Putative environmental levels of levofloxacin facilitate the dissemination of antibiotic-resistant Escherichia coli via plasmid-mediated transformability

Antibiotic residues in the environment pose a great risk to global public health. They increase antibiotic resistance by enhancing plasmid conjugation among bacteria or mutations within bacterial genomes. However, little is known about whether the putative environmental levels of antibiotics are sufficient to influence plasmid-mediated transformability. In this study, we explored the effect of eight kinds of representative antibiotics and several other compounds on the plasmid transformability of competent Escherichia coli. Only levofloxacin (LEV) at the putative environmental levels was found to facilitate the frequency of PBR322-or RP4-plasmid-mediated transformation by up to 5.3-fold. Additionally, PBR322 transformation frequency could be further enhanced by copper ion or ammonia nitrogen but inhibited by humic acid. However, when competent E. coli was exposed to the minimal inhibitory concentrations (MIC) of the antibiotics, an enhanced plasmid-assimilation ability was observed and plasmid transformation frequency was increased by up to 98.6-fold for all the tested antibiotics. Furthermore, E. coli exhibited a preference for the uptake of plasmids harbouring the resistance genes to the antibiotics it had been exposed to. Among these antibiotics, cephalexin, tetracycline, and kanamycin induced the highest uptake of RP4. The putative environmental levels of LEV enhanced plasmid transformability regardless of the presence of corresponding antibiotic resistance gene (ARG) on the genetic elements, suggesting environmental LEV residues may facilitate dissemination of antibiotic resistance by any plasmid-mediated transformability, thereby posing a great risk to health.

Femtoliter droplet confinement of Streptococcus pneumoniae: bacterial genetic transformation by cell-cell interaction in droplets

Streptococcus pneumoniae (pneumococcus), a deadly bacterial human pathogen, uses genetic transformation to gain antibiotic resistance. Genetic transformation begins when a pneumococcal strain in a transient specialized physiological state called competence, attacks and lyses another strain, releasing DNA, taking up fragments of the liberated DNA, and integrating divergent genes into its genome. While many steps of the process are known and generally understood, the precise mechanism of this natural genetic transformation is not fully understood and the current standard strategies to study it have limitations in specifically controlling and observing the process in detail. To overcome these limitations, we have developed a droplet microfluidic system for isolating individual episodes of bacterial transformation between two confined cells of pneumococcus. By encapsulating the cells in a 10 μm diameter aqueous droplet, we provide an improved experimental model of genetic transformation, as both participating cells can be identified, and the released DNA is spatially restricted near the attacking strain. Specifically, the bacterial cells, one rifampicin (R) resistant, the other novobiocin (N) and spectinomycin (S) resistant were encapsulated in droplets carried by the fluorinated oil FC-40 with 5% surfactant and allowed to carry out competence-specific attack and DNA uptake (and consequently gain antibiotic resistances) within the droplets. The droplets were then broken, and recombinants were recovered by selective plating with antibiotics. The new droplet system encapsulated 2 or more cells in a droplet with a probability up to 71%, supporting gene transfer rates comparable to standard mixtures of unconfined cells. Thus, confinement in droplets allows characterization of natural genetic transformation during a strictly defined interaction between two confined cells.

A practical random mutagenesis system for Ralstonia solanacearum strains causing bacterial wilt of Pogostemon cablin using Tn5 transposon

A practical random mutagenesis system of Ralstonia solanacearum by electroporation with Tn5 transposon was established, which may be utilized to provide genetic approach to study virulence genes of R. solanacearum strains and create nonpathogenic mutants for biological control of bacterial wilt in Pogostemon cablin. R. solanacearum strain PRS-84 used in this study was isolated from P. cablin plants infected with bacterial wilt. The bacterial suspension of R. solanacearum strain PRS-84 was mixed with Tn5 transposome complex and the mixture was transformed by electroporation. The electroporated cells were then spread on the 2, 3, 5-triphenyltetrazolium chloride agar plates containing kanamycin to select the kanamycin-resistant colonies. Several factors which determined the bacterial transformation efficiency were optimized. The transformation process was shown to be optimal at the electric field strength of 12.5 kV cm^-1. Bacterial cells harvested at mid-exponential phase gave the highest transformation efficiency. 10 µg mL^-1 kanamycin was found to be the optimal concentration for transformant selection. Tn5 insertion mutants of R. solanacearum strain PRS-84 were identified by PCR amplification and Southern blot analysis. Mutants subcultured for 100 passages were also detected by PCR amplification and Southern blot analysis. Furthermore, pathogenicity screening test of mutants was performed by inoculating in vitro regenerated patchouli plants. Results revealed that mutants with a single Tn5 insertion in their genomes were obtained from R. solanacearum strain PRS-84, and the Tn5 insertion could be stably inherited in the mutants. Then, mutants with reduced pathogenicity were selected.

Fine-tuning cellular levels of DprA ensures transformant fitness in the human pathogen Streptococcus pneumoniae

Natural genetic transformation is a widespread mechanism of horizontal gene transfer. It involves the internalization of exogenous DNA as single strands and chromosomal integration via homologous recombination, promoting acquisition of new genetic traits. Transformation occurs during a distinct physiological state called competence. In Streptococcus pneumoniae, competence is controlled by ComDE, a two-component system induced by an exported peptide pheromone. DprA is universal among transformable species, strongly induced during pneumococcal competence, and crucial for pneumococcal transformation. Pneumococcal DprA plays three crucial roles in transformation and competence. Firstly, DprA protects internalized DNA from degradation. Secondly, DprA loads the homologous recombinase RecA onto transforming DNA to promote transformation. Finally, DprA interacts with the response regulator ComE to shut-off competence. Here, we explored the effect of altering the cellular levels of DprA on these three roles. High cellular levels of DprA were not required for the primary role of DprA as a transformation-dedicated recombinase loader or for protection of transforming DNA. In contrast, full expression of dprA was required for optimal competence shut-off and transformant fitness. High cellular levels of DprA thus ensure the fitness of pneumococcal transformants by mediating competence shut-off. This promotes survival and propagation of transformants, maximizing pneumococcal adaptive potential.

Effect of Host Human Products on Natural Transformation in Acinetobacter baumannii

Our previous data show that serum albumin can trigger natural transformation in Acinetobacter baumannii. However, extracellular matrix/basal membrane components, norepinephrine, and mucin did not have a significant effect on this process. Therefore, the effect of human products appears to be albumin specific, as both BSA and HSA have been shown to increase of natural transformation.

FabV/Triclosan Is an Antibiotic-Free and Cost-Effective Selection System for Efficient Maintenance of High and Medium-Copy Number Plasmids in Escherichia coli

Antibiotic resistance genes and antibiotics are frequently used to maintain plasmid vectors in bacterial hosts such as Escherichia coli. Due to the risk of spread of antibiotic resistance, the regulatory authorities discourage the use of antibiotic resistance genes/antibiotics for the maintenance of plasmid vectors in certain biotechnology applications. Overexpression of E. coli endogenous fabI gene and subsequent selection on Triclosan has been proposed as a practical alternative to traditional antibiotic selection systems. Unfortunately, overexpression of fabI cannot be used to select medium –copy number plasmids, typically used for the expression of heterologous proteins in E. coli. Here we report that Vibrio cholera FabV, a functional homologue of E. coli FabI, can be used as a suitable marker for the selection and maintenance of both high and medium -copy number plasmid vectors in E. coli.

N. elongata produces type IV pili that mediate interspecies gene transfer with N. gonorrhoeae

The genus Neisseria contains at least eight commensal and two pathogenic species. According to the Neisseria phylogenetic tree, commensals are basal to the pathogens. N. elongata, which is at the opposite end of the tree from N. gonorrhoeae, has been observed to be fimbriated, and these fimbriae are correlated with genetic competence in this organism. We tested the hypothesis that the fimbriae of N. elongata are Type IV pili (Tfp), and that Tfp functions in genetic competence. We provide evidence that the N. elongata fimbriae are indeed Tfp. Tfp, as well as the DNA Uptake Sequence (DUS), greatly enhance N. elongata DNA transformation. Tfp allows N. elongata to make intimate contact with N. gonorrhoeae and to mediate the transfer of antibiotic resistance markers between these two species. We conclude that Tfp functional for genetic competence is a trait of a commensal member of the Neisseria genus. Our findings provide a mechanism for the horizontal gene transfer that has been observed among Neisseria species.

Analysis of comparative efficiencies of different transformation methods of E. coli using two common plasmid vectors

The efficiencies of different transformation methods of E. coli DH5Qalpha train, induced by several cations like Mg2+, Mn2+ Rb+ and especially Ca2+, with or without polyethylene glycol (PEG) and dimethyl sulfoxide (DMSO) were compared using the two commonly used plasmid vectors pCAMBIA1201 and pBI121. The widely used calcium chloride (CaCl2) method appeared to be the most efficient procedure, while rubidium chloride (RbCl) method was the least effective. The improvements in the classical CaCl2 method were found to further augment the transformation efficiency (TR)E for both the vectors like repeated alternate cycles of heat shock, followed by immediate cold, at least up to the third cycle; replacement of the heat shock step by a single microwave pulse and even more by double microwave treatment and administration of combined heat shock-microwave treatments. The pre-treatment of CaCl2-competent cells with 5% (v/v) ethanol, accompanied by single heat shock also triggered the (TR)E, which was further enhanced, when combined heat shock-microwave was applied. The minor alterations or improved approaches in CaCl2 method suggested in the present study may thus find use in more efficient E. coli transformation.

[Biological activity of Ungernia victoris extract in the Escherichia coli CaCl2-transformation system in the presence of calcium channel modulators]

Furosemid and verapamil appear to have an effect on the yield of E. coli plasmid transformants according to their properties as calcium channel regulators in eukaryotes. It means that furosemid stimulates and verapamil inhibits transforming DNA penetration into competent cell. PHB/Ca2+ polyP complex does not function only as a channel for transforming DNA penetration but appears to be one of the targets for transformation blocking U. victoris extract.

Synthesis and DNA cleaving activity of water-soluble non-conjugated thienyl tetraynes

Water-soluble non-conjugated thienyl tetraynes (3-6) were synthesized and their DNA cleaving activity was evaluated using electrophoresis, atomic force microscopy (AFM) and Escherichia coli (E. coli) transformation techniques. The amino-functionalized compound 4 was shown to possess an activity to cleave plasmid DNA by both electrophoresis and E. coli transformation techniques. AFM also showed a cleavage of the circular DNA into a linear form with a formation of burst-star-shaped architectures, which were envisaged to be cross-linked DNA oligomers.

First characterization of heterogeneous resistance to imipenem in invasive nontypeable Haemophilus influenzae isolates

This study describes the first two reported invasive nontypeable Haemophilus influenzae (NTHI) isolates (strains 183 and 184) with heterogeneous resistance to imipenem. For both isolates, Etest showed imipenem MICs of > or =32 microg/ml. When the two strains were examined by the quantitative method of population analysis, both strain populations were heterogeneously resistant to imipenem and contained subpopulations growing in the presence of up to 32 microg of imipenem/ml at frequencies of 1.7 x 10(-5) and 1.5 x 10(-7), respectively. By pulsed-field gel electrophoresis analysis, the two isolates appeared to be genetically closely related. The sequencing of the ftsI gene encoding penicillin-binding protein 3 (PBP 3) and comparison with the sequence of the imipenem-susceptible H. influenzae strain Rd identified a pattern of six amino acid substitutions shared between strains 183 and 184; an additional change was unique to strain 183. No relationship between mutations in the dacB gene encoding PBP 4 and imipenem resistance was found. The replacement of the ftsI gene in the imipenem-susceptible strain Rd (for which the MIC of imipenem is 0.38 to 1 microg/ml) with ftsI from strain 183 resulted in a transformant for which the MIC of imipenem ranged from 4 to 8 microg/ml as determined by Etest. The Rd/183 transformant population showed heterogeneous resistance to imipenem; it contained subpopulations growing in the presence of up to 32 mug of imipenem/ml at a frequency of 3.3 x10(-8). The presence of additional resistance mechanisms, such as the overexpression of the AcrAB efflux pump, was investigated and does not seem to be involved. These data indicate that the heterogeneous imipenem resistance phenotype of our NTHI clone depends largely on the PBP 3 amino acid substitutions. We speculated that bacterial regulatory networks may play a role in the control of the heterogeneous expression of the resistance phenotype.

Acrylamide effects on kinesin-related proteins of the mitotic/meiotic spindle

The microtubule (MT) motor protein kinesin is a vital component of cells and organs expressing acrylamide (ACR) toxicity. As a mechanism of its potential carcinogenicity, we determined whether kinesins involved in cell division are inhibited by ACR similar to neuronal kinesin [Sickles, D.W., Brady, S.T., Testino, A.R., Friedman, M.A., and Wrenn, R.A. (1996). Direct effect of the neurotoxicant acrylamide on kinesin-based microtubule motility. Journal of Neuroscience Research 46, 7-17.] Kinesin-related genes were isolated from rat testes [Navolanic, P.M., and Sperry, A.O. (2000). Identification of isoforms of a mitotic motor in mammalian spermatogenesis. Biology of Reproduction 62, 1360-1369.], their kinesin-like proteins expressed in bacteria using recombinant DNA techniques and the effects of ACR, glycidamide (GLY) and propionamide (a non-neurotoxic metabolite) on the function of two of the identified kinesin motors were tested. KIFC5A MT bundling activity, required for mitotic spindle formation, was measured in an MT-binding assay. Both ACR and GLY caused a similar concentration-dependent reduction in the binding of MT; concentrations of 100 microM ACR or GLY reduced its activity by 60%. KRP2 MT disassembling activity was assayed using the quantity of tubulin disassembled from taxol-stabilized MT. Both ACR and GLY inhibited KRP2-induced MT disassembly. GLY was substantially more potent; significant reductions of 60% were achieved by 500 microM, a comparable inhibition by ACR required a 5 mM concentration. Propionamide had no significant effect on either kinesin, except KRP2 at 10 mM. This is the first report of ACR inhibition of a mitotic/meiotic motor protein. ACR (or GLY) inhibition of kinesin may be an alternative mechanism to DNA adduction in the production of cell division defects and potential carcinogenicity. We conclude that ACR may act on multiple kinesin family members and produce toxicities in organs highly dependent on microtubule-based functions.

Assessment of Aloe vera (L.) genotoxic potential on Escherichia coli and plasmid DNA

Aloe vera is a tropical plant, known in Brazil as babosa and several reputable suppliers produce a stabilized aloe gel for topic use. Since people use Aloe vera topically, they could be exposed to solar ultraviolet light in addition and it might cause a cross damage effect between these agents. The aim of this work was to investigate the biological effects of Aloe vera pulp extract, associated or not to UVA radiation, on Escherichia coli-deficient repair mutants and plasmid DNA, in order to test its genotoxic potential. Data obtained from analysis of survival fractions, bacterial transformation and agarose gel electrophoresis suggest that Aloe vera has genotoxic properties, but it seems not to be able to damage the cell membrane.

[The influence of the extract of Ungernia victoris cultured cells and of some metal cations on Escherichia coli transformation with plasmid DNA]

In the system of cation-induced E. coli transformation by the plasmid pBR322 the effects of the extract derived from the biomass of cultured cells of U. victoris on the correlation between yield of transformants, viability of CaCl2 - treated cells and plasmid DNA conformation alterations has been investigated. The data obtained have been compared with effects of some salts of one- and divalent metals on the same parameters. The presence of different mechanisms of the variations of cell population viability and yield of transformants depending on utilization of salts or plant extracts has been shown.

Genotoxic potentiality of aqueous extract prepared from Chrysobalanus icaco L. leaves

Plants have been related to our lives, being used as medicine, regardless of scientific evidence of side effects. This work analyses the toxicological effects of Chrysobalanus icaco L. aqueous extract, used in different pathologies. It was studied through: (i) alteration of plasmid pUC 9.1 topology; (ii) survival of bacterial strains submitted, or not, to previous treatment with SnCl2; (iii) transformation efficiency of E. coli strain by the treatment with the plasmid pUC 9.1. In (i), the treatment of the plasmid resulted in DNA single-strand breaks (SSB). A decrease of the lethal effect induced by SnCl2 in presence of the extract was found, while no C. icaco bacterial survival reduction was observed. The transformation efficiency of the plasmid was also reduced. Results suggest that the extract could present a potential genotoxic effect, as demonstrated either by the induction of SSB in plasmid or in transformation efficiency experiments. Finally, it presents an antioxidant action.

Inhibition of Resistance Plasmid Transfer in Escherichia coli by Ionophores, Chlortetracycline, Bacitracin, and Ionophore/Antimicrobial Combinations

Medicinal feed additives bacitracin, chlortetracycline (CTC), laidlomycin, lasalocid, and salinomycin inhibited the transfer of multiresistance-conferring plasmid pBR325 (Tet(r) Amp(r) Cp(r), 6.0 kb) into selected gram-negative strains with the use of an in vitro model. High concentrations of ampicillin-sensitive competence-pretreated Escherichia coli HB 101 cells were exposed to 10% (v/v) of 1:10 dimethyl sulfoxide/agent : water containing test mixtures for 0.5 hr prior to plasmid addition and transforming conditions. Transformation was inhibited for all antimicrobials and showed a positive association wich higher concentration. Additional testing of ionophore compounds separately and in combination with bacitracin, chlortetracycline, lincomycin, roxarsone, tylosin, and virginiamycin at representative feed concentrations demonstrated 80.6% to >99.9% inhibition (P < 0.001) of resistance transfer. Bacitracin alone inhibited transformation within the range of 50-500 ppm. No increase in resistance transfer was observed when poultry-derived and reference gram-negative isolates having low or no transformation efficiency were additionally tested. The results suggest that these compounds, at relevant concentrations used in animal feed, may interfere with cell envelope-associated DNA uptake channels or other transformation competence mechanisms. Through these mechanisms, ionophores and cell membrane-interactive feed agents such as CTC and bacitracin may act to inhibit resistance transfer mechanisms within poultry and livestock.

