salmonella typhimurium, which to date had not been electrotransformed by plasmids.

Bacterial genetics by electric shock

Subjecting bacteria to a high-voltage electric discharge renders the cells permeable to DNA. This powerful method allows the genetic manipulation of bacterial species that cannot easily be transformed by conventional techniques.

Genetic transformation of Lactobacillus casei by electroporation

Lactobacillus casei IAM1045 was transformed with a plasmid pAM beta 1-1, a tra deleted derivative of pAM beta 1, by electroporation. Effective transformation was achieved in electroporation buffers of a wide range of pH values, and in all phases of cell growth tested, with highest frequency in the early log phase. Polyethylene glycol increased the transformation frequency, whereas divalent cations such as Mg2+, Ca2+ and Mn2+ at 0.25 mM decreased the frequency by 2 to 3 orders. Highly efficient transformation of approximately 10(-4)/viable cell was achieved under optimal conditions. A plasmid harboring the trpD, C, F, B and A genes from L casei RNL7 was introduced by electroporation into tryptophan auxotrophic L casei JCM1053. The resulting transformant was found to express the trp genes introduced.

Electroporation-mediated transformation of lysostaphin-treated Clostridium perfringens

A reliable and efficient method has been developed for the electroporation-mediated transformation of Clostridium perfringens with plasmid DNA. Transformation of vegetative cells of C. perfringens strain 13 with the 7.9-kb Escherichia coli-C. perfringens shuttle plasmid pHR 106 required pretreatment with lysostaphin (2 to 20 micrograms/ml) for 1 h at 37 degrees C. Cells harvested early in the logarithmic stage of growth were transformed more efficiently than cells at other growth phases. The transformation frequency increased with the DNA concentration, to a saturating level at 5 to 10 micrograms DNA/ml. The transformation frequency was proportional to the field strength and time constant of the electroporation pulse; however, the field strength was a far more important parameter. A cell density between 1 x 10(8) and 5 x 10(8) cells/ml proved to be optimal for transformation. The procedure was capable of generating up to 3.0 x 10(5) transformants per micrograms DNA. The potential value of the method for the cloning of C. perfringens genes was demonstrated by the cloning of the clostridial tetracycline-resistance determinant, tetP, from the E. coli recombinant plasmid pJIR71, into C. perfringens strain 13.

Efficient transformation of Bacillus thuringiensis and B. cereus via electroporation: transformation of acrystalliferous strains with a cloned delta-endotoxin gene

Electroporation was used as a method to transform intact cells of Bacillus thuringiensis and B. cereus. With our optimized method a range of plasmid vectors could be transformed into strains of B. thuringiensis at frequencies of up to 10(7) transformants/micrograms DNA. This high frequency allows cloning experiments to be done directly in B. thuringiensis. A bifunctional vector capable of replicating in Escherichia coli and in Bacillus spp. was constructed. The kurhd1 protoxin gene was cloned into this shuttle vector to produce plasmid pX193, then transformed into B. thuringiensis HD1 cryB and B. cereus 569K. The cloned protoxin gene was expressed in sporulating cultures of both strain HD1 cryB (pX193) and 569K (pXI93), producing crystal protein active in biotests against larvae of Heliothis virescens. This demonstrates the usefulness of the electroporation method for the introduction of cloned toxin genes, in either their native or modified form, into a variety of host strains.

Transient increase of the intracellular Ca2+ concentration during chemotactic signal transduction in Dictyostelium discoideum cells

In general, calcium has been believed to control a variety of cellular processes as a signal transducer, with a high degree of spatial and temporal precision. For the determination of intracellular free-calcium concentrations [( Ca2+]i), the highly selective Ca2+ indicators, quin2/AM and fura2/AM, have been widely used in many mammalian and plant cells. However, intact cells of the cellular slime mold Dictyostelium discoideum Ax-2 are generally impermeable to externally added drugs, thus resulting in a failure to determine [Ca2+]i. Introduction of quin2/AM and fura2/AM by electroporation allowed us to measure [Ca2+]i in D. discoideum cells. The fluorescence images of fura2-loaded single cells showed that resting [Ca2+]i in vegetative and aggregation-competent cells is around 50 nM. Caffeine (10 mM) gave a transient increase in [Ca2+]i, which illustrated a normal responsive ability of electroporated cells to the externally added stimulus. Application of the chemoattractant, cAMP (20 nM), to aggregation-competent cells induced a rapid increase in [Ca2+]i within 1-2 s, and the [Ca2+]i level increased to about four-fold higher than the resting [Ca2+]i within 30 s of chemotactic stimulation. This was followed by a gradual decrease of [Ca2+]i to the basal level. These results strongly suggest that [Ca2+]i is a primary messenger in signal transduction, particularly during the chemotactic response of Dictyostelium cells.

Application of electroporation for transfer of plasmid DNA to Lactobacillus, Lactococcus, Leuconostoc, Listeria, Pediococcus, Bacillus, Staphylococcus, Enterococcus and Propionibacterium

Plasmid DNA was introduced by electroporation into Bacillus, Enterococcus, Lactobacillus, Lactococcus, Leuconostoc, Listeria, Pediococcus, Propionibacterium and Staphylococcus as an alternative to competent-cell or protoplast transformation. Plasmid-containing transformants were recovered in these recipients at frequencies ranging from 10(1) to 10(5) transformants micrograms-1 of pGK12. Several parameters of the protocol, including DNA concentration, voltage, plating regimen and electroporation buffers were evaluated to determine conditions that improved transformation frequencies for Lactobacillus acidophilus. Using optimized conditions, the following plasmids were introduced into L. acidophilus: pAMB1, pC194, pGB354, pGKV1, pSA3, pTRK13, pTV1 and pVA797. The ability to transfer plasmid DNA via eletroporation will greatly facilitate the application of recombinant DNA methodology and transposon technology to Gram-positive bacteria for cloning and analysis of significant genes.

Electro-stimulated transformation of E. coli cells pretreated by EDTA solution

The phenomenon of transformation of E. coli cells under electric treatment has been studied. The cells of strains MH 1, HB 101 and DH 1 after EDTA treatment in an isotonic medium were transformed with DNA pBR322 by applying a single exponential pulse (E = 10 kV/cm, T = 1.5 ms) to the suspension. The maximum transformation efficiency obtained was 4 X 10(6) colonies/micrograms DNA. The maximum transformation frequency was 0.4% at a DNA concentration of 15 micrograms/ml.

Transformation of bacteria with plasmid DNA by electroporation

The possibility of electric field-mediated transformation ("electroporation") of a gram-positive bacterium (Enterococcus faecalis) and two gram-negative bacteria (Escherichia coli and Pseudomonas putida) with plasmid DNA was investigated. E. faecalis protoplasts could be transformed by electroporation with a transformation frequency of 10(4) to 10(5) transformants/micrograms plasmid. Untreated--i.e., washed--cells of E. coli could be transformed with rates of 1 X 10(5) transformants/micrograms plasmid DNA. Transformation rates for P. putida cells were up to 3 X 10(4) if the method developed for E. coli was used. Detailed protocols for these systems, including the results of various optimization experiments, are given.

DNA transfection of Escherichia coli by electroporation

Electroporation was applied to transfection and transformation of Escherichia coli. Efficient transfer of DNA was achieved by a single voltage pulse at 2.5 kV (initial electric field strength = 6.25 kV/cm), with a 25 microF capacitor. As the recipient for transfecting DNA in the electroporation, spheroplasts, EDTA-treated cells and osmotically shocked bacteria were inferior to intact E. coli. Various parameters affecting the transfection efficiency were defined including growth phase of recipient cells, concentrations of DNA and cells, temperature and additions. In most strains tested, electroporation was far more efficient than Ca2+-dependent transfection (transformation). Various aspects of the electroporation-mediated DNA uptake are discussed.

Gene transfer into intact plant cells by electroinjection through cell walls and membranes

Tobacco mosaic virus (TMV) RNA was introduced directly into mesophyll cells of Nicotiana tabacum var. Samsun using electric-field pulses (electroinjection). The injected gene was successfully expressed in the recipient cells as judged by the assay for the virus coat protein using immunofluorescence and by the virus infectivity assay of the homogenate of the electroinjected cells for local lesions on tobacco leaves. As much as 50% of the cells that survived 24 days after electroinjection showed immunofluorescent specks.

Electric field mediated stable transformation of carrot protoplasts with naked DNA

We have developed an electroporation procedure for the transformation of carrot protoplasts with Ti-plasmid DNA from Agrobacterium tumefaciens. The uptake of pTiC58 into carrot protoplasts was mediated by high voltage electrical pulses at field strengths from 0.5 to 3.8 kV/cm. Protoplast regeneration, somatic embryogenesis and plantlet regeneration were unaffected by the electroporation conditions selected for DNA uptake. Uptake of plasmid pTiC58 resulted in hormone independent regeneration of carrot protoplasts. Transformed somatic embryos were detected in carrot cultures 45 days after electroporation. The transformed somatic embryos developed into teratomas which synthesized nopaline. Hybridization was obtained between a labeled T-DNA fragment from pTiC58 and DNA fragments from 4 month old teratomas regenerated from electro-transformed protoplasts. Based on the number of somatic embryos regenerated after electro-transformation, a frequency of 1.6×10(2) transformants/10(4) somatic embryos/μg pTiC58 DNA was obtained.

Electric field mediated transformation: isolation and characterization of a TK+ subclone

Transformation of mammalian TK- cells by a plasmid carrying the TK gene from Herpes virus simplex 1 (pAGO) was mediated by electroporation. The cells were treated either in suspension or growing in monolayers directly in the petri dish. The yield of transformation was between 8.10(-5) and 2.10(-4) per microgram DNA depending on the experimental conditions. The structure of the integrated DNA was investigated proving the occurrence of a duplication process that affected preferentially the pBR322 part of the pAGO DNA (60 copies per cell). The TK gene that gave the TK+ phenotype to the selected clone was present in less than 6 copies.