Effects of metal ion chelators on DNA strand breaks and inactivation produced by hydrogen peroxide in Escherichia coli: detection of iron-independent lesions

Iron Deficiency Generates Oxidative Stress and Activation of the SOS Response in Caulobacter crescentus

In C. crescentus, iron metabolism is mainly controlled by the transcription factor Fur (ferric uptake regulator). Iron-bound Fur represses genes related to iron uptake and can directly activate the expression of genes for iron-containing proteins. In this work, we used total RNA sequencing (RNA-seq) of wild type C. crescentus growing in minimal medium under iron limitation and a fur mutant strain to expand the known Fur regulon, and to identify novel iron-regulated genes. The RNA-seq of cultures treated with the iron chelator 2-2-dypiridyl (DP) allowed identifying 256 upregulated genes and 236 downregulated genes, being 176 and 204 newly identified, respectively. Sixteen transcription factors and seven sRNAs were upregulated in iron limitation, suggesting that the response to low iron triggers a complex regulatory network. Notably, lexA along with most of its target genes were upregulated, suggesting that DP treatment caused DNA damage, and the SOS DNA repair response was activated in a RecA-dependent manner, as confirmed by RT-qPCR. Fluorescence microscopy assays using an oxidation-sensitive dye showed that wild type cells in iron limitation and the fur mutant were under endogenous oxidative stress, and a direct measurement of cellular H₂O₂ showed that cells in iron-limited media present a higher amount of endogenous H₂O₂. A mutagenesis assay using the rpoB gene as a reporter showed that iron limitation led to an increase in the mutagenesis rate. These results showed that iron deficiency causes C. crescentus cells to suffer oxidative stress and to activate the SOS response, indicating an increase in DNA damage.

Effect of the FE2+ chelation by 2,2'-dipyridyl in the doxorubicin-induced lethality in breast tumor cell lines

Breast cancer cells may exhibit changes in iron homeostasis, which results in increased labile iron pool (LIP) levels. Several studies highlight the crucial role of high LIP levels in the maintenance of tumor cell physiology. Iron chelators have been tested in anticancer therapy in combination with chemotherapeutic agents, to improve drug efficacy. Thus, the aim of this study was to evaluate the effect of 2,2'-dipyridyl (DIP), a Fe²⁺ chelator, in combination with doxorubicin (DOX) in breast tumor cells. The maximum concentration of DIP that did not significantly reduce the viability of MDA-MB-231 cells was 10μM and for MCF-7 cells was 50μM. We observed that MCF-7 had higher LIP levels than MDA-MB-231 cells. DIP alone increased ROS generation in MCF-7 cells, and DIP pretreatment reduced ROS generation induced by DOX treatment. In conclusion, the increase in MCF-7 cell viability induced by DIP pretreatment in DOX-treated cells seems to be related to an increase in the cellular antioxidant capacity and the iron chelator did not improve drug efficacy in the two breast tumor cell lines analyzed.

Bipyridine (2,2'-dipyridyl) potentiates Escherichia coli lethality induced by nitrogen mustard mechlorethamine

Alkylating agents are used in anti-tumor chemotherapy because they bind covalently to DNA and generate adducts that may lead to cell death. Bifunctional (HN2) and monofunctional (HN1) nitrogen are two such agents, and HN2 was the first drug successfully employed in anti-leukemia chemotherapy. Currently, HN2 is used either alone or combined with other drugs to treat Hodgkin's disease. It is well known that several crosslinking agents require metabolic activation via reactive oxygen species (ROS) to exert their lethal effects. The objective of this work was therefore to determine whether the abovementioned mustards would also require metabolic activation to exert lethal action against Escherichia coli. For this purpose, we measured survival following exposure to HN2 in E. coli strains that were deficient in nucleotide excision repair (uvrA NER mutant), base excision repair (xthA nfo nth fpg BER mutant) or superoxide dismutase (sodAB mutant) activity. We also performed the same experiments in cells pretreated with an iron chelator (2,2'-dipyridyl, DIP). The NER and BER mutants were only sensitive to HN2 treatment (survival rates similar to those of the wild-type were achieved with 5-fold lower HN2 doses). However, wild-type and sodAB strains were not sensitive to treatment with HN2. In all tested strains, survival dropped by 2.5-fold following pretreatment with DIP compared to treatment with HN2 alone. Furthermore, DIP treatment increased ROS generation in both wild type and sodAB-deficient strains. Based on these data and on the survival of the SOD-deficient strain, we suggest that the increased production of ROS caused by Fe(2+) chelation may potentiate the lethal effects of HN2 but not HN1. This potentiation may arise as a consequence of enhancement in the number of or modification of the type of lesions formed. No sensitization was observed for the non-crosslinkable HN2 analog, HN1.

Inactivation of Escherichia coli by polychromatic simulated sunlight: evidence for and implications of a fenton mechanism involving iron, hydrogen peroxide, and superoxide

Sunlight inactivation of Escherichia coli has previously been shown to accelerate in the presence of oxygen, exogenously added hydrogen peroxide, and bioavailable forms of exogenously added iron. In this study, mutants unable to effectively scavenge hydrogen peroxide or superoxide were found to be more sensitive to polychromatic simulated sunlight (without UVB wavelengths) than wild-type cells, while wild-type cells grown under low-iron conditions were less sensitive than cells grown in the presence of abundant iron. Furthermore, prior exposure to simulated sunlight was found to sensitize cells to subsequent hydrogen peroxide exposure in the dark, but this effect was attenuated for cells grown with low iron. Mutants deficient in recombination DNA repair were sensitized to simulated sunlight (without UVB wavelengths), but growth in the presence of iron chelators reduced the degree of sensitization conferred by this mutation. These findings support the hypothesis that hydrogen peroxide, superoxide, and intracellular iron all participate in the photoinactivation of E. coli and further suggest that the inactivation rate of enteric bacteria in the environment may be strongly dependent on iron availability and growth conditions.

Intracellular free iron and its potential role in ultrahigh-pressure-induced inactivation of Escherichia coli

Intracellular free iron of Escherichia coli was determined by whole-cell electron paramagnetic resonance spectrometry. Ultrahigh pressure (UHP) increased both intracellular free iron and cell lethality in a pressure-dose-dependent manner. The iron chelator 2,2'-dipyridyl protected cells against UHP treatments. A mutation that produced iron overload conditions sensitized E. coli to UHP treatment.

DNA-damaging effects of dental bleaching agents

We studied DNA-damaging effects of dental bleaching systems containing hydrogen peroxide and/or carbamide peroxide by the "comet assay" (alkaline version). Dental bleaching systems in a hydrogen peroxide concentration range from 0.03 to 30 mM produced a genotoxic effect on isolated HeLa cells in vitro comparable with the effects of pharmacopoeial hydrogen peroxide or urea peroxide. Catalase protected the cells against products containing hydrogen peroxide and had no effect on the genotoxicity of samples containing carbamide peroxide.

H2S: a universal defense against antibiotics in bacteria

Many prokaryotic species generate hydrogen sulfide (H(2)S) in their natural environments. However, the biochemistry and physiological role of this gas in nonsulfur bacteria remain largely unknown. Here we demonstrate that inactivation of putative cystathionine β-synthase, cystathionine γ-lyase, or 3-mercaptopyruvate sulfurtransferase in Bacillus anthracis, Pseudomonas aeruginosa, Staphylococcus aureus, and Escherichia coli suppresses H(2)S production, rendering these pathogens highly sensitive to a multitude of antibiotics. Exogenous H(2)S suppresses this effect. Moreover, in bacteria that normally produce H(2)S and nitric oxide, these two gases act synergistically to sustain growth. The mechanism of gas-mediated antibiotic resistance relies on mitigation of oxidative stress imposed by antibiotics.

Effects of antibacterial mineral leachates on the cellular ultrastructure, morphology, and membrane integrity of Escherichia coli and methicillin-resistant Staphylococcus aureus

Background

We have previously identified two mineral mixtures, CB07 and BY07, and their respective aqueous leachates that exhibit in vitro antibacterial activity against a broad spectrum of pathogens. The present study assesses cellular ultrastructure and membrane integrity of methicillin-resistant Staphylococcus aureus (MRSA) and Escherichia coli after exposure to CB07 and BY07 aqueous leachates.

Methods

We used scanning and transmission electron microscopy to evaluate E. coli and MRSA ultrastructure and morphology following exposure to antibacterial leachates. Additionally, we employed Baclight LIVE/DEAD staining and flow cytometry to investigate the cellular membrane as a possible target for antibacterial activity.

Results

Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) imaging of E. coli and MRSA revealed intact cells following exposure to antibacterial mineral leachates. TEM images of MRSA showed disruption of the cytoplasmic contents, distorted cell shape, irregular membranes, and distorted septa of dividing cells. TEM images of E. coli exposed to leachates exhibited different patterns of cytoplasmic condensation with respect to the controls and no apparent change in cell envelope structure. Although bactericidal activity of the leachates occurs more rapidly in E. coli than in MRSA, LIVE/DEAD staining demonstrated that the membrane of E. coli remains intact, while the MRSA membrane is permeabilized following exposure to the leachates.

Conclusions

These data suggest that the leachate antibacterial mechanism of action differs for Gram-positive and Gram-negative organisms. Upon antibacterial mineral leachate exposure, structural integrity is retained, however, compromised membrane integrity accounts for bactericidal activity in Gram-positive, but not in Gram-negative cells.

pH-Dependent metal ion toxicity influences the antibacterial activity of two natural mineral mixtures

Background

Recent studies have demonstrated that several mineral products sold for medicinal purposes demonstrate antimicrobial activity, but little is known about the physicochemical properties involved in antibacterial activity.

Methodology/Principal Findings

Using in vitro mineral suspension testing, we have identified two natural mineral mixtures, arbitrarily designated BY07 and CB07, with antibacterial activity against a broad-spectrum of bacterial pathogens. Mineral-derived aqueous leachates also exhibited antibacterial activity, revealing that chemical, not physical, mineral characteristics were responsible for the observed activity. The chemical properties essential for bactericidal activity against Escherichia coli were probed by testing antibacterial activity in the presence of metal chelators, the hydroxyl radical scavenger, thiourea, and varying pH levels. Chelation of the BY07 minerals with EDTA or desferrioxamine eliminated or reduced BY07 toxicity, respectively, suggesting a role of an acid-soluble metal species, particularly Fe³⁺ or other sequestered metal cations, in mineral toxicity. This conclusion was supported by NMR relaxation data, which indicated that BY07 and CB07 leachates contained higher concentrations of chemically accessible metal ions than leachates from non-bactericidal mineral samples.

Conclusions/Significance

We conclude that the acidic environment of the hydrated minerals significantly contributes to antibacterial activity by increasing the availability and toxicity of metal ions. These findings provide impetus for further investigation of the physiological effects of mineral products and their applications in complementary antibacterial therapies.

Endogenous nitric oxide protects bacteria against a wide spectrum of antibiotics

Bacterial nitric oxide synthases (bNOS) are present in many Gram-positive species and have been demonstrated to synthesize NO from arginine in vitro and in vivo. However, the physiological role of bNOS remains largely unknown. We show that NO generated by bNOS increases the resistance of bacteria to a broad spectrum of antibiotics, enabling the bacteria to survive and share habitats with antibiotic-producing microorganisms. NO-mediated resistance is achieved through both the chemical modification of toxic compounds and the alleviation of the oxidative stress imposed by many antibiotics. Our results suggest that the inhibition of NOS activity may increase the effectiveness of antimicrobial therapy.

[New promising antioxidants based on 2,6-dimethylphenol]

Three new sulfur-containing derivatives of 2,6-dimethylphenol were synthesized. Their antioxidative activity, mutagenicity, and genotoxicity were examined by bacterial tests and by calculating the dominant lethal mutations in murine embryonic cells. It was shown that all the compounds synthesized have a marked antioxidative effect and no genotoxic or mutagenic properties. One of the antioxidants, 4-(3-dodecylthiopropyl)-2,6-dimethylphenol, increases the survival of cells of both the wild-type Escherichia coli strain and bacterial strains defective in the genes of repair enzymes and has a more distinct antioxidative effect than the classic antioxidants alpha-tocopherol and trolox, increasing the survival of cells devoid of repair enzymes.

Gyrase inhibitors induce an oxidative damage cellular death pathway in Escherichia coli

Modulation of bacterial chromosomal supercoiling is a function of DNA gyrase-catalyzed strand breakage and rejoining. This reaction is exploited by both antibiotic and proteic gyrase inhibitors, which trap the gyrase molecule at the DNA cleavage stage. Owing to this interaction, double-stranded DNA breaks are introduced and replication machinery is arrested at blocked replication forks. This immediately results in bacteriostasis and ultimately induces cell death. Here we demonstrate, through a series of phenotypic and gene expression analyses, that superoxide and hydroxyl radical oxidative species are generated following gyrase poisoning and play an important role in cell killing by gyrase inhibitors. We show that superoxide-mediated oxidation of iron–sulfur clusters promotes a breakdown of iron regulatory dynamics; in turn, iron misregulation drives the generation of highly destructive hydroxyl radicals via the Fenton reaction. Importantly, our data reveal that blockage of hydroxyl radical formation increases the survival of gyrase-poisoned cells. Together, this series of biochemical reactions appears to compose a maladaptive response, that serves to amplify the primary effect of gyrase inhibition by oxidatively damaging DNA, proteins and lipids.

Study of DNA damage induced by dental bleaching agents in vitro

Dental bleaching is a simple and conservative procedure for aesthetic restoration of vital and non-vital discolored teeth. Nevertheless, a number of studies have demonstrated the risk of tissue damage from the contact of these agents with the oral mucosa. In the current study, the genotoxic potential associated with exposure to dental bleaching agents was assessed by the single cell gel (comet) assay in vitro. Chinese hamster ovary (CHO) cells in vitro were exposed to six commercial dental bleaching agents (Clarigel Gold - Dentsply; Whitespeed - Discus Dental; Nite White - Discus Dental; Magic Bleaching - Vigodent; Whiteness HP - FGM and Lase Peroxide - DMC). The results pointed out that all dental bleaching agents tested contributed to DNA damage as depicted by the mean tail moment, being the strongest effect observed with the highest dose of hydrogen peroxide (Whiteness HP and Lase Peroxide, at a 35% concentration). On the other hand, Magic Bleaching (Vigodent) induced the lowest level of DNA breakage. Negative and positive controls displayed absence and presence of DNA-damaging, respectively. Taken together, these results suggest that dental bleaching agents may be a factor that increases the level of DNA damage. A higher concentration of hydrogen peroxide produced higher noxious activities in the genome as detected by single cell gel (comet) assay.

Endonuclease IV and exonuclease III are involved in the repair and mutagenesis of DNA lesions induced by UVB in Escherichia coli

Exonuclease III (Exo III) and endonuclease IV (Endo IV) play a critical role in the base excision repair (BER) of Escherichia coli. Both are endowed with AP endonucleolytic activity, cleaving the 5' phosphodiester bond adjacent to spontaneous or induced abasic sites in DNA. Although mutants defective in Exo III (xthA) are usually hypersensitive to oxidative agents such as hydrogen peroxide, near-UV-light and X-rays, mutants defective in Endo IV (nfo) are not as sensitive as the xthA strain. To further investigate the roles of these AP endonucleases in DNA repair, we evaluated the sensitivity and mutagenesis of xthA and nfo strains after UVB and compared with UVC light. Our results revealed that xthA but not nfo strain was hypersensitive to UVB. The use of Fe(+2) ion chelator (dipyridyl), prior to irradiation, completely protected the xthA mutant against UVB lethal lesions, suggesting the generation of toxic oxidative lesions mediated by transition metal reactions. The nfo strain displayed increased UVB-induced mutagenesis, which was significantly suppressed by pre-treatment with dipyridyl. Although xthA strain did not display increased mutagenesis after UVC and UVB treatments, this phenotype was not related to xthA mutation, but rather to an unknown secondary mutation specifying an antimutator phenotype. After UVB irradiation, the base substitution spectra of nfo strain revealed a bias towards AT-->GC transitions and GC-->CG transversions, which were also suppressed by previous treatment with the iron chelator. Overall, on the basis of the differential sensitivities and mutational spectra displayed after UVC and UVB treatments, we propose a role for Endo IV and Exo III to counteract DNA damage induced by the oxidative counterpart of UVB in E.coli.

Assessment of genetic damage induced by dental bleaching agents on mouse lymphoma cells by single cell gel (comet) assay

Dental bleaching is a simple and conservative procedure for aesthetic restoration of vital discoloured teeth. However, dental bleaching agents may represent a hazard to human health, especially by causing DNA strand breaks. Genotoxicity tests form an important part of cancer research and risk assessment of potential carcinogens. In the current study, the genotoxic potential associated with exposure to dental bleaching agents was assessed by the single cell gel (comet) assay in vitro. Six commercial dental bleaching agents (Clarigel Gold - Dentsply; Whitespeed - Discus Dental; Nite White - Discus Dental; Magic Bleaching - Vigodent; Whiteness HP - FGM and Lase Peroxide - DMC) were exposed to mouse lymphoma cells in vitro. The results pointed out that all dental bleaching agents tested contributed to the DNA damage as depicted by the mean tail moment. Clear concentration-related effects were obtained for DNA damaging, being the strongest effect observed at the highest dose of the hydrogen peroxide (Whiteness HP and Lase Peroxide, at 35% concentration). On the contrary, Whitespeed (Discus Dental) induced the lowest level of DNA breakage. Taken together, these results suggest that dental bleaching agents may be a factor that increases the level of DNA damage as detected by the single cell gel (comet) assay.

NO-mediated cytoprotection: instant adaptation to oxidative stress in bacteria

Numerous sophisticated systems have been described that protect bacteria from increased levels of reactive oxygen species. Although indispensable during prolonged oxidative stress, these response systems depend on newly synthesized proteins, and are hence both time and energy consuming. Here, we describe an "express" cytoprotective system in Bacillus subtilis which depends on nitric oxide (NO). We show that NO immediately protects bacterial cells from reactive oxygen species by two independent mechanisms. NO transiently suppresses the enzymatic reduction of free cysteine that fuels the damaging Fenton reaction. In addition, NO directly reactivates catalase, a major antioxidant enzyme that has been inhibited in vivo by endogenous cysteine. Our data also reveal a critical role for bacterial NO-synthase in adaptation to oxidative stress associated with fast metabolic changes, and suggest a possible role for NO in defending pathogens against immune oxidative attack.

Does UVB radiation induce SoxS gene expression in Escherichia coli cells?

The SoxRS regulon is induced when bacterial cells are exposed to redox-cycling agents such as menadione or paraquat. In this paper it is shown that a physical agent, such as ultraviolet radiation with a wavelength of 312 nm (UVB) can induce soxS gene expression. The results indicate that this induction involves the RpoS protein. Moreover, an unexpected increase of soxS gene expression independent of a functional soxR gene in UVB-irradiated cells has been verified. This increase could be explained by transcription of soxS gene in a rpoS-dependent pathway.

Effects of low iron conditions on the repair of DNA lesions induced by Cumene hydroperoxide in Escherichia coli cells

In the present study, we evaluated the sensitivity of different Escherichia coli strains to Cumene hydroperoxide (CHP) treatment under distinct conditions of Fe2+ availability. Our results showed that the pretreatment with an iron chelator (dipyridyl) protects all the tested strains against CHP toxic effects, but it was not sufficient to abolish the CHP induced mutagenesis. On the other hand, simultaneous pretreatment with both dipyridyl and neocuproine (copper chelator) leads to a complete protection against CHP mutagenic effects. Our data suggest the participation of copper ion in the CHP mutagenesis induced in E. coli.

Copper-mediated toxicity of 2,4,5-trichlorophenol: biphasic effect of the copper(I)-specific chelator neocuproine

The lipophilic copper(I)-specific chelator neocuproine has been frequently used as an inhibitor of copper-mediated damage in biological systems. In this communication we report that the copper-mediated toxicity of 2,4,5-trichlorophenol is markedly potentiated by neocuproine at levels which are near-stoichiometric with respect to the copper concentration but is inhibited at higher concentrations. However, no potentiation was observed when neocuproine was substituted by bathocuproinedisulfonic acid, a negative charged ligand with essentially the same copper-binding characteristics as neocuproine. We found that the potentiation by neocuproine was due to the formation of a lipophilic copper complex, while the inhibition by bathocuproinedisulfonic acid was due to the formation of a hydrophilic one. Caution in the use of neocuproine to study copper-mediated toxicity is advised.

Efficient repair of hydrogen peroxide-induced DNA damage by Escherichia coli requires SOS induction of RecA and RuvA proteins

The survival of Escherichia coli following treatment with a low dose (1-3 mM) of hydrogen peroxide (H(2)O(2)) that causes extensive mode-one killing of DNA repair mutants is stimulated by the induction of the SOS regulon. Results for various mutants indicate that induction of recA and RecA protein-mediated recombination are critical factors contributing to the repair of H(2)O(2)-induced oxidative DNA damage. However, because DNA damage activates RecA protein's coprotease activity essential to cleavage of LexA repressor protein and derepression of all SOS genes, it is unclear to what extent induction of RecA protein stimulates this repair. To make this determination, we examined mode-one killing of DeltarecA cells carrying plasmid-borne recA (P(tac)-recA(+)) and constitutively expressing a fully induced level of wild-type RecA protein when SOS genes other than recA are non-inducible in a lexA3 (Ind(-)) genetic background or inducible in a lexA(+) background. At a H(2)O(2) dose resulting in maximal killing, DeltarecA lexA3 (Ind(-)) cells with P(tac)-recA(+) show 40-fold greater survival than lexA3 (Ind(-)) cells with chromosomal recA having a low, non-induced level of RecA protein. However, they still show 10- to 15-fold lower survival than wild-type cells and DeltarecA lexA(+) cells with P(tac)-recA(+). To determine if the inducible RuvA protein stimulates survival, we examined a ruvA60 mutant that is defective for the repair of UV-induced DNA damage. This mutant also shows 10- to 15-fold lower survival than wild-type cells. We conclude that while induction of RecA protein has a pronounced stimulatory effect on the recombinational repair of H(2)O(2)-induced oxidative DNA damage, the induction of other SOS proteins such as RuvA is essential for wild-type repair.

Repair of DNA lesions induced by hydrogen peroxide in the presence of iron chelators in Escherichia coli: participation of endonuclease IV and Fpg

In Escherichia coli, the repair of lethal DNA damage induced by H(2)O(2) requires exonuclease III, the xthA gene product. Here, we report that both endonuclease IV (the nfo gene product) and exonuclease III can mediate the repair of lesions induced by H(2)O(2) under low-iron conditions. Neither the xthA nor the nfo mutants was sensitive to H(2)O(2) in the presence of iron chelators, while the xthA nfo double mutant was significantly sensitive to this treatment, suggesting that both exonuclease III and endonuclease IV can mediate the repair of DNA lesions formed under such conditions. Sedimentation studies in alkaline sucrose gradients also demonstrated that both xthA and nfo mutants, but not the xthA nfo double mutant, can carry out complete repair of DNA strand breaks and alkali-labile bonds generated by H(2)O(2) under low-iron conditions. We also found indications that the formation of substrates for exonuclease III and endonuclease IV is mediated by the Fpg DNA glycosylase, as suggested by experiments in which the fpg mutation increased the level of cell survival, as well as repair of DNA strand breaks, in an AP endonuclease-null background.

H2O2-induced cross-protection against UV-C killing in Escherichia coli is blocked in a lexA (Def) background

Pretreatment with 2.5 mM H2O2 protects E. coli cells against UV-C killing, a phenomenon independent of LexA cleavage. In this paper, we observe that this cross-protection response is neither dependent on the dinY gene product nor on the system that controls dinY, since H2O2 is able to induce cross-protection but not to induce the dinY gene in a lexA-noninducible strain [lexA (Ind-)]. Moreover, this response is not induced in a lexA (Def) background, suggesting that the expression of the SOS regulon may inhibit this cross-protection response.

Synergistic lethal effect between hydrogen peroxide and neocuproine (2,9-dimethyl 1,10-phenanthroline) in Escherichia coli

Despite 2,9-dimethyl 1,10-phenanthroline (NC) has been extensively used as a potential inhibitor of damage due to oxidative stress in biological systems, the incubation of E. coli cultures with the copper ion chelator NC prior to the challenge with hydrogen peroxide caused a lethal synergistic effect. The SOS response seems to be involved in the repair of the synergistic lesions through the recombination pathway. Furthermore, there is evidence for the UvrABC excinuclease participation in the repair of the synergistic lesions, and the base excision repair may also be required for bacterial survival to the synergistic effect mainly at high concentrations of H2O2, being the action of Fpg protein an important event. Incubation of lexA (Ind-) cultures with iron (II) ion chelator 2,2'-dipyridyl simultaneously with NC prevented the lethal synergistic effect. This result suggests an important role of the Fenton reaction on the phenomenon. NC treatment was able to increase the number of DNA strand breaks (DNAsb) induced by 10 mM of H2O2 in lexA (Ind-) strain and the simultaneous treatment with 2,2'-dipyridyl was able to block this effect.

Hydrogen peroxide induces protection against lethal effects of cumene hydroperoxide in Escherichia coli cells: an Ahp dependent and OxyR independent system?

Pretreatment with 2.5 mM H2O2 protects bacterial cells against cumene hydroperoxide killing. This response is independent of the OxyR system, but possibly involves the participation of Ahp protein, since ahp mutants are not protected. Treatment of bacterial cells with high H2O2 concentrations caused an alteration on the electrophoretic profile of the smaller subunit (22-kDa) of Ahp. This alteration does not require novel gene products and is not dependent on the OxyR protein. In this way, we propose that the modification of the 22-kDa subunit of Ahp by high H2O2 concentration may be responsible for the protection against the lethal effects of cumene hydroperoxide.

The role of thiamine in Yersinia kristensenii resistance to antibiotics and heavy metals

The resistance to divalent metal ions, antibiotics and H2O2 was investigated in Yersinia kristensenii strains 13, 15, 18 by performing subcultivations with CdSO4 (20 and 100 mg/L) in nutrient agar (NA) and M9 medium with thiamine. Metal resistance of all three strains in NA was the same and decreased in the following sequence: Ni > Zn = Co > Cd. The chloramphenicol (Cmp) resistance ranged between 32 and 256 mg/L and the H2O2 sensitivity was very low or even zero. In the presence of thiamine the metal resistance sequence changed to Zn = Cd > Ni, Co, Ni and Co tolerance being 10-20 mg/L. Cmp resistance of all strains increased to 256 mg/L and H2O2 sensitivity also rose. In Cd-treated cultures, the ratio of glucose to thiamine in culture medium affected Cd resistance. At normal content of glucose and thiamine (5 g/L and 5 mg/L), Cd resistance markedly decreased coincident with thiamine exhaustion in these slowly-growing cultures. The Cmp resistance decreased to 16 mg/L, Ni and Co intolerance and H2O2 hypersensitivity appeared. At lowered glucose or thiamine levels (5 g/L and 2.5 mg/L or 2.5 g/L and 5 mg/L) a marginal decrease of Cd resistance took place in response to limited glucose uptake. Low thiamine or low-glucose cultures were resistant to H2O2, and exhibited a small decrease in Cmp resistance and a low Ni, Co tolerance. The adaptation of strain 15 to Cd induced only a small decrease of Cd resistance. Lowered glucose-to-thiamine ratio in culture medium probably induced in Cd-treated cultures a response triggering Cd resistance.

Evaluation of technetium-99m decay on Escherichia coli inactivation: effects of physical or chemical agents

Technetium-99m (99mTc) has been used in nuclear medicine and in biomedical research to label molecular and cellular structures employed as radiotracers. Here, we have evaluated, on a DNA repair proficient Escherichia coli strain, the 99mTc decay inactivation and the influence of the (i) pre-treatment with metal ion chelators or of the (ii) treatment with a free radical scavenger on the protection of the cells against the lethal effect of the 99mTc. As SnCl2 is frequently used as a reducing agent in the 99mTc-labeling process, we have also studied the capability of SnCl2 to alter the biological effects induced by the 99mTc decay. As we are exposed to either chemical or physical agents in the nature, we have decided to study a possible influence of the ultraviolet solar radiation in the biological phenomena induced by the 99mTc decay. Our data point out (i) a very important role of the Auger and/or conversion electrons in the cytotoxicity induced by the 99mTc decay; (ii) SnCl2, the metal ion chelators and the free radical scavenger protect the cells against the lethal effect of the 99mTc; and (iii) near-UV does not alter the lethal effect of the 99mTc decay.

Effects of 1,10-phenanthroline and hydrogen peroxide in Escherichia coli: lethal interaction

It has been observed that when Escherichia coli cells are treated simultaneously with phenanthroline and H2O2, there is a lethal interaction. In order to analyze the mechanism of this lethal interaction, wild-type and xthA mutant cells of E. coli were treated with 2.5 mM H2O2 and 1 mM phenanthroline. This treatment was preceded by treatments with different metal chelators (dipyridyl for Fe2+, desferal for Fe3+ and neocuproine for Cu2+) or conducted simultaneously to other treatments with chelators and radical scavengers (thiourea, ethanol and sodium benzoate). The lethal interaction was observed in both the E. coli wild-type strain and xthA mutant strain, which is deficient in the exonuclease III repair enzyme. Nevertheless, the mutant strain was much more sensitive than the wild-type one. Dipyridyl pretreatment protected the cells against the lethal interaction, while desferal pretreament was unable to do so. This suggests that the lethal interaction requires Fe2+ and not Fe3+ ions. Ethanol and sodium benzoate were incapable of protecting bacterial cells against the lethal interaction. Even a 20-min pretreatment with benzoate did not confer protection. On the other hand, thiourea protected the cells completely. Based on our results, we propose that the lethal interaction may be caused not only by the reaction kinetics of phenanthroline and Fe, but also by the ability of phenanthroline to intercalate in DNA. After forming the mono and bis complexes, phenanthroline would serve as a shuttle and take the Fe2+ ions to the DNA. So, the Fenton reaction would take its course with the consequent generation of OH. radicals near DNA. This proximity to the DNA would protect the OH. radicals against the scavengers' action, thus optimizing the Fenton reaction.

Structural and physiological determinants of resistance ofAeromonas salmonicidato reactive radicals

The facultative intracellular pathogen Aeromonas salmonicida survives and replicates in macrophages, a virulence trait presumed to be associated with its ability to resist reactive radicals. The mechanisms used by A. salmonicida to resist reactive radicals in vitro were shown to have both structural and physiological determinants. The sensitivity of A. salmonicida to exogenous H2O2, superoxide, and nitrogen radicals, as well as endogenous oxygen radicals, differed depending on growth conditions, cell surface structure, and preexposure to sublethal doses of radicals. Whereas sensitivities to exogenous oxygen radicals did not correlate with basal levels of catalase or Fe-superoxide dismutase, under similar culture conditions S-layer positive cells were more resistant to oxygen radicals than S-layer mutants. S-layer mutants recovered resistance when physically reconstituted with S-layer sheets. Hemin-coated S-layers, while protective against nitrogen radicals, sensitized A. salmonicida to H2O2. Sublethal concentrations of H2O2or superoxide induced a highly protective response characterized by de novo synthesis of both catalase and Mn-superoxide dismutase. It is proposed that for A. salmonicida the constitutive S-layer provides a first line of defense and the inducible catalase and Mn-superoxide dismutase provide a powerful second line of defense against macrophage-mediated killing via reactive oxygen species.Key words: Aeromonas salmonicida, oxygen radicals, nitrogen radicals, oxidative stress, S-layers.

Protection of DNA during oxidative stress by the nonspecific DNA-binding protein Dps

Reactive oxygen species can damage most cellular components, but DNA appears to be the most sensitive target of these agents. Here we present the first evidence of DNA protection against the toxic and mutagenic effects of oxidative damage in metabolically active cells: direct protection of DNA by Dps, an inducible nonspecific DNA-binding protein from Escherichia coli. We demonstrate that in a recA-deficient strain, expression of Dps from an inducible promoter prior to hydrogen peroxide challenge increases survival and reduces the number of chromosomal single-strand breaks. dps mutants exhibit increased levels of the G x C-->T x A mutations characteristic of oxidative damage after treatment with hydrogen peroxide. In addition, expression of Dps from the inducible plasmid reduces the frequency of spontaneous G x C-->T x A and A x T-->T x A mutations and can partially suppress the mutator phenotype of mutM (fpg) and mutY alleles. In a purified in vitro system, Dps reduces the number of DNA single-strand breaks and Fpg-sensitive sites introduced by hydrogen peroxide treatment, indicating that the protection observed in vivo is a direct effect of DNA binding by Dps. The widespread conservation of Dps homologs among prokaryotes suggests that this may be a general strategy for coping with oxidative stress.

Role of SOS and OxyR systems in the repair of Escherichia coli submitted to hydrogen peroxide under low iron conditions

There are at least two mechanisms by which H2O2 induces DNA lesions in Escherichia coli: one in the presence of physiological iron levels and the other in low iron conditions. The survival as well as the induction of SOS response in different DNA repair mutant strains of E coli was evaluated after H2O2 treatment under low iron conditions (pretreatment with an iron chelator). Our results indicate that, in normal iron conditions RecA protein has a relevant role in recombination repair events, while in low iron conditions RecA protein is important as a positive regulator of the SOS response. On the other hand, the oxy delta R mutant is sensitive to the lethal effects of H2O2 only in low iron conditions and this sensitivity cannot be correlated with DNA strand breaks.

Mutagenic activity and specificity of hydrogen peroxide in the ad-3 forward-mutation test in two-component heterokaryons of Neurospora crassa

In the ad-3 forward-mutation test, hydrogen peroxide was at best a weak mutagen in nongrowing conidia from a DNA repair-proficient heterokaryon (H-12, uvs-2+/uvs-2+) but was a moderate mutagen in nongrowing conidia from a DNA-repair-deficient heterokaryon (H-59, uvs-2/uvs-2) over a narrow range of high concentrations. H-59 also was more sensitive than H-12 to the killing activity of hydrogen peroxide at high concentrations. Thus, a DNA-repair pathway, of which the gene product of the uvs-2+ allele is a part, appears to be involved in the repair of hydrogen peroxide-induced DNA lesions at low survival in these strains. There was slightly, but significantly, more killing by hydrogen peroxide of nongrowing conidia from H-12 and H-59 in the presence of O2 than in the absence of O2 (presence of N2). Thus, the killing activity of hydrogen peroxide was enhanced by O2. The Mutational Spectra of hydrogen peroxide-induced ad-3 mutants shows that hydrogen peroxide induced mainly gene/point mutations but also some multilocus deletion mutations in H-12 and H-59. Multiple-locus mutations occurred only in H-59, but the frequency was very low. The frequencies of the 3 kinds of intracistronic complementation pattern among ad-3BR mutants (gene/point mutations) suggest that hydrogen peroxide induced both base-pair substitutions and frameshift mutations in both strains.

Hydrogen peroxide induces protection against N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) effects in Escherichia coli

Cross-adaptive response is defined as the capacity of cells to become resistant to a lethal agent when pretreated with a different lethal substance. In the present paper, the cross-adaptive response between hydrogen peroxide and N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) was studied in Escherichia coli repair mutants. Our results suggest that high doses of H2O2 induces protection against the lethal effects of MNNG in wild-type strain, ada, ogt, ada-ogt, aidB and alkA mutants. On the other hand, the MNNG induced mutagenesis is reduced by H2O2 pretreatment in wild-type and ogt mutant strains, but not in ada mutant. Furthermore, the protecting effect induced by H2O2 is time dependent: it decreases 15 min after the pretreatment and, after 30 min, is almost abolished. This reduction in the protecting effect is followed by an augmentation in the mutation frequency when MNNG is added 30 min after H2O2 pretreatment. This cross-adaptive response may be due to a modification of the MNNG alkylation pattern in the oxidized DNA.

Fpg and UvrA proteins participate in the repair of DNA lesions induced by hydrogen peroxide in low iron level in Escherichia coli

The survival of different DNA repair mutant strains of Escherichia coli treated with H2O2 was evaluated in the presence or absence of an iron chelator (dipyridyl). Our results suggest that Fpg and UvrA proteins participate in vivo in the repair of DNA lesions produced by higher H2O2 concentrations in the presence of an iron chelator while UvrB and UvrC proteins seem to be ineffective in the repair of these lesions.

Generation of reactive-oxygen species induced by electropermeabilization of Chinese hamster ovary cells and their consequence on cell viability

Cells can be permeabilized transiently when pulsed by a brief intense external electric field. The molecular and cellular bases of cell electropermeabilization are still unclear. This process can be described by a local transient membrane organisation in which high permeability exists. In this study, using the chemiluminescent probe lucigenin, we showed that electropulsation of Chinese hamster ovary cells induced generation of reactive oxygen species at the electropermeabilized cell level. This generation was directly associated with the part of the membrane surface which is electrically restructured, as shown by its dependence on electric parameters. The electroinduced cell process was activated by Ca2+ and Mg2+ ions, and by exogenous adenosine 5'-triphosphate. A metal-ion-catalyzed Haber-Weiss reaction was thought to occur in the process, as shown by the action of effector molecules of Haber-Weiss reaction such as the chelating agent EDTA, and the hydroxyl radical scavenger dimethylsulfoxide. The modulation of the oxygen species electroinduction and the use of antioxidant products (dimethylsulfoxide, sodium L-ascorbate) showed that cell survival after electric treatment was directly correlated to the oxidative jump intensity. This observation had to be associated with the cell-damaging action of oxygen-reactive species.

Iron released from an erythrocyte lysate by oxidative stress is diffusible and in redox active form

The incubation of a ghost-free erythrocyte lysate with the oxidizing agent phenylhydrazine resulted in both methemoglobin formation and release of iron in a desferrioxamine (DFO)-chelatable form. The released iron was diffusible, as shown by a dialysis carried out simultaneously with the incubation. When the dialysate was added to erythrocyte ghosts or to microsomes from liver or brain, lipid peroxidation developed in the membranes, indicating that the diffusible iron was in a redox active form. The addition of ATP to the lysate markedly increased both iron diffusion and lipid peroxidation in the membranes subsequently added to the dialysate. The possible implication of these data in some well known pathologies is discussed.

Modulation of the H2O2-induced SOS response in Escherichia coli PQ300 by amino acids, metal chelators, antioxidants, and scavengers of reactive oxygen species

The SOS chromotest is a simple colorimetric genotoxicity assay that monitors DNA repair by measuring the induction of the gene sfiA in Escherichia coli K-12. E. coli PQ300, a diagnostic SOS tester strain for the detection of oxidative genotoxins, carries a mutation in a key gene for antioxidative defense, oxyR. This mutation renders PQ300 more sensitive to oxidative genotoxins, particularly to H2O2. We found that induction of the SOS response by H2O2 in E. coli PQ300 is dependent on the composition of the incubation medium; a substantially reduced response was obtained in minimal phosphate buffered saline (PBS) as opposed to complex Luria broth (LB) medium. Supplementation of PBS with histidine or cysteine stimulated H2O2-induced SOS induction to levels exceeding those found in LB medium. Low concentrations of glutathione (20-70 microM) also enhanced the H2O2-induced SOS response in E. coli PQ300, whereas higher concentrations (> 150 microM) were protective. Preincubation of tester cells with the chelators o-phenanthroline, 2,2-dipyridyl, and ethylenediaminetetraacetic acid (EDTA) protected cells from the effects of H2O2, although EDTA was only partially effective. Pretreatment of PQ300 with the antioxidant ascorbic acid or the hydroxyl radical scavenger dimethyl sulfoxide also diminished the SOS response, whereas mannitol and glucose were ineffective. The results show that the net effect of H2O2-induced DNA damage is influenced by the balance of oxidative and antioxidative factors and, furthermore, can be modulated by constituents of the extracellular milieu.