Mutagenesis of benzo[a]pyrene diol epoxide in yeast: requirement for DNA polymerase zeta and involvement of DNA polymerase eta

Benzo[a]pyrene is a potent environmental carcinogen, which can be metabolized in cells to the DNA damaging agent anti-benzo[a]pyrene-7,8-dihydrodiol-9,10-epoxide (anti-BPDE). We hypothesize that mutations induced by BPDE DNA adducts are mainly generated through an error-prone translesion synthesis that requires a specialized DNA polymerase (Pol). Using an in vivo mutagenesis assay in the yeast model system, we have examined the potential roles of Pol(zeta) and Pol(eta) in (+/-)-anti-BPDE-induced mutagenesis. In cells proficient in mutagenesis, (+/-)-anti-BPDE induced 85% base substitutions with predominant G --> C followed by G --> T transversions, 9% deletions of 1-3 nucleotides, and 6% insertions of 1-3 nucleotides. In rad30 mutant cells lacking Pol(eta), (+/-)-anti-BPDE-induced mutagenesis was reduced and accompanied by a moderate decrease in base substitutions and more significant decrease in deletions and insertions of 1-3 nucleotides. In rev3 mutant cells lacking Pol(zeta), (+/-)-anti-BPDE-induced mutagenesis was mostly abolished, leading to a great decrease in both base substitutions and deletions/insertions of 1-3 nucleotides. In contrast, large deletions/insertions were significantly increased in cells lacking Pol(zeta). Consistent with the in vivo results, purified yeast Pol(zeta) performed limited translesion synthesis opposite (+)- and (-)-trans-anti-BPDE-N(2)-dG DNA adducts with predominant G incorporation opposite the lesion. These results show that (+/-)-anti-BPDE-induced mutagenesis in yeast requires Pol(zeta) and partially involves Pol(eta) and suggest that Pol(zeta) directly participates in nucleotide insertions opposite the lesion, while Pol(eta) significantly contributes to deletions and insertions of 1-3 nucleotides.

Effects of La3+ on growth, transformation, and gene expression of Escherichia coli

Rare earth elements have been emitted into the environment largely as fertilizer components. This has caused much fear about whether they would influence our environment, especially on the metabolism and genetics of microorganisms. In this article, the trivalent ion of a rare earth element, lanthanum, was studied for the effects on growth, transformation, and gene expression of Escherichia coli. The results showed that La3+ at concentrations from 50 to 150 microg/mL stimulated the endogenic metabolism and ectogenic metabolism, but had few effects on gene expression. La3+ at lower concentrations from 0.5 to 30 microg/mL inhibit intensively E. coli-absorbing external DNA, decreasing the transformation efficiency. It is also supported by observations using transmission electron microscopy. Our results are significant in understanding the function of rare earth elements to microorganisms and assessing the risk of application of rare earth compounds.

Multidrug resistance in Serratia marcescens and cloning of genes responsible for the resistance

Six clinically isolated strains of Serratia marcescens were tested for their drug resistance. All showed fairly high resistance to many antimicrobial agents tested including norfloxacin, streptomycin, ampicillin, erythromycin, tetracycline, chloramphenicol, and antimicrobial dyes. Using the drug-hypersensitive strain of Escherichia coli KAM32 as the host, we cloned the genes responsible for multidrug resistance from chromosomal DNA of one of the strains of S. marcescens, NUSM8906. We obtained 28 hybrid plasmids that made host cells resistant to several antimicrobial agents. Many of the transformants harboring each of the plasmids showed multidrug resistance, and some showed resistance to specific drugs. The hybrid plasmids were classified into several groups based on their drug specificity. It appears that each class of plasmid carries different types of drug resistance genes. Analysis of such genes will reveal the multiple mechanisms involved in multidrug resistance in S. marcescens.

Optimization of electroporation conditions for Mycobacterium avium

Successful transformation and subsequent genetic manipulation of Mycobacterium avium requires suitable vectors, efficient transformation systems, and reliable selectable markers. A systematic analysis of the parameters involved in the transformation of M. avium was performed to optimize DNA transfer. Factors examined included the composition of the growth medium, growth medium additives, variations in washing of the bacteria prior to electroporation, and conditions of electroporation. Of the parameters assayed, the frequency of transformation (defined as the number of transformants per 10(6) transformed bacteria) showed the greatest increase with the addition of 1.5% glycine to the M. avium culture medium and the use of higher concentrations of plasmid DNA. The addition of 0.5 M sucrose to the growth medium and wash solution yielded a modest increase in transformation frequency, but more importantly afforded greater consistency of results between different batches of cells with no decrease in transformation yields following freezing and thawing. We also confirmed that gfp could be used as a selective marker for M. avium, even as a single copy integrant, and allowed for rapid discrimination between false and true transformants. Using this protocol, we were able to transform nine of 11 clinical strains of M. avium.

Escherichia coli cells penetrated by chrysotile fibers are transformed to antibiotic resistance by incorporation of exogenous plasmid DNA

A suspension of recipient Escherichia coli cells in stationary phase, chrysotile asbestos, and pUC18 donor DNA spread over the surface of a Luria-Bertani agar plate using a streak bar several times, resulted in intracellular uptake of the plasmid DNA by the E. coli cells. The transformation efficiency was highest with a duration of cell exposure to chrysotile of more than 60 s and an agar concentration of 2%. To improve chrysotile-mediated transformation efficiency, we systematically optimized various conditions and parameters. In comparison to chrysotile exposure without cations, exposure with cations produced up to 100-fold more transformants. Optimized conditions resulted in 10(6) transformants/ micro g pUC18 DNA. The drastic physical change due to 'quick drying on the surface of the agar plate' when cells were exposed to chrysotile, was essential for chrysotile-mediated transformation. We suggest that DNA uptake mediated by chrysotile asbestos is the result of a mechanical physical transformation of E. coli, since the E. coli cells are not chemically competent. Electron microscopy of cells exposed to chrysotile suggested penetration of the E. coli membrane by chrysotile fibers. It is suggested that E. coli transformation by the plasmid DNA was the result of penetration by chrysotile fibers to which plasmid DNA is bound or adsorbed.

Mutagenicity of the 1-nitropyrene-DNA adduct N-(deoxyguanosin-8-yl)-1-aminopyrene in Escherichia coli located in a nonrepetitive CGC sequence

1-Nitropyrene, a common environmental pollutant, forms a major DNA adduct, N-(deoxyguanosin-8-yl)-1-aminopyrene (dG(AP)). Mutational spectra of randomly introduced dG(AP) in Escherichia coli included many different types of mutations. However, a prior site-specific study in a CGCG(AP)CG sequence showed only CpG deletions and +1 frame shifts. To further explore the context effects of dG(AP) in mutagenesis, in this work this adduct was incorporated into a nonrepetitive CGC sequence in single-stranded M13mp7L2 DNA. Upon replication of this construct in repair-competent E. coli, one-base deletions and base substitutions were detected. The -1 frame shifts, whose frequency increased 3-6-fold with SOS (to an average frequency of 1.5%), involved deletion of the adjacent C residues. The base substitutions ( approximately 2.2%) included targeted G-to-T and G-to-C transversions, whose frequencies did not increase with SOS. This suggests that dG(AP) mutagenesis is highly dependent on the local DNA sequence.

Detection of dopaminergic modulators in a tier I screening battery for identifying endocrine-active compounds (EACs)

Apomorphine (APO; D(2) receptor agonist), haloperidol (HAL; D(2) receptor antagonist), and reserpine (RES; a dopamine depletor that acts to lower brain dopamine levels by depleting central nervous system monoamines via disrupting storage vesicle function) have been examined in a Tier I screening battery, which has been designed to detect endocrine-active compounds (EACs). The Tier I battery incorporates two short-term in vivo tests (a 5-day ovariectomized female battery and a 15-day intact male battery using Sprague-Dawley rats) and an in vitro yeast transactivation system (YTS). In addition, two blood collection procedures were evaluated for their utility in detecting HAL-induced increases in serum prolactin (PRL) levels (i.e., the stress associated with each procedure). In the in vivo female battery, both HAL and RES increased serum PRL concentrations as expected, although the increase caused by RES was marginal. Increases in serum PRL levels are enhanced when daily dosages are administered via multiple-daily dosing of the test compound, which results in higher sustained blood levels of the test compounds. APO failed to decrease serum PRL concentrations in the female battery. In the in vivo male battery, HAL increased serum PRL concentrations as expected. However, APO and RES failed to affect serum PRL concentrations. The blood collection comparison experiment demonstrated that possible confounding of the data can occur with serum PRL concentrations when animals are exposed to stress. Basal levels of PRL were approximately fourfold higher in animals that were bled via the tail vein procedure when compared to PRL levels from animals that were bled under CO(2) anesthesia at euthanization. As a result of the higher basal PRL levels, the HAL-induced increase in serum PRL concentrations was completely attenuated in the tail-vein bled animals (1.3-fold). In contrast, HAL produced a fivefold increase in serum PRL in animals where blood was collected under CO(2) anesthesia at euthanization. Hence, collection of blood from animals under CO(2) anesthesia at euthanization is an acceptable approach for detection of compounds that increase PRL. In summary, HAL-like compounds would be identified in the Tier I male and female battery primarily via increased serum PRL concentrations. RES-like compounds would be identified in the Tier I male battery via decreased gonadotropins and steroids and possibly in the Tier I female battery by a minimal increase in serum PRL concentrations. Compounds that produce a marginal increase in serum PRL when administered using single daily dosing can also be confirmed in an in vivo female battery with multiple dosing because this regimen increases the magnitude of the PRL increase. APO, a D(2) receptor agonist, was not detected in the in vivo male or female batteries, but in both instances the top dosage produced minimal decreases in body weight (99 to 96% of control). Hence, the proposed Tier I battery needs to be further evaluated with higher dosages of APO and other D(2) receptor agonists to determine whether it is capable of detecting such agents.

Opa expression correlates with elevated transformation rates in Neisseria gonorrhoeae

Neisseria gonorrhoeae is naturally competent for DNA transformation. Under most conditions encountered in vivo, gonococci express one or more opacity (Opa) proteins on their surfaces. Recently, it was shown that DNA preferentially binds to the surfaces of Opa-expressing organisms compared to those of isogenic Opa-negative strains, presumably due to the numerous cationic residues in the predicted surface-exposed loops of the Opa protein. This study examined whether Opa-DNA interactions actually influence DNA transformation of the gonococcus. The data show that Opa-expressing gonococci are more efficient recipients of DNA for transformation and are more susceptible to exogenous DNase I treatment at early stages during the DNA transformation process than non-Opa expressors. Furthermore, inhibition of the transformation process was demonstrable for Opa(+) populations when either nonspecific DNA or the polyanion heparin was used. Overall, the data suggest that Opa expression, with its presumptive positive surface charge contribution, promotes DNA transformation by causing a more prolonged sequestration of donor DNA at the cell surface, which translates into more efficient transformation over time.

Transformation of a type 4 encapsulated strain of Streptococcus pneumoniae

Streptococcus pneumoniae strain JNR.7/87 is a highly virulent, type 4 encapsulated Gram-positive bacterium whose transformability has not been tested previously, and whose genome is currently being sequenced. The strain was transformed at very low efficiency by addition of exogenous competence-stimulating peptide: However, the efficiency was too low and irreproducible to be useful in many genetic studies. Therefore, the effects on transformation efficiency of changing different components of competence-stimulating peptide-induced transformation have been examined. Screening of growth media was followed by optimization of pre-induction culture acidification, glycine concentration, and induction time. An optimized protocol was developed whereby S. pneumoniae strain JNR.7/87 was transformed reproducibly with a streptomycin resistance (SmR) marker at an efficiency of approximately 10(5) colony forming units per 10(8) cells.

[Selection of drug-labelled Bacillus sphaericus and studies on transformation and expression of foreign DNA]

NTG was used to make chemical mutation for Bacillus sphaericus, RifR and SmR labelled strains were selected, which could resist drug as much as 100 u/ml. The resistance to drug was stably inherited. The RifR strain was used as recipient and the plasmid containing the lysostaphin gene was transfered into it by protoplasts. Results showed that the lysostaphin gene could be expressed stably at high level in Bacillus sphaericus and the lysostaphin activity was about 122 u/ml medium after shaking culture.

Factors affecting transformation efficiency of BCG with a Mycobacterium-Escherichia coli shuttle vector pYUB18 by electroporation

BCG has been one of the vehicles for multi-recombinant vaccine. However, low transformation efficiency of BCG with plasmid DNA hampered studies involving expression of foreign antigens in BCG. In an effort to determine the optimal conditions, this study was initiated to investigate factors involved in the transformation of BCG with a Mycobacterium-Escherichia coli shuttle vector, pYUB18, by electroporation. Mycobacterium bovis BCG (strain 1173P2) was grown in Middlebrook (M) 7H9 broth containing albumin-dextrose-catalase and 0.05% tween 80, and transformed BCG was grown in M7H10 agar containing kanamycin for counting viable cells. Pretreatment of BCG with 10 mM CaCl2 improved the transformation efficiency, but overnight incubation of BCG with 1% glycine did not. The transformation efficiency in BCG also varied depending on voltage, resistance, and DNA concentration. The maximum transformation efficiency was obtained when the infinity resistance, 12.5 Kv/cm, and 100 ng of DNA were used, and reached 1.4 x 10(5) CFU/microgram of plasmid DNA, which is about 3-100 times greater than those from previous reports. The transformation conditions described in this study, therefore, will give us a better position for employing BCG as a vehicle for developing multi-recombinant vaccines.

A simple method for the production of highly competent cells of Agrobacterium for transformation via electroporation

The introduction of binary plasmids into Agrobacterium hosts for Agrobacterium-mediated transformation of plants is most readily achieved by electroporation. However, occasionally, no transformed colonies are recovered and the transformation program is delayed. Poor transformation rates are commonly associated with particular combinations of Agrobacterium strains and plasmid-selection markers. In order to avoid this problem, it is important for the bacteria to have a highly competent status for reception of plasmid DNA. It is also important to optimize the level of antibiotic for the selection of transformed colonies. In this article, we demonstrate that transformation competence is strongly related to the phase of growth at which a bacterial culture is prepared for electroporation, and we describe a simple procedure that allows the level of transformation-competent cells to be maximized. We have observed that there is significant variation between transformed Agrobacterium strains in the levels of antibiotic tolerance; we define the antibiotic levels that are appropriate for selection of three Argobacterium tumefaciens (EHA101, LBA4404, C58) and two Agrobacterium rhizogenes (LBA9402, Ar2626) strains, transformed with three alternative resistance markers (spectinomycin(res), kanamycin(res), and gentamycin(res)).

Polyethylene glycol-mediated bacterial colony transformation

A moderately efficient and quick method of bacterial colony transformation is described. Plasmid DNA was added to bacteria suspended in a solution of polyethylene glycol/calcium chloride (PEG/CaCl2). After a brief incubation and heat shock, the cells were directly plated. Transformation efficiencies up to 8.6 +/- 1.28 x 10(6) transformants per microgram of pUC18 were obtained. We have found that the reverse of the transformation process could also take place. Suspending a bacterial pellet harboring the plasmid of interest in PEG/CaCl2 results in the release of the plasmid DNA, and thus indirectly lends support to the transformation process.

Transformation of Helicobacter pylori by chromosomal metronidazole resistance and by a plasmid with a selectable chloramphenicol resistance marker

Most strains of Helicobacter pylori are naturally competent for uptake of chromosomal DNA. Transformation frequencies for streptomycin resistance or rifampicin resistance markers ranged from 1 x 10(-4) to 1 x 10(-3) per viable cell using a plate transformation procedure. Transformation of a metronidazole resistance marker (MtrR) was demonstrated when either a laboratory-derived mutant or a MtrR clinical isolate were used as the source of donor DNA. MtrR was transformed at a frequency of 3 x 10(-5) per viable cell. All H. pylori strains tested produce large amounts of DNAase, which may reduce DNA available for transformation. Four H. pylori plasmids were isolated. DNA fragments from H. pylori plasmids were deleted or rearranged when cloned in pUC19 and propagated in Escherichia coli DH5 alpha. An H. pylori plasmid, pUOA26 which contained a chloramphenicol resistance determinant from Campylobacter coli, was constructed in H. pylori. This plasmid could be successfully introduced by natural transformation only into H. pylori recipients which contained a homologous resident plasmid. Transformation of pUOA26 into plasmid-free cells of H. pylori was achieved by electroporation. Transformation frequencies were 1 x 10(-4) transformants per viable cell when plasmid DNA was isolated from the same strain; however, introduction of pUOA26 DNA derived from H. pylori 8091 into a different H. pylori strain, NCTC 11639, resulted in transformation at much lower frequencies (< or = 1 x 10(-7) per viable cell).(ABSTRACT TRUNCATED AT 250 WORDS)

[L-transforming effect of biseptol on cells of Shigella sonnei and flexneri]

Possible L-transformation of Shigella under the action of biseptol on an artificial nutrient medium was investigated. The L-forms were isolated from 6 out of 35 cultures. By the morphological properties the isolates were similar to the L-forms of Shigella flexneri induced by furazolidone. The growth of the L-forms proceeded by the pattern of the rapid mass L-conversion of bacteria and was accompanied by the development of resistance to biseptol. It was inhibited by folic acid added to the medium.

Physiological characterization of natural transformation in Acinetobacter calcoaceticus

Acinetobacter calcoaceticus BD413 develops competence for natural transformation immediately after the start of the exponential growth-phase and remains competent up to e few hours into the stationary phase, after which competence gradually declines. The transformation frequencies obtained strongly depend on the kind of transforming DNA and the incubation time with DNA. Up to 25% of the cells in a culture can be transformed. DNA uptake in Acinetobacter does not display sequence specificity, is Mg(2+)-, Mn(2+)- or Ca(2+)-dependent and is uncoupler sensitive. The transforming DNA enters the cells in single-stranded form. These properties constitute a unique combination, not previously observed in other bacteria, and make A. caloaceticus ideally suited for detailed studies of the bioenergetics of DNA translocation.

Mechanism of action of tricyclic drugs on Escherichia coli and Yersinia enterocolitica plasmid maintenance and replication

Tricyclic medical compounds like many other non-antibiotics exhibit antimicrobial activities. Two chemically representative groups were tested in plasmid DNA transformation and replication to assign intracellular target sites responsible for the multiple effects in Escherichia coli and Yersinia enterocolitica cells. To analyse the mechanism of action at the molecular level, the effects of chlorpromazine, 7,8 dioxochlorpromazine, promethazine, methylene blue, imipramine, cannabidiolic acid and tetrahydrocannabidiolic acid were examined at several points in the course of transformation, in plasmid replication and on the topological state of plasmid DNA. Two possible target sites were identified, one of them involving membrane binding sites which participate in plasmid DNA replication. Drug binding at these sites interfered with the replicating plasmid DNA and membrane protein complex, preventing the proper processing of the replication that resulted in plasmid loss. The other in vivo and in vitro effect was observed on the topological state of plasmid DNA. Tricyclic drugs intefered with energy dependent gyrase activity and promoted the relaxation of plasmid DNA, causing disturbances in gene expression and in plasmid replication. The results give insight into the chemical structures connected with significant specific antimicrobial effects.

Transformation system for Amycolatopsis (Nocardia) mediterranei: direct transformation of mycelium with plasmid DNA

A new procedure for transformation of Amycolatopsis (Nocardia) mediterranei LBG A3136 was developed. The method makes use of polyethylene glycol and alkaline cations and enables direct transformation of the A. mediterranei mycelium with high efficiency: more than 10(6) transformants per microgram of DNA were obtained. Transformation of A. mediterranei is stimulated by the ionophore antibiotic valinomycin and abolished by arsenate and p-chloromercuribenzenesulfonate. pMEA123, a vector based on the indigenous plasmid pMEA100 and containing the erythromycin resistance gene, was constructed.

Transposon mutagenesis in Staphylococcus epidermidis using the Enterococcus faecalis transposon Tn917

We transformed a clinical Staphylococcus epidermidis isolate with the Enterococcus faecalis transposon Tn917-carrying plasmid pTV1. Loss of plasmid replication was observed at 47 degrees C. Tn917 transposes efficiently and apparently randomly. The transposition frequency could be stimulated by erythromycin. Transposon mutagenesis in S. epidermidis provides a means for genetic study of the various virulence factors of this pathogen.

High-efficiency transformation of Listeria monocytogenes by electroporation of penicillin-treated cells

A procedure has been developed for electroporation-mediated transformation of Listeria monocytogenes with plasmid DNA. The method was optimized for intact cells of L. monocytogenes 23074 by determining the effects of field strength, cell density, and plasmid DNA topology. Transformation efficiencies were dramatically increased when cells were treated with penicillin. Optimum frequencies of transformation (4 x 10(6) transformants/microgram DNA) were obtained when cells were grown in 10 micrograms/ml of penicillin G and electroporated at a field strength of 10 kV/cm. Using this procedure, transformation of relaxed plasmid DNA from ligation reactions provided 1 x 10(4) transformants/microgram DNA, allowing direct molecular cloning of DNA into this organism.

Factors involved in the electroporation-induced transformation of Clostridium perfringens

The following factors were found to improve the efficiency of transformation of Clostridium perfringens 3624A Rifr Strr: (1) a reduction in cuvette sample volume (DNA and cell suspension) to 0.8 ml, (2) use of a 1 microgram/ml concentration of transforming DNA, (3) use of late-logarithmic phase cells, (4) 3-fold concentration of cell density (3.0 x 10(8) CFU/ml), and (5) a reduction in the pH of the expression and selective plating medium to 6.4. Application of the improved conditions resulted in transformation efficiencies for C. perfringens 3624A Rifr Strr ranging from 7.1 transformants/microgram DNA for plasmic pIP401 to 9.2 x 10(4) transformants per microgram DNA for plasmid pAK201. The greatest transformation efficiency obtained using pAK201 was 9.8 x 10(6) transformants/micrograms DNA for C. perfringens strain 13. Using the improved protocol, pAM beta 1 was transformed at a 42-fold greater level when compared with the values reported earlier [1]. In addition to C. perfringens 3624A Rifr Strr, strains 13, 10543A, 3628C, NTG-4, and 3624A were successfully transformed. Nuclease does not appear to be a factor in the C. perfringens strain-specific electro-transformation protocol.

Transformation of Lactobacillus helveticus subsp. jugurti with plasmid pLHR by electroporation

Lactobacillus helveticus subsp. jugurti was transformed with plasmid, pLHR (8.5 kilobases), by electroporation. The plasmid, pLHR, consists of a cryptic plasmid, pLJ1, from L. helveticus subsp. jugurti, the Escherichia coli vector pBR329, and the erythromycin resistance gene of pAM beta 1 from Enterococcus faecalis. Maximum transformation efficiency of 1.3 x 10(4) transformants per microgram of DNA was obtained by exposure to a pulse with an exponential decay waveform at 4 kV/cm with 25 microF capacitance. The presence of glycine in the growth medium was essential for transformation. Plasmid DNA isolated from transformants had not undergone detectable rearrangements or deletions. In addition, it was found that L. helveticus subsp. jugurti has a restriction and modification system.

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-transformation system for coryneform bacteria by auxotrophic complementation

We evaluated electroporation as an alternative system for genetic exchange for one of the coryneform bacteria, Brevibacterium flavum MJ233. The maximum number of transformants, 6 x 10(4) cells, was obtained when cells were cultured with Penicillin G (1 U/ml) and harvested at the middle-log phase. Electroporation was done using 12.5 kV/cm of pulse field strength, 1 x 10(10) cells, and 1 microgram of plasmid DNA. Other coryneform bacteria, Brevibacterium lactofermentum ATCC 13869, Corynebacterium glutamicum ATCC 31830, and B. stationis IFO 12144 were also transformed by electroporation. Electroporation has the advantage that intact cells can be used as host cells without the need for protoplast formation and regeneration. Moreover, minimal medium can be used, so auxotrophic complementation of the transformants is possible.

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.

One-step preparation of competent Escherichia coli: transformation and storage of bacterial cells in the same solution

We have developed a simple, one-step procedure for the preparation of competent Escherichia coli that uses a transformation and storage solution [TSS; 1 x TSS is LB broth containing 10% (wt/vol) polyethylene glycol, 5% (vol/vol) dimethyl sulfoxide, and 50 mM Mg2+ at pH 6.5]. Cells are mixed with an equal volume of ice-cold 2 x TSS and are immediately ready for use. Genetic transformation is equally simple: plasmid DNA is added and the cells are incubated for 5-60 min at 4 degrees C. A heat pulse is not necessary and the incubation time at 4 degrees C is not crucial, so there are no critical timing steps in the transformation procedure. Transformed bacteria are grown and selected by standard methods. Thus, this procedure eliminates the centrifugation, washing, and long-term incubation steps of current methods. Although cells taken early in the growth cycle (OD600 0.3-0.4) yield the highest transformation efficiencies (10(7)-10(8) transformants per micrograms of plasmid DNA), cells harvested at other stages in the growth cycle (including stationary phase) are capable of undergoing transformation (10(5)-10(7) transformants per micrograms of DNA). For long-term storage of competent cells, bacteria can be frozen in TSS without addition of other components. Our procedure represents a simple and convenient method for the preparation, transformation, and storage of competent bacterial cells.

Differential effects of DNA gyrase inhibitors on the genetic transformation of Neisseria gonorrhoeae

Inhibitors of DNA gyrase in Escherichia coli exerted differential effects on the genetic transformation of Neisseria gonorrhoeae. When competent cells of the gonococcus were exposed to novobiocin before the uptake of transforming antibiotic resistance DNA, there was a 50 to 60% reduction in the number of transformants compared with the number of control untreated cells. Norfloxacin, a more potent inhibitor of DNA gyrase and an analog of nalidixic acid, nearly abolished the production of transformants by recipient cells. On the contrary, exposure of competent cells to nalidixic acid had no effect on transformant yield. The target of these inhibitors appears to be at the level of recombination. Possible mechanisms are discussed.

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.

[Detection in an experiment of a previously unknown phenomenon of the malignant transformation of bacteria as affected by antibiotics or phenol]

The experimental study of bacteria exposed to antibiotics or phenol has revealed the hitherto unknown process of their malignant transformation. These facts are of universal importance for life sciences, as they bring about changes in the knowledge of the main processes in the life and development of organisms. The discovery of this phenomenon will help in achieving the correct solutions of cardinal problems in biology and medicine.

Loss of transforming activity of plasmid DNA (pBR322) in E. coli caused by singlet molecular oxygen

Plasmid DNA pBR322 in aqueous solution was exposed to singlet molecular oxygen (1O2) generated by microwave discharge. DNA damage was detected as loss of transforming activity of pBR322 in E. coli (CMK) dependent on the time of exposure. DNA damage was effectively decreased by singlet-oxygen quenchers such as sodium azide and methionine. Replacement of water in the incubation buffer by D2O led to an increase in DNA damage. 9,10-Bis(2-ethylene)anthracene disulfate was used as a chemical trap for 1O2 quantitation by HPLC analysis of the endoperoxide formed.

Comparison of the inactivation of Bacillus subtilis transforming DNA by the potassium superoxide and xanthine-xanthine oxidase systems for generating superoxide

Potassium superoxide (KO2) and xanthine-xanthine oxidase (X-XO), which are known generating systems for the superoxide anion, have different inactivating actions on Bacillus subtilis transforming DNA in vitro. Superoxide dismutase and CuSO4 enhanced the inactivation for KO2, but not for X-XO. Mannitol, a hydroxyl radical scavenger, protected against the inactivation by X-XO, but not by KO2. The results obtained with X-XO were consistent with the involvement of Fenton reactions, in which hydroxyl radical is the reactive species that ultimately causes damage. On the other hand, KO2-induced inactivation was partly due to the effect of H2O2. Differences in inactivation between the KO2 and X-XO systems may result from the different rates of production of the superoxide anion.

Transformation of encapsulated Streptococcus pneumoniae

We describe the high-efficiency transformation of several virulent, encapsulated isolates of Streptococcus pneumoniae. Transformation was effected by the induction of competence with competence factor and was apparently the result both of inducing noncompetent recipients and overcoming the inhibition imposed by the capsule.