Staphylococcus aureus Uses the Bacilliredoxin (BrxAB)/Bacillithiol Disulfide Reductase (YpdA) Redox Pathway to Defend Against Oxidative Stress Under Infections

The neutrophil oxidant hypothiocyanous acid causes a thiol-specific stress response and an oxidative shift of the bacillithiol redox potential in Staphylococcus aureus

IMPORTANCE

Staphylococcus aureus colonizes the skin and the airways but can also lead to life-threatening systemic and chronic infections. During colonization and phagocytosis by immune cells, S. aureus encounters the thiol-reactive oxidant HOSCN. The understanding of the adaptation mechanisms of S. aureus toward HOSCN stress is important to identify novel drug targets to combat multi-resistant S. aureus isolates. As a defense mechanism, S. aureus uses the flavin disulfide reductase MerA, which functions as HOSCN reductase and protects against HOSCN stress. Moreover, MerA homologs have conserved functions in HOSCN detoxification in other bacteria, including intestinal and respiratory pathogens. In this work, we studied the comprehensive thiol-reactive mode of action of HOSCN and its effect on the reversible shift of the E BSH to discover new defense mechanisms against the neutrophil oxidant. These findings provide new leads for future drug design to fight the pathogen at the sites of colonization and infections.

Quantifying mitochondrial redox and bilirubin content in intact primary hepatocytes of obese mice using fluorescent reporters

Assessing the physiological role of H2O2 requires sensitive techniques to quantify H2O2 and antioxidants in live cells. Here, we present a protocol to assess the mitochondrial redox state and unconjugated bilirubin levels in intact live primary hepatocytes from obese mice. We described steps to quantify H2O2, GSSG/GSH, and bilirubin content in the mitochondrial matrix and the cytosol using the fluorescent reporters roGFP2-ORP1, GRX1-roGFP2, and UnaG, respectively. We detail hepatocyte isolation, plating, and transduction and live-cell imaging using a high-content imaging reader. For complete details on the use and execution of this protocol, please refer to Shum et al.1.

Antioxidant defense of Deinococcus radiodurans: how does it contribute to extreme radiation resistance?

PURPOSE

Deinococcus radiodurans is an extremely radioresistant bacterium characterized by D10 of 10 kGy, and able to grow luxuriantly under chronic ionizing radiation of 60 Gy/h. The aim of this article is to review the antioxidant system of D. radiodurans and its possible role in the unusual resistance of this bacterium to ionizing radiation.

CONCLUSIONS

The unusual radiation resistance of D. radiodurans has apparently evolved as a side effect of the adaptation of this extremophile to other damaging environmental factors, especially desiccation. The antioxidant proteins and low-molecular antioxidants (especially low-molecular weight Mn2+ complexes and carotenoids, in particular, deinoxanthin), as well as protein and non-protein regulators, are important for the antioxidant defense of this species. Antioxidant protection of proteins from radiation inactivation enables the repair of DNA damage caused by ionizing radiation.

Functional Diversity of Homologous Oxidoreductases-Tuning of Substrate Specificity by a FAD-Stacking Residue for Iron Acquisition and Flavodoxin Reduction

Although bacterial thioredoxin reductase-like ferredoxin/flavodoxin NAD(P)⁺ oxidoreductases (FNRs) are similar in terms of primary sequences and structures, they participate in diverse biological processes by catalyzing a range of different redox reactions. Many of the reactions are critical for the growth, survival of, and infection by pathogens, and insight into the structural basis for substrate preference, specificity, and reaction kinetics is crucial for the detailed understanding of these redox pathways. Bacillus cereus (Bc) encodes three FNR paralogs, two of which have assigned distinct biological functions in bacillithiol disulfide reduction and flavodoxin (Fld) reduction. Bc FNR2, the endogenous reductase of the Fld-like protein NrdI, belongs to a distinct phylogenetic cluster of homologous oxidoreductases containing a conserved His residue stacking the FAD cofactor. In this study, we have assigned a function to FNR1, in which the His residue is replaced by a conserved Val, in the reduction of the heme-degrading monooxygenase IsdG, ultimately facilitating the release of iron in an important iron acquisition pathway. The Bc IsdG structure was solved, and IsdG-FNR1 interactions were proposed through protein-protein docking. Mutational studies and bioinformatics analyses confirmed the importance of the conserved FAD-stacking residues on the respective reaction rates, proposing a division of FNRs into four functionally unique sequence similarity clusters likely related to the nature of this residue.

MerA functions as a hypothiocyanous acid reductase and defense mechanism in Staphylococcus aureus

The major pathogen Staphylococcus aureus has to cope with host-derived oxidative stress to cause infections in humans. Here, we report that S. aureus tolerates high concentrations of hypothiocyanous acid (HOSCN), a key antimicrobial oxidant produced in the respiratory tract. We discovered that the flavoprotein disulfide reductase (FDR) MerA protects S. aureus from this oxidant by functioning as a HOSCN reductase, with its deletion sensitizing bacteria to HOSCN. Crystal structures of homodimeric MerA (2.4 Å) with a Cys₄₃ -Cys₄₈ intramolecular disulfide, and reduced MerACys₄₃ S (1.6 Å) showed the FAD cofactor close to the active site, supporting that MerA functions as a group I FDR. MerA is controlled by the redox-sensitive repressor HypR, which we show to be oxidized to intermolecular disulfides under HOSCN stress, resulting in its inactivation and derepression of merA transcription to promote HOSCN tolerance. Our study highlights the HOSCN tolerance of S. aureus and characterizes the structure and function of MerA as a major HOSCN defense mechanism. Crippling the capacity to respond to HOSCN may be a novel strategy for treating S. aureus infections.

Inside the phagosome: A bacterial perspective

The neutrophil phagosome is one of the most hostile environments that bacteria must face and overcome if they are to succeed as pathogens. Targeting bacterial defense mechanisms should lead to new therapies that assist neutrophils to kill pathogens, but this has not yet come to fruition. One of the limiting factors in this effort has been our incomplete knowledge of the complex biochemistry that occurs within the rapidly changing environment of the phagosome. The same compartmentalization that protects host tissue also limits our ability to measure events within the phagosome. In this review, we highlight the limitations in our knowledge, and how the contribution of bacteria to the phagosomal environment is often ignored. There appears to be significant heterogeneity among phagosomes, and it is important to determine whether survivors have more efficient defenses or whether they are ingested into less threatening environments than other bacteria. As part of these efforts, we discuss how monitoring or recovering bacteria from phagosomes can provide insight into the conditions they have faced. We also encourage the use of unbiased screening approaches to identify bacterial genes that are essential for survival inside neutrophil phagosomes.

Oxidation of bacillithiol during killing of Staphylococcus aureus USA300 inside neutrophil phagosomes

Targeting immune evasion tactics of pathogenic bacteria may hold the key to treating recalcitrant bacterial infections. Staphylococcus aureus produces bacillithiol (BSH), its major low-molecular-weight thiol, which is thought to protect this opportunistic human pathogen against the bombardment of oxidants inside neutrophil phagosomes. Here, we show that BSH was oxidized when human neutrophils phagocytosed S. aureus, but provided limited protection to the bacteria. We used mass spectrometry to measure the oxidation of BSH upon exposure of S. aureus USA300 to either a bolus of hypochlorous acid (HOCl) or a flux generated by the neutrophil enzyme myeloperoxidase. Oxidation of BSH and loss of bacterial viability were strongly correlated (r = 0.99, p < 0.001). BSH was fully oxidized after exposure of S. aureus to lethal doses of HