Endogenous superoxide dismutase levels regulate iron-dependent hydroxyl radical formation in Escherichia coli exposed to hydrogen peroxide

Iron(III) Binding Properties of PF1140, a Fungal N-Hydroxypyridone, and Activity against Mycoplasma genitalium

Mycoplasma genitalium is a sexually transmitted bacterium associated with urogenital disease syndromes in the US and worldwide. The global rise in drug resistance in M. genitalium necessitates the development of novel drugs to treat this pathogen. To address this need, we have screened extracts from a library of fungal isolates assembled through the University of Oklahoma Citizen Science Soil Collection Program. Analysis of one of the bioactive extracts using bioassay-guided fractionation led to the purification of the compound PF1140 (1) along with a new and several other known pyridones. The N-hydroxy pyridones are generally regarded as siderophores with high binding affinity for iron(III) under physiological conditions. Results from UV-vis absorption spectroscopy-based titration experiments revealed that 1 complexes with Fe3+. As M. genitalium does not utilize iron, we propose that the PF1140-iron complex induces cytotoxicity by facilitating the cellular uptake of iron, which reacts with endogenous hydrogen peroxide to produce toxic hydroxyl radicals.

Metal Ions Modify In Vitro DNA Damage Yields with High-LET Radiation

Cu2+ and Co2+ are metals known to increase DNA damage in the presence of hydrogen peroxide through a Fenton-type reaction. We hypothesized that these metals could increase DNA damage following irradiations of increasing LET values as hydrogen peroxide is a product of the radiolysis of water. The reaction mixtures contain either double- or single-stranded DNA in solution with Cu2+ or Co2+ and were irradiated either with X-ray, carbon-ion or iron-ion beams, or they were treated with hydrogen peroxide or bleomycin at increasing radiation dosages or chemical concentrations. DNA damage was then assessed via gel electrophoresis followed with a band intensity analysis. DNA damage was the greatest when DNA in the solution with either metal was treated with only hydrogen peroxide followed by the DNA damage of DNA in the solution with either metal post irradiation of low-LET (X-Ray) or high-LET (carbon-ion and iron-ion), respectively, and demonstrated the least damage after treatment with bleomycin. Cu2+ portrayed greater DNA damage than Co2+ following all experimental conditions. The metals' effect caused more DNA damage and was observed to be LET-dependent for single-strand break formation but inversely dependent for double-strand break formation. These results suggest that Cu2+ is more efficient than Co2+ at inducing both DNA single-strand and double-strand breaks following all irradiations and chemical treatments.

Combined Application of Aminoglycosides and Ascorbic Acid in the Elimination of Proteus mirabilis Rods Responsible for Causing Catheter-Associated Urinary Tract Infections (CAUTIs)-A Molecular Approach

Proteus mirabilis is a common cause of catheter-associated urinary tract infections (CAUTIs). In this study, we verified the effectiveness of amikacin or gentamicin and ascorbic acid (AA) co-therapy in eliminating uropathogenic cells, as well as searched for the molecular basis of AA activity by applying chromatographic and fluorescent techniques. Under simulated physiological conditions, a combined activity of the antibiotic and AA supported the growth (threefold) of the P. mirabilis C12 strain, but reduced catheter colonization (≤30%) in comparison to the drug monotherapy. Slight modifications in the phospholipid and fatty acid profiles, as well as limited (≤62%) 2',7'-dichlorofluorescein fluorescence, corresponding to the hydroxyl radical level, allowed for the exclusion of the hypothesis that the anti-biofilm effect of AA was related to membrane perturbations of the C12 strain. However, the reduced (≤20%) fluorescence intensity of propidium iodide, as a result of a decrease in membrane permeability, may be evidence of P. mirabilis cell defense against AA activity. Quantitative analyses of ascorbic acid over time with a simultaneous measurement of the pH values proved that AA can be an effective urine acidifier, provided that it is devoid of the presence of urease-positive cells. Therefore, it could be useful in a prevention of recurrent CAUTIs, rather than in their treatment.

Utilization of Pharmacological Ascorbate to Enhance Hydrogen Peroxide-Mediated Radiosensitivity in Cancer Therapy

Interest in the use of pharmacological ascorbate as a treatment for cancer has increased considerably since it was introduced by Cameron and Pauling in the 1970s. Recently, pharmacological ascorbate has been used in preclinical and early-phase clinical trials as a selective radiation sensitizer in cancer. The results of these studies are promising. This review summarizes data on pharmacological ascorbate (1) as a safe and efficacious adjuvant to cancer therapy; (2) as a selective radiosensitizer of cancer via a mechanism involving hydrogen peroxide; and (3) as a radioprotector in normal tissues. Additionally, we present new data demonstrating the ability of pharmacological ascorbate to enhance radiation-induced DNA damage in glioblastoma cells, facilitating cancer cell death. We propose that pharmacological ascorbate may be a general radiosensitizer in cancer therapy and simultaneously a radioprotector of normal tissue.

Coping with Reactive Oxygen Species to Ensure Genome Stability in Escherichia coli

The facultative aerobic bacterium Escherichia coli adjusts its cell cycle to environmental conditions. Because of its lifestyle, the bacterium has to balance the use of oxygen with the potential lethal effects of its poisonous derivatives. Oxidative damages perpetrated by molecules such as hydrogen peroxide and superoxide anions directly incapacitate metabolic activities relying on enzymes co-factored with iron and flavins. Consequently, growth is inhibited when the bacterium faces substantial reactive oxygen insults coming from environmental or cellular sources. Although hydrogen peroxide and superoxide anions do not oxidize DNA directly, these molecules feed directly or indirectly the generation of the highly reactive hydroxyl radical that damages the bacterial chromosome. Oxidized bases are normally excised and the single strand gap repaired by the base excision repair pathway (BER). This process is especially problematic in E. coli because replication forks do not sense the presence of damages or a stalled fork ahead of them. As consequence, single-strand breaks are turned into double-strand breaks (DSB) through replication. Since E. coli tolerates the presence of DSBs poorly, BER can become toxic during oxidative stress. Here we review the repair strategies that E. coli adopts to preserve genome integrity during oxidative stress and their relation to cell cycle control of DNA replication.

Iron chelation increases the tolerance of Escherichia coli to hyper-replication stress

In Escherichia coli, an increase in the frequency of chromosome replication is lethal. In order to identify compounds that affect chromosome replication, we screened for molecules capable of restoring the viability of hyper-replicating cells. We made use of two E. coli strains that over-initiate DNA replication by keeping the DnaA initiator protein in its active ATP bound state. While viable under anaerobic growth or when grown on poor media, these strains become inviable when grown in rich media. Extracts from actinomycetes strains were screened, leading to the identification of deferoxamine (DFO) as the active compound in one of them. We show that DFO does not affect chromosomal replication initiation and suggest that it was identified due to its ability to chelate cellular iron. This limits the formation of reactive oxygen species, reduce oxidative DNA damage and promote processivity of DNA replication. We argue that the benzazepine derivate (±)-6-Chloro-PB hydrobromide acts in a similar manner.

Eradication of Pseudomonas aeruginosa cells by cathodic electrochemical currents delivered with graphite electrodes

Antibiotic resistance is a major challenge to the treatment of bacterial infections associated with medical devices and biomaterials. One important intrinsic mechanism of such resistance is the formation of persister cells that are phenotypic variants of microorganisms and highly tolerant to antibiotics. Recently, we reported a new approach to eradicating persister cells of Pseudomonas aeruginosa using low-level direct electrochemical current (DC) and synergy with the antibiotic tobramycin. To further understand the underlying mechanism and develop this technology toward possible medical applications, we investigated the electricidal activities of non-metallic biomaterial on persister and biofilm cells of P. aeruginosa using graphite-based TGON™ 805 electrodes. We employed both single and dual chamber systems to compare electrochemical factors of TGON and stainless steel 304 electrodes. The results revealed that TGON-based treatments were highly effective against P. aeruginosa persister cells. In the single chamber system, complete eradication of planktonic persister cells (corresponding to a 7-log killing) was achieved with 70μA/cm² DC using TGON electrodes within 40min of treatment, while the cell viability in biofilms was reduced by 2 logs within 1h. The killing effects were dose and time dependent with higher current densities requiring less time. Moreover, reduction reactions were found more effective than oxidation reactions, confirming that metal cations are not indispensable, although they may facilitate cell killing. The findings of this study can help develop electrochemical technologies to eradicate persister and biofilm cells for more effective treatment of medical device and biomaterial associated infections.

STATEMENT OF SIGNIFICANCE

Infections associated with medical devices and biomaterials present a major challenge due to high-level tolerance of microbes to conventional antibiotics. It is well established that such tolerance is due to the formation of dormant persister cells and multicellular structures known as biofilms. Recent studies have demonstrated electrochemical treatment as a promising alternative to eradicate bacterial infections, since the killing mechanism is independent of the growth phase of bacterial cells, but relies on various electrochemical species interplaying during the treatment. The current study investigated major bactericidal properties of the electrochemical currents mediated via TGON, a carbon-based electrode material. Up to total eradication of Pseudomonas aeruginosa persister cells was achieved. The new knowledge of electrochemical properties and the bioactivity of TGON may help develop new methods/devices to eradicate bacterial infections by delivering safe levels of electrochemical currents.

Involvement of multiple stressors induced by non-thermal plasma-charged aerosols during inactivation of airborne bacteria

A lab-scale, tunable, single-filament, point-to-point nonthermal dieletric-barrier discharge (DBD) plasma device was built to study the mechanisms of inactivation of aerosolized bacterial pathogens. The system inactivates airborne antibiotic-resistant pathogens efficiently. Nebulization mediated pre-optimized (4 log and 7 log) bacterial loads were challenged to plasma-charged aerosols, and lethal and sublethal doses determined using colony assay, and cell viability assay; and the loss of membrane potential and cellular respiration were determined using cell membrane potential assay and XTT assay. Using the strategies of Escherichia coli wildtype, over-expression mutant, deletion mutants, and peroxide and heat stress scavenging, we analyzed activation of intracellular reactive oxygen species (ROS) and heat shock protein (hsp) chaperons. Superoxide dismutase deletion mutants (ΔsodA, ΔsodB, ΔsodAΔsodB) and catalase mutants ΔkatG and ΔkatEΔkatG did not show significant difference from wildtype strain, and ΔkatE and ΔahpC was found significantly more susceptible to cell death than wildtype. The oxyR regulon was found to mediate plasma-charged aerosol-induced oxidative stress in bacteria. Hsp deficient E. coli (ΔhtpG, ΔgroEL, ΔclpX, ΔgrpE) showed complete inactivation of cells at ambient temperature, and the treatment at cold temperature (4°C) significantly protected hsp deletion mutants and wildtype cells, and indicate a direct involvement of hsp in plasma-charged aerosol mediated E. coli cell death.

Overexpressed Superoxide Dismutase and Catalase Act Synergistically to Protect the Repair of PSII during Photoinhibition in Synechococcus elongatus PCC 7942

The repair of PSII under strong light is particularly sensitive to reactive oxygen species (ROS), such as the superoxide radical and hydrogen peroxide, and these ROS are efficiently scavenged by superoxide dismutase (SOD) and catalase. In the present study, we generated transformants of the cyanobacterium Synechococcus elongatus PCC 7942 that overexpressed an iron superoxide dismutase (Fe-SOD) from Synechocystis sp. PCC 6803; a highly active catalase (VktA) from Vibrio rumoiensis; and both enzymes together. Then we examined the sensitivity of PSII to photoinhibition in the three strains. In cells that overexpressed either Fe-SOD or VktA, PSII was more tolerant to strong light than it was in wild-type cells. Moreover, in cells that overexpressed both Fe-SOD and VktA, PSII was even more tolerant to strong light. However, the rate of photodamage to PSII, as monitored in the presence of chloramphenicol, was similar in all three transformant strains and in wild-type cells, suggesting that the overexpression of these ROS-scavenging enzymes might not protect PSII from photodamage but might protect the repair of PSII. Under strong light, intracellular levels of ROS fell significantly, and the synthesis de novo of proteins that are required for the repair of PSII, such as the D1 protein, was enhanced. Our observations suggest that overexpressed Fe-SOD and VktA might act synergistically to alleviate the photoinhibition of PSII by reducing intracellular levels of ROS, with resultant protection of the repair of PSII from oxidative inhibition.

Macrophage effector responses of horses are influenced by expression of CD154

Reactive intermediates contribute to innate immunity by providing phagocytes with a mechanism of defense against bacteria, viruses and parasites. To better characterize the role of CD154 in the production of reactive intermediates, we cloned and expressed recombinant equine CD154 (reqCD154) in Chinese Hamster Ovary (CHO). In co-culture experiments, CHO cells ectopically expressing reqCD154 elicited superoxide production in monocyte-derived macrophages (MDM). Collectively, our results indicate that regulation of CD154 expression plays a role in innate host defenses.

Chloro-benzoquinones cause oxidative DNA damage through iron-mediated ROS production in Escherichia coli

Chloro-benzoquinones (CBQs) are a group of disinfection byproducts that are suspected to be potentially carcinogenic. Here, the mechanism of DNA damage caused by CBQs in the presence of ferrous ions was investigated in an Escherichia coli wild type M5 strain and a mutant L5 (ahpCF katEG mutant) strain that carried an enhanced green fluorescent protein reporter under the control of a SOS response gene (recA) promoter. All tested CBQs (including para-benzoquinone, 2-chloro-para-benzoquinone, and dichloro-para-benzoquinones with different substitutes) caused substantial oxidative DNA damage with EC50 values in the micromolar range. Moreover, 2,5-dichloro-para-benzoquinone (2,5-DCBQ), a typical CBQ, caused substantial ROS production in E. coli mutant cells. And ROS scavengers provided partial protective effects on genotoxicity of 2,5-DCBQ to E. coli mutant cells. The addition of Fe(2+) to the 2,5-DCBQ exposure system caused an increase in DNA oxidative damage; iron-chelating agents could partially prevent these cells from DNA damage. Finally, intracellular AhpCF, catalase E, and catalase G were all found to play an important role in the survival of E. coli cells exposed to CBQs, as indicated by an increased sensitivity of the ahpCF katEG mutant L5 strain to treatment compared with wild type M5 cells. Taken together, these results suggest that CBQs cause oxidative DNA damage in E. coli cells through the participation of iron-mediated ROS production.

Cyanide enhances hydrogen peroxide toxicity by recruiting endogenous iron to trigger catastrophic chromosomal fragmentation

Hydrogen peroxide (HP) or cyanide (CN) are bacteriostatic at low-millimolar concentrations for growing Escherichia coli, whereas CN + HP mixture is strongly bactericidal. We show that this synergistic toxicity is associated with catastrophic chromosomal fragmentation. Since CN alone does not kill at any concentration, while HP alone kills at 20 mM, CN must potentiate HP poisoning. The CN + HP killing is blocked by iron chelators, suggesting Fenton's reaction. Indeed, we show that CN enhances plasmid DNA relaxation due to Fenton's reaction in vitro. However, mutants with elevated iron or HP pools are not acutely sensitive to HP-alone treatment, suggesting that, in addition, in vivo CN recruits iron from intracellular depots. We found that part of the CN-recruited iron pool is managed by ferritin and Dps: ferritin releases iron on cue from CN, while Dps sequesters it, quelling Fenton's reaction. We propose that disrupting intracellular iron trafficking is a common strategy employed by the immune system to kill microbes.

Metal ions, not metal-catalyzed oxidative stress, cause clay leachate antibacterial activity

Aqueous leachates prepared from natural antibacterial clays, arbitrarily designated CB-L, release metal ions into suspension, have a low pH (3.4–5), generate reactive oxygen species (ROS) and H₂O₂, and have a high oxidation-reduction potential. To isolate the role of pH in the antibacterial activity of CB clay mixtures, we exposed three different strains of Escherichia coli O157:H7 to 10% clay suspensions. The clay suspension completely killed acid-sensitive and acid-tolerant E. coli O157:H7 strains, whereas incubation in a low-pH buffer resulted in a minimal decrease in viability, demonstrating that low pH alone does not mediate antibacterial activity. The prevailing hypothesis is that metal ions participate in redox cycling and produce ROS, leading to oxidative damage to macromolecules and resulting in cellular death. However, E. coli cells showed no increase in DNA or protein oxidative lesions and a slight increase in lipid peroxidation following exposure to the antibacterial leachate. Further, supplementation with numerous ROS scavengers eliminated lipid peroxidation, but did not rescue the cells from CB-L-mediated killing. In contrast, supplementing CB-L with EDTA, a broad-spectrum metal chelator, reduced killing. Finally, CB-L was equally lethal to cells in an anoxic environment as compared to the aerobic environment. Thus, ROS were not required for lethal activity and did not contribute to toxicity of CB-L. We conclude that clay-mediated killing was not due to oxidative damage, but rather, was due to toxicity associated directly with released metal ions.

The immediate protective response to microbial challenge

The innate immune system detects infection and tissue injury through different families of pattern-recognition receptors (PRRs), such as Toll-like receptors. Most PRR-mediated responses initiate elaborate processes of signaling, transcription, translation, and secretion of effector mediators, which together require time to achieve. Therefore, PRR-mediated processes are not active in the early phases of infection. These considerations raise the question of how the host limits microbial replication and invasion during this critical period. Here, we examine the crucial defense mechanisms, such as antimicrobial peptides or extracellular traps, typically activated within minutes of the initial infection phase, which we term the "immediate protective response". Deficiencies in different components of the immediate protective response are often associated with severe and recurrent infectious diseases in humans, highlighting their physiologic importance.

Escherichia coli pyruvate:flavodoxin oxidoreductase, YdbK - regulation of expression and biological roles in protection against oxidative stress

E. coli YdbK is predicted to be a pyruvate:flavodoxin oxidoreductase (PFOR). However, enzymatic activity and the regulation of gene expression of it are not well understood. In this study, we found that E. coli cells overexpressing the ydbK gene had enhanced PFOR activity, indicating the product of ydbK to be a PFOR. The PFOR was labile to oxygen. The expression of ydbK was induced by superoxide generators such as methyl viologen (MV) in a SoxS-dependent manner after a lag period. We identified a critical element upstream of ydbK gene required for the induction by MV and proved direct binding of SoxS to the element. E. coli ydbK mutant was highly sensitive to MV, which was enhanced by additional inactivation of fpr gene encoding ferredoxin (flavodoxin):NADP(H) reductase (FPR). Aconitase activity, a superoxide sensor, was more extensively decreased by MV in the E. coli ydbK mutant than in wild-type strain. The induction level of soxS gene was higher in E. coli ydbK fpr double mutant than in wild-type strain. These results indicate that YdbK helps to protect cells from oxidative stress. It is possible that YdbK maintains the cellular redox state together with FPR and is involved in the reduction of oxidized proteins including SoxR in the late stages of the oxidative stress response in E. coli.

Gimme shelter: how Vibrio fischeri successfully navigates an animal's multiple environments

Bacteria successfully colonize distinct niches because they can sense and appropriately respond to a variety of environmental signals. Of particular interest is how a bacterium negotiates the multiple, complex environments posed during successful infection of an animal host. One tractable model system to study how a bacterium manages a host’s multiple environments is the symbiotic relationship between the marine bacterium, Vibrio fischeri, and its squid host, Euprymna scolopes. V. fischeri encounters many different host surroundings ranging from initial contact with the squid to ultimate colonization of a specialized organ known as the light organ. For example, upon recognition of the squid, V. fischeri forms a biofilm aggregate outside the light organ that is required for efficient colonization. The bacteria then disperse from this biofilm to enter the organ, where they are exposed to nitric oxide, a molecule that can act as both a signal and an antimicrobial. After successfully managing this potentially hostile environment, V. fischeri cells finally establish their niche in the deep crypts of the light organ where the bacteria bioluminesce in a pheromone-dependent fashion, a phenotype that E. scolopes utilizes for anti-predation purposes. The mechanism by which V. fischeri manages these environments to outcompete all other bacterial species for colonization of E. scolopes is an important and intriguing question that will permit valuable insights into how a bacterium successfully associates with a host. This review focuses on specific molecular pathways that allow V. fischeri to establish this exquisite bacteria–host interaction.

dpr and sod in Streptococcus mutans are involved in coexistence with S. sanguinis, and PerR is associated with resistance to H2O2

Large numbers of bacteria coexist in the oral cavity. Streptococcus sanguinis, one of the major bacteria in dental plaque, produces hydrogen peroxide (H(2)O(2)), which interferes with the growth of other bacteria. Streptococcus mutans, a cariogenic bacterium, can coexist with S. sanguinis in dental plaque, but to do so, it needs a means of detoxifying the H(2)O(2) produced by S. sanguinis. In this study, we investigated the association of three oxidative stress factors, Dpr, superoxide dismutase (SOD), and AhpCF, with the resistance of S. sanguinis to H(2)O(2). The knockout of dpr and sod significantly increased susceptibility to H(2)O(2), while the knockout of ahpCF had no apparent effect on susceptibility. In particular, dpr inactivation resulted in hypersensitivity to H(2)O(2). Next, we sought to identify the factor(s) involved in the regulation of these oxidative stress genes and found that PerR negatively regulated dpr expression. The knockout of perR caused increased dpr expression levels, resulting in low-level susceptibility to H(2)O(2) compared with the wild type. Furthermore, we evaluated the roles of perR, dpr, and sod when S. mutans was cocultured with S. sanguinis. Culturing of the dpr or sod mutant with S. sanguinis showed a significant decrease in the S. mutans population ratio compared with the wild type, while the perR mutant increased the ratio. Our results suggest that dpr and sod in S. mutans are involved in coexistence with S. sanguinis, and PerR is associated with resistance to H(2)O(2) in regulating the expression of Dpr.

VP2118 has major roles in Vibrio parahaemolyticus response to oxidative stress

BACKGROUND

Reactive oxygen species (ROS), including superoxide anion radical, induce chronic risk of oxidative damage to many cellular macromolecules resulting in damage to cells. Superoxide dismutases (SODs) catalyze the dismutation of superoxide to oxygen and hydrogen peroxide and are a primary defense against ROS. Vibrio parahaemolyticus, a marine bacterium that causes acute gastroenteritis following consumption of raw or undercooked seafood, can survive ROS generated by intestinal inflammatory cells. However, there is little information concerning SODs in V. parahaemolyticus. This study aims to clarify the role of V. parahaemolyticus SODs against ROS.

METHODS

V. parahaemolyticus SOD gene promoter activities were measured by a GFP reporter assay. Mutants of V. parahaemolyticus SOD genes were constructed and their SOD activity and resistance to oxidative stresses were measured.

RESULTS

Bioinformatic analysis showed that V. parahaemolyticus SODs were distinguished by their metal cofactors, FeSOD (VP2118), MnSOD (VP2860), and CuZnSOD (VPA1514). VP2118 gene promoter activity was significantly higher than the other SOD genes. In a VP2118 gene deletion mutant, SOD activity was significantly decreased and could be recovered by VP2118 gene complementation. The absence of VP2118 resulted in significantly lowered resistance to ROS generated by hydrogen peroxide, hypoxanthine-xanthine oxidase, or Paraquat. Furthermore, both the N- and C-terminal SOD domains of VP2118 were necessary for ROS resistance.

CONCLUSION

VP2118 is the primary V. parahaemolyticus SOD and is vital for anti-oxidative stress responses.

GENERAL SIGNIFICANCE

The V. parahaemolyticus FeSOD VP2118 may enhance ROS resistance and could promote its survival in the intestinal tract to facilitate host tissue infection.

Suppression of pleiotropic phenotypes of a Burkholderia multivorans fur mutant by oxyR mutation

Fur (ferric uptake regulator) is an iron-responsive transcriptional regulator in many bacterial species, and the fur mutant of Burkholderia multivorans ATCC 17616 exhibits pleiotropic phenotypes, such as an inability to efficiently use several carbon sources, as well as high sensitivity to hydrogen peroxide (H(2)O(2)), paraquat (a superoxide-producing compound) and nitric oxide (NO). To gain more insight into the pleiotropic role of the Fur protein of ATCC 17616, spontaneous suppressor mutants of the ATCC 17616 fur mutant that restored tolerance to NO were isolated and characterized in this study. The microarray-based comparative genomic analysis and subsequent sequencing analysis indicated that such suppressor mutants had a 2 bp deletion in the oxyR gene, whose orthologues encode H(2)O(2)-responsive transcriptional regulators in other bacterial species. The suppressor mutants and the reconstructed fur-oxyR double-deletion mutant showed indistinguishable phenotypes in that they were all (i) more resistant than the fur mutant to H(2)O(2), superoxide, NO and streptonigrin (an iron-activated antibiotic) and (ii) able to use carbon sources that cannot efficiently support the growth of the fur mutant. These results clearly indicate that the oxyR mutation suppressed the pleiotropic effect of the B. multivorans fur mutant. The fur-oxyR double mutants were found to overexpress the KatG (catalase/peroxidase) and AhpC1 and AhpD (alkyl hydroperoxide reductase subunits C and D) proteins, and their enzymic activities to remove reactive oxygen and nitrogen species were suggested to be responsible for the suppression of phenotypes caused by the fur mutation.

Highly reactive oxygen species: detection, formation, and possible functions

The so-called reactive oxygen species (ROS) are defined as oxygen-containing species that are more reactive than O(2) itself, which include hydrogen peroxide and superoxide. Although these are quite stable, they may be converted in the presence of transition metal ions, such as Fe(II), to the highly reactive oxygen species (hROS). hROS may exist as free hydroxyl radicals (HO·), as bound ("crypto") radicals or as Fe(IV)-oxo (ferryl) species and the somewhat less reactive, non-radical species, singlet oxygen. This review outlines the processes by which hROS may be formed, their damaging potential, and the evidence that they might have signaling functions. Since our understanding of the formation and actions of hROS depends on reliable procedures for their detection, particular attention is given to procedures for hROS detection and quantitation and their applicability to in vivo studies.

A combination of two antioxidants (an SOD mimic and ascorbate) produces a pro-oxidative effect forcing Escherichia coli to adapt via induction of oxyR regulon

Cationic Mn(III) N-alkylpyridyl (MnTalkyl-2(or 3)-PyP(5+)) and N, N'-dialkylimidazolylporphyrins (MnTDalkyl-2-ImP(5+)) have been regarded as the most powerful SOD mimics/peroxynitrite scavengers - i. e. antioxidants. The ethyl-, MnTE-2-PyP(5+) (AEOL10113), and hexylpyridyl-, MnTnHex-2-PyP(5+) and diethylimidazolylporphyrin, MnTDE-2-ImP(5+) (AEOL10150) have been mostly studied in vitro and in vivo. Given the in vivo abundance of cellular reductants, MnPs can couple with them in removing superoxide. Thus, they could be readily reduced from Mn(III)P to Mn(II)P with ascorbate and glutathione, and in a subsequent step reduce either O(2)(.-) (while acting as superoxide reductase) or oxygen (while exerting pro-oxidative action). Moreover, MnPs can catalyze ascorbate oxidation and in turn hydrogen peroxide production. The in vivo type of MnP action (anti- or pro-oxidative) will depend upon the cellular levels of reactive species, endogenous antioxidants, availability of oxygen, ratio of O(2)(.-)- to peroxide-removing systems, redox ability of MnPs and their cellular localization/bioavailibility. To exemplify the switch from an anti- to pro-oxidative action we have explored a very simple and straightforward system - the superoxide-specific aerobic growth of SOD-deficient E. coli. In such a system, cationic MnPs, ortho and meta MnTE-2-(or 3)-PyP(5+) act as powerful SOD mimics. Yet, in the presence of exogenous ascorbate, the SOD mimics catalyze the H(2)O(2) production, causing oxidative damage to both wild and SOD-deficient strains and inhibiting their growth. Catalase added to the medium reversed the effect indicating that H(2)O(2) is a major damaging/signaling species involved in cell growth suppression. The experiments with oxyR- and soxRS-deficient E. coli were conducted to show that E. coli responds to increased oxidative stress exerted by MnP/ascorbate system by induction of oxyR regulon and thus upregulation of antioxidative defenses such as catalases and peroxidases. As anticipated, when catalase was added into medium to remove H(2)O(2), E. coli did not respond with upregulation of its own antioxidant systems.

Role of antioxidant enzymes in bacterial resistance to organic acids

Growth in aerobic environments has been shown to generate reactive oxygen species (ROS) and to cause oxidative stress in most organisms. Antioxidant enzymes (i.e., superoxide dismutases and hydroperoxidases) and DNA repair mechanisms provide protection against ROS. Acid stress has been shown to be associated with the induction of Mn superoxide dismutase (MnSOD) in Lactococcus lactis and Staphylococcus aureus. However, the relationship between acid stress and oxidative stress is not well understood. In the present study, we showed that mutations in the gene coding for MnSOD (sodA) increased the toxicity of lactic acid at pH 3.5 in Streptococcus thermophilus. The inclusion of the iron chelators 2,2'-dipyridyl (DIP), diethienetriamine-pentaacetic acid (DTPA), and O-phenanthroline (O-Phe) provided partial protection against 330 mM lactic acid at pH 3.5. The results suggested that acid stress triggers an iron-mediated oxidative stress that can be ameliorated by MnSOD and iron chelators. These findings were further validated in Escherichia coli strains lacking both MnSOD and iron SOD (FeSOD) but expressing a heterologous MnSOD from S. thermophilus. We also found that, in E. coli, FeSOD did not provide the same protection afforded by MnSOD and that hydroperoxidases are equally important in protecting the cells against acid stress. These findings may explain the ability of some microorganisms to survive better in acidified environments, as in acid foods, during fermentation and accumulation of lactic acid or during passage through the low pH of the stomach.

Exposure to polychlorinated biphenyls enhances lipid peroxidation in human normal peritoneal and adhesion fibroblasts: a potential role for myeloperoxidase

Nitric oxide, superoxide, and lipid peroxidation (LPO) produced under oxidative stress may contribute to the development of postoperative adhesions. The objective of this study was to determine the effects of polychlorinated biphenyls (PCBs) on LPO, superoxide dismutase, myeloperoxidase (MPO), and nitrite/nitrate in human normal peritoneal and adhesion fibroblasts. PCB treatment reduced inducible nitric oxide synthase (iNOS) expression as well as levels of nitrite/nitrate in both cell lines. Although there was no difference in iNOS expression between the two cell lines, adhesion fibroblasts manifested lower basal levels of MPO compared to normal peritoneal fibroblasts. There was a reduction in MPO expression and its activity in response to PCB treatment in normal peritoneal fibroblasts; however, this effect was minimal in adhesion fibroblasts. Moreover, adhesion fibroblasts manifested higher levels of LPO compared to normal peritoneal fibroblasts, whereas PCB treatment increased LPO levels in both cell types. We conclude that PCBs promote the development of the adhesion phenotype by generating an oxidative stress environment. This is evident by lower iNOS, MPO, and nitrite/nitrate and a simultaneous increase in LPO. Loss of MPO activity, possibly through a mechanism involving MPO heme depletion and free iron release, is yet another source of oxidative stress.

Microarray analysis of Mycobacterium bovis BCG revealed induction of iron acquisition related genes in response to hydrogen peroxide

Mycobacterium bovis BCG strain Pasteur 1173P2 responds with adaptive and protective strategies against oxidative stress. Despite advances in our understanding of the responses to oxidative stress in many specific cases, the connectivity between targeted protective genes and the rest of cell metabolism remains obscure. This study was therefore carried out to investigate the genome-wide response of M. bovis BCG to hydrogen peroxide after 10 and 60 min of treatment. ATP measurements were carried out in order to monitor the changes in M. bovis BCG growth over a 1 h period. The furA gene in Mycobacterium bovis, a pleiotropic regulator that couples iron metabolism to the oxidative stress response was involved in the response to hydrogen peroxide stress. There were also increased levels of catalase/ peroxidase (KatG) and the biosynthesis operon of mycobactin. This study revealed significant upregulation of the oxidative response group of M. bovis, amino acid transport and metabolism, defense mechanisms, DNA replication, recombination and repair, and downregulation of cell cycle control, mitosis, and meiosis, lipid transport and metabolism, and cell wall/membrane biogenesis. This study shows that the treatment of M. bovis BCG with hydrogen peroxide induces iron acquisition related genes and oxidative stress response genes within one hour of treatment.

Oxidative stress and mitochondrial dysfunction in neurodegenerative diseases

In recent years, it has become increasingly clear that mitochondrial dysfunction and oxidative damage are major contributors to neuronal loss. Free radicals, typically generated from mitochondrial respiration, cause oxidative damage of nucleic acids, lipids, carbohydrates and proteins. Despite enormous amount of effort, however, the mechanism by which oxidative damage causes neuronal death is not well understood. Emerging data from a number of neurodegenerative diseases suggest that there may be common features of toxicity that are related to oxidative damage. In this review, while focusing on Huntington's disease (HD), we discuss similarities among HD, Friedreich ataxia and xeroderma pigmentosum, which provide insight into shared mechanisms of neuronal death.

Thein vitroeffect of hydrogen peroxide on vaginal microbial communities

This study presents a series of experiments carried out in order to elucidate the role of H2O2 in antimicrobial activity of lactobacilli. Vaginal swabs were collected from 60 premenopausal women and checked for pH and Nugent score, and Lactobacillus species were cultured, phenotyped and genotyped. The main outcome measures involved: (1) species of vaginal lactobacilli most effective in liberating H2O2, (2) minimal microbicidal concentrations of added H2O2, (3) kinetics of H2O2 liberation in relation to oxygen tension, (4) antimicrobial activity of pure H2O2 versus one produced by selected vaginal lactobacilli and the total activity of their culture supernatants. Results showed that H2O2 was liberated especially by: Lactobacillus delbrueckii, Lactobacillus acidophilus, Lactobacillus crispatus, Lactobacillus johnsonii and L. gasseri. Hydrogen peroxide reached concentrations from 0.05 to 1.0 mM, which under intensive aeration increased even up to 1.8 mM. Microorganisms related to vaginal pathologies show varied resistance to the action of pure H2O2. Most potent inhibitory activity against bacteria and yeasts was presented by Lactobacillus culture supernate producing H2O2, followed by the nonproducing strain and pure H2O2. To conclude - the antimicrobial activity of lactobacilli is a summation of various inhibitory mechanisms in which H2O2 plays some but not a crucial role, in addition to other substances.

Superoxide dismutase B gene (sodB)-deficient mutants of Francisella tularensis demonstrate hypersensitivity to oxidative stress and attenuated virulence

A Francisella tularensis live vaccine strain mutant (sodB(Ft)) with reduced Fe-superoxide dismutase gene expression was generated and found to exhibit decreased sodB activity and increased sensitivity to redox cycling compounds compared to wild-type bacteria. The sodB(Ft) mutant also was significantly attenuated for virulence in mice. Thus, this study has identified sodB as an important F. tularensis virulence factor.

Microarray analysis of Pseudomonas aeruginosa reveals induction of pyocin genes in response to hydrogen peroxide

Background

Pseudomonas aeruginosa, a pathogen infecting those with cystic fibrosis, encounters toxicity from phagocyte-derived reactive oxidants including hydrogen peroxide during active infection. P. aeruginosa responds with adaptive and protective strategies against these toxic species to effectively infect humans. Despite advances in our understanding of the responses to oxidative stress in many specific cases, the connectivity between targeted protective genes and the rest of cell metabolism remains obscure.

Results

Herein, we performed a genome-wide transcriptome analysis of the cellular responses to hydrogen peroxide in order to determine a more complete picture of how oxidative stress-induced genes are related and regulated.

Our data reinforce the previous conclusion that DNA repair proteins and catalases may be among the most vital antioxidant defense systems of P. aeruginosa. Our results also suggest that sublethal oxidative damage reduces active and/or facilitated transport and that intracellular iron might be a key factor for a relationship between oxidative stress and iron regulation. Perhaps most intriguingly, we revealed that the transcription of all F-, R-, and S-type pyocins was upregulated by oxidative stress and at the same time, a cell immunity protein (pyocin S2 immunity protein) was downregulated, possibly leading to self-killing activity.

Conclusion

This finding proposes that pyocin production might be another novel defensive scheme against oxidative attack by host cells.

Substantial DNA damage from submicromolar intracellular hydrogen peroxide detected in Hpx- mutants of Escherichia coli

Since the discovery of catalase, it has been postulated that aerobic organisms generate enough oxidants to threaten their own fitness and, in particular, their genetic stability. An alternative is that these enzymes exist to defend the cell against more-abundant oxidants imposed by external sources. These hypotheses were tested directly through study of Hpx- (katG katE ahpCF) mutants of Escherichia coli, which lack enzymes to scavenge hydrogen peroxide (H2O2). These strains grew well in anaerobic medium but poorly when they were aerated. The Hpx- bacteria formed filaments and exhibited high rates of mutagenesis, both indicators of DNA damage. An additional recA mutation caused Hpx- cells to die rapidly upon aeration, even though the intracellular H2O2 was <1 microM. Spin-trap experiments detected substantial hydroxyl radicals, and cell-permeable iron chelators eliminated both the phenotypic defects and hydroxyl-radical formation, confirming that the Fenton reaction was responsible. An Hpx- oxyR strain exhibited even more DNA lesions than did the Hpx- mutant, indicating that the OxyR stress response induced protein(s) that suppressed DNA damage. One critical protein was Dps, an iron-sequestration protein, because Hpx- dps mutants exhibited sensitivity similar to that of the Hpx- oxyR mutant. These results reveal that aerobic E. coli generates sufficient H2O2 to create toxic levels of DNA damage. Scavenging enzymes and controls on free iron are required to avoid that fate. The rate constant of the Fenton reaction measured at physiological pH was much higher than under the acidic conditions that were used to determine the commonly cited value.

Involvement of reactive oxygen species in the bactericidal activity of activated carbon fibre supporting silver; Bactericidal activity of ACF(Ag) mediated by ROS

An activated carbon fibre supporting silver (ACF(Ag)) was tested for its antibacterial capacity against Escherichia coli (E. coli). Water that has passed through ACF(Ag) demonstrated strong bactericidal ability. This activity decreased over the time suggesting that generated bactericidal species were short lifespan. Since formation of reactive oxygen species (ROS) might be catalysed by silver impregnated and/or ACF itself, implication of ROS and silver was evaluated by the use of ROS scavengers and a silver ions neutralizing agent. The role of ROS in the E. coli mortality was confirmed by the use of a molecular approach which revealed a strong expression of oxidative stress genes.

Vitreoscilla hemoglobin renders Enterobacter aerogenes highly susceptible to heavy metals

When expressed in heterologous microorganisms Vitreoscilla hemoglobin (VHb) acts as oxygen storage and causes a higher oxygen uptake. In this study, the effect of this protein on growth, sensitivity and antioxidant properties of Enterobacter aerogenes exposed to metal stress was investigated. The strain expressing VHb was more sensitive to mercury and cadmium as the minimal inhibitory concentration (MIC) for these metals was up to 2-fold lower in this strain than the host and the recombinant strain carrying a comparable plasmid. At lower concentrations than MIC, the metals partially limited growth and caused an inhibition proportional to metal concentration applied. The growth pattern of VHb expressing strain was also distinctly different from other two non-hemoglobin strains. The hemoglobin containing strain showed substantially higher superoxide dismuates (SOD) activity than the non-hemoglobin strains, while catalase levels were similar in all strains. All strains exposed to copper, however, showed similar MIC values, growth patterns, and SOD and catalase levels.

Antimicrobial reactive oxygen and nitrogen species: concepts and controversies

Phagocyte-derived reactive oxygen and nitrogen species are of crucial importance for host resistance to microbial pathogens. Decades of research have provided a detailed understanding of the regulation, generation and actions of these molecular mediators, as well as their roles in resisting infection. However, differences of opinion remain with regard to their host specificity, cell biology, sources and interactions with one another or with myeloperoxidase and granule proteases. More than a century after Metchnikoff first described phagocytosis, and more than four decades after the discovery of the burst of oxygen consumption that is associated with microbial killing, the seemingly elementary question of how phagocytes inhibit, kill and degrade microorganisms remains controversial. This review updates the reader on these concepts and the topical questions in the field.

Expression of a heterologous manganese superoxide dismutase gene in intestinal lactobacilli provides protection against hydrogen peroxide toxicity

In living organisms, exposure to oxygen provokes oxidative stress. A widespread mechanism for protection against oxidative stress is provided by the antioxidant enzymes: superoxide dismutases (SODs) and hydroperoxidases. Generally, these enzymes are not present in Lactobacillus spp. In this study, we examined the potential advantages of providing a heterologous SOD to some of the intestinal lactobacilli. Thus, the gene encoding the manganese-containing SOD (sodA) was cloned from Streptococcus thermophilus AO54 and expressed in four intestinal lactobacilli. A 1.2-kb PCR product containing the sodA gene was cloned into the shuttle vector pTRK563, to yield pSodA, which was functionally expressed and complemented an Escherichia coli strain deficient in Mn and FeSODs. The plasmid, pSodA, was subsequently introduced and expressed in Lactobacillus gasseri NCK334, Lactobacillus johnsonii NCK89, Lactobacillus acidophilus NCK56, and Lactobacillus reuteri NCK932. Molecular and biochemical analyses confirmed the presence of the gene (sodA) and the expression of an active gene product (MnSOD) in these strains of lactobacilli. The specific activities of MnSOD were 6.7, 3.8, 5.8, and 60.7 U/mg of protein for L. gasseri, L. johnsonii, L. acidophilus, and L. reuteri, respectively. The expression of S. thermophilus MnSOD in L. gasseri and L. acidophilus provided protection against hydrogen peroxide stress. The data show that MnSOD protects cells against hydrogen peroxide by removing O(2)(.-) and preventing the redox cycling of iron. To our best knowledge, this is the first report of a sodA from S. thermophilus being expressed in other lactic acid bacteria.

Environmental stress responses in Lactobacillus: a review

Environmental stress responses in Lactobacillus, which have been investigated mainly by proteomics approaches, are reviewed. The physiological and molecular mechanisms of responses to heat, cold, acid, osmotic, oxygen, high pressure and starvation stresses are described. Specific examples of the repercussions of these effects in food processing are given. Molecular mechanisms of stress responses in lactobacilli and other bacteria are compared.

Factors contributing to hydrogen peroxide resistance in Streptococcus pneumoniae include pyruvate oxidase (SpxB) and avoidance of the toxic effects of the fenton reaction

Aerobic growth of Streptococcus pneumoniae results in production of amounts of hydrogen peroxide (H(2)O(2)) that may exceed 1 mM in the surrounding media. H(2)O(2) production by S. pneumoniae has been shown to kill or inhibit the growth of other respiratory tract flora, as well as to have cytotoxic effects on host cells and tissue. The mechanisms allowing S. pneumoniae, a catalase-deficient species, to survive endogenously generated concentrations of H(2)O(2) that are sufficient to kill other bacterial species is unknown. In the present study, pyruvate oxidase (SpxB), the enzyme responsible for endogenous H(2)O(2) production, was required for survival during exposure to high levels (20 mM) of exogenously added H(2)O(2). Pretreatment with H(2)O(2) did not increase H(2)O(2) resistance in the mutant, suggesting that SpxB activity itself is required, rather than an H(2)O(2)-inducible pathway. SpxB mutants synthesized 85% less acetyl-phosphate, a potential source of ATP. During H(2)O(2) exposure, ATP levels decreased more rapidly in spxB mutants than in wild-type cells, suggesting that the increased killing of spxB mutants was due to more rapid ATP depletion. Together, these data support the hypothesis that S. pneumoniae SpxB contributes to an H(2)O(2)-resistant energy source that maintains viability during oxidative stress. Thus, SpxB is required for resistance to the toxic by-product of its own activity. Although H(2)O(2)-dependent hydroxyl radical production and the intracellular concentration of free iron were similar to that of Escherichia coli, killing by H(2)O(2) was unaffected by iron chelators, suggesting that S. pneumoniae has a novel mechanism to avoid the toxic effects of the Fenton reaction.

Genetic engineering of Enterobacter aerogenes with the Vitreoscilla hemoglobin gene: cell growth, survival, and antioxidant enzyme status under oxidative stress

Hemoglobins in unicellular organisms, like the one here in the bacterium Vitreoscilla, have greater chemical reactivity than their homologues in multicellular organisms. They can catalyze redox reactions and may protect cells against oxidative stress. The ability of Vitreoscilla hemoglobin to complement deficiencies of terminal cytochrome oxidases in Escherichia coli also suggests that this hemoglobin can receive electrons during respiration. In this study, a recombinant strain of Enterobacter aerogenes engineered to produce the Vitreoscilla Hb was investigated with regard to its susceptibility to oxidative stress. The culture response to oxidative stress produced by exogenously applied hydrogen peroxide was characterized in terms of cell growth, survival and the activities of two key antioxidant enzymes (catalase and superoxide dismutase). The influence of the physiological state of the cells and different media upon these culture dynamics was determined. Results showed that the hemoglobin-expressing strain is quite distinct in terms of growth/survival properties and activity of antioxidant enzymes from that of non-hemoglobin counterparts.

Responses to reactive oxygen intermediates and virulence of Salmonella typhimurium

Salmonella typhimurium is an intracellular pathogen that can survive and replicate in macrophages. One of the host defense mechanisms that S. typhimurium encounters upon infection is superoxide produced by the phagocytes' NADPH-oxidase. Salmonella has evolved numerous ways of coping with superoxide in the extracellular environment. In addition, Salmonella has to defend itself against superoxide produced as a by-product of aerobic respiration. Over the last decade, research on bacterial mutants has led to the identification of Salmonella strains that differ from their parental strain in susceptibility to superoxide in vitro. However, the consequences of such mutations for bacterial virulence are highly variable, indicating that superoxide sensitivity per se is not a characteristic that renders Salmonella less virulent. By discussing various bacterial mutants classified according to their in vitro sensitivity to superoxide, we will exemplify the complex mechanisms that Salmonella has evolved to cope with superoxide stress.

Frataxin deficiency and mitochondrial dysfunction

Friedreich ataxia (FA) is an inherited recessive disorder characterized by progressive neurological disability and heart abnormalities. The Friedreich ataxia gene (FRDA) encodes a small mitochondrial protein, frataxin, which is produced in insufficient amounts in the disease as a consequence of a GAA triplet repeat expansion in the first intron of the gene. Frataxin deficiency leads to excessive free radical production, dysfunction of Fe-S center containing enzymes (in particular respiratory complexes I, II and III, and aconitase), and progressive iron accumulation in mitochondria. Frataxin may be a mitochondrial iron-binding protein that prevents this metal from participating in Fenton chemistry to generate toxic hydroxyl radicals. We investigated whether frataxin deficiency may in addition interfere with signaling pathways. First, we showed that exposure of FA fibroblasts to iron fails to produce the normally observed increase in expression of the stress defense protein manganese superoxide dismutase. This impaired induction involves a nuclear factor-kappaB-independent pathway that does not require free radical signaling intermediates. We also examined the role of frataxin in neuronal differentiation by using stably transfected clones of P19 embryonic carcinoma cells with antisense or sense frataxin constructs. We found that during retinoic acid-induced neurogenesis frataxin deficiency enhances apoptosis and reduces the number of terminally differentiated neuronal-like cells. The addition of the antioxidant N-acetyl-cysteine only rescues cells non-committed to the neuronal lineage, indicating that frataxin deficiency impairs differentiation mechanisms and survival responses through different mechanisms. Both studies suggest that some abnormalities in frataxin-deficient cells are related to free radical independent signaling pathways.

Mutagenic target for hydroxyl radicals generated in Escherichia coli mutant deficient in Mn- and Fe- superoxide dismutases and Fur, a repressor for iron-uptake systems

We previously reported that mutations in Mn- and Fe-superoxide dismutases and Fur, a repressor for iron uptake systems, simulated generation of hydroxyl radicals, and caused hypermutability in Escherichia coli. The predominant type of spontaneous mutation was GC --> TA, followed by AT --> CG, suggesting the involvement of 7,8-dihydro-8-oxoguanine (8-oxoG) and 1,2-dihydro-2-oxoadenine (2-oxoA) in DNA as well as 7,8-dihydro-8-oxodeoxyguanosine triphosphate (8-oxodGTP) and 1,2-dihydro-2-oxodeoxyadenosine triphosphate (2-oxodATP) in the nucleotide pool. To determine the targets contributing to oxidative mutagenesis, DNA or nucleotides, we characterized spontaneous mutations and compared the distribution to those in mutMY and mutT strains, in which GC --> TA and AT --> CG were predominantly induced, respectively. The hotspots and sequence contexts where AT --> CG occurred frequently in sodAB fur strain were almost identical to those in mutT strain,whereas, those where GC --> TA occurred frequently in sodAB fur strain were quite different from those in mutMY strain. These observations suggested that AT --> CG is due to 8-oxodGTP, while GC --> TA is produced by some other lesion(s). The 2-oxodATP is also a major oxidative lesion in nucleotides, and strongly induces GC --> TA. The expression of cDNA for MTH1, which can hydrolyze 2-oxodATP as well as 8-oxodGTP, partially but significantly, suppressed the GC --> TA mutator phenotype of the sodAB fur strain, whereas, it did not for the mutMY strain. Additionally, the sequence contextby 2-oxodATP in E. coli was similar to that in sodAB fur strain. These results suggested that the targets contributing to oxidative mutagenesis in sodAB fur strain are nucleotides such as dGTP and dATP, rather than DNA.

Antioxidative properties of Lactobacillus sake upon exposure to elevated oxygen concentrations

The ability of bacteria to overcome oxidative stress is related to the levels and types of antioxidative mechanisms which they possess. In this study, the antioxidative properties in Lactobacillus sake strains from different food origins were determined at low temperature (8 degrees C) and upon exposure to oxygen levels between 20 and 90% O(2). The L. sake strains tested grew well at 8 degrees C and in the presence of 20% O(2), however, most of the strains could not grow at O(2) levels as high as 50 and/or 90%. Cell-free extracts of all strains possessed certain levels of hydroxyl radical scavenging, metal chelating and reducing capacities essential for growth of cells at ambient O(2). At elevated O(2) concentrations, a high H(2)O(2) splitting capacity and low specific rates of H(2)O(2) production were demonstrated in the O(2)-insensitive strain L. sake NCFB 2813, which could grow at elevated O(2) conditions. Although H(2)O(2) was generated in the O(2)-sensitive L. sake DSM 6333 at levels which were not directly toxic to the cells (<0.2 mM), we can conclude that its removal is essential for cell protection at elevated O(2) conditions.

Superoxide dismutase-deficient mutants of Helicobacter pylori are hypersensitive to oxidative stress and defective in host colonization

Superoxide dismutase (SOD) is a nearly ubiquitous enzyme among organisms that are exposed to oxic environments. The single SOD of Helicobacter pylori, encoded by the sodB gene, has been suspected to be a virulence factor for this pathogenic microaerophile, but mutations in this gene have not been reported previously. We have isolated mutants with interruptions in the sodB gene and have characterized them with respect to their response to oxidative stress and ability to colonize the mouse stomach. The sodB mutants are devoid of SOD activity, based on activity staining in nondenaturing gels and quantitative assays of cell extracts. Though wild-type H. pylori is microaerophilic, the mutants are even more sensitive to O(2) for both growth and viability. While the wild-type strain is routinely grown at 12% O(2), growth of the mutant strains is severely inhibited at above 5 to 6% O(2). The effect of O(2) on viability was determined by subjecting nongrowing cells to atmospheric levels of O(2) and plating for survivors at 2-h time intervals. Wild-type cell viability dropped by about 1 order of magnitude after 6 h, while viability of the sodB mutant decreased by more than 6 orders of magnitude at the same time point. The mutants are also more sensitive to H(2)O(2), and this sensitivity is exacerbated by increased O(2) concentrations. Since oxidative stress has been correlated with DNA damage, the frequency of spontaneous mutation to rifampin resistance was studied. The frequency of mutagenesis of an sodB mutant strain is about 15-fold greater than that of the wild-type strain. In the mouse colonization model, only 1 out of 23 mice inoculated with an SOD-deficient mutant of a mouse-adapted strain became H. pylori positive, while 15 out of 17 mice inoculated with the wild-type strain were shown to harbor the organism. Therefore, SOD is a virulence factor which affects the ability of this organism to colonize the mouse stomach and is important for the growth and survival of H. pylori under conditions of oxidative stress.

Mechanism of oxidative DNA damage induction in a strict anaerobe, Prevotella melaninogenica

We investigated the mechanism of the oxidative DNA damage induction by exposure to O(2) in Prevotella melaninogenica, a strict anaerobe. Flow cytometry with hydroethidine and dichlorofluorescein diacetate showed that O(2) exposure generated O(2)*-) and H(2)O(2). Results of electron spin resonance with alpha-(4-pyridyl-1-oxide)-N-tert-butylnitrone and ethanol showed that O(2) exposure also induced *OH radical generation in P. melaninogenica loaded with FeCl(2) but not in samples without FeCl(2) loading. In P. melaninogenica, O(2) exposure increased 8-hydroxydeoxyguanosine (8OHdG), typical of oxidative DNA damage. Catalase inhibited the increase, but the *OH radical scavengers did not. Phenanthroline, a membrane-permeable Fe and Cu chelator, increased the 8OHdG induction. In FeCl(2)-loaded samples, induction of 8OHdG decreased. Addition of H(2)O(2) markedly increased 8OHdG levels. These results indicate that in P. melaninogenica, exposure to O(2) generated and accumulated O(2)* and H(2)O(2), and that a crypto-OH radical generated through H(2)O(2) was the active species in the 8OHdG induction.

Critical protective role of bacterial superoxide dismutase in rhizobium-legume symbiosis

In nitrogen-poor soils, rhizobia elicit nodule formation on legume roots, within which they differentiate into bacteroids that fix atmospheric nitrogen. Protection against reactive oxygen species (ROS) was anticipated to play an important role in Rhizobium-legume symbiosis because nitrogenase is extremely oxygen sensitive. We deleted the sodA gene encoding the sole cytoplasmic superoxide dismutase (SOD) of Sinorhizobium meliloti. The resulting mutant, deficient in superoxide dismutase, grew almost normally and was only moderately sensitive to oxidative stress when free living. In contrast, its symbiotic properties in alfalfa were drastically affected. Nitrogen-fixing ability was severely impaired. More strikingly, most SOD-deficient bacteria did not reach the differentiation stage of nitrogen-fixing bacteroids. The SOD-deficient mutant nodulated poorly and displayed abnormal infection. After release into plant cells, a large number of bacteria failed to differentiate into bacteroids and rapidly underwent senescence. Thus, bacterial SOD plays a key protective role in the symbiotic process.

Ferritin from the obligate anaerobe Porphyromonas gingivalis: purification, gene cloning and mutant studies

Porphyromonas gingivalis is an obligate anaerobe that utilizes haem, transferrin and haemoglobin efficiently as sources of iron for growth, and has the ability to store haem on its cell surface, resulting in black pigmentation of colonies on blood agar plates. However, little is known about intracellular iron storage in this organism. Ferritin is one of the intracellular iron-storage proteins and may also contribute to the protection of organisms against oxidative stresses generated by intracellular free iron. A ferritin-like protein was purified from P. gingivalis and the encoding gene (ftn) was cloned from chromosomal DNA using information on its amino-terminal amino acid sequence. Comparison of the amino acid sequence deduced from the nucleotide sequence of ftn with those of known ferritins and bacterioferritins identified the protein as a ferritin and positioned it between proteins from the Proteobacteria and Thermotogales. The P. gingivalis ferritin was found to contain non-haem iron, thus confirming its identity. Construction and characterization of a P. gingivalis ferritin-deficient mutant revealed that the ferritin was particularly important for the bacterium to survive under iron-depleted conditions (both haemin and transferrin starvation), indicating that intracellular iron is stored in ferritin regardless of the iron source and that the iron stored in ferritin is utilized under iron-restricted conditions. However, the ferritin appeared not to contribute to protection against oxidative stresses caused by peroxides and atmospheric oxygen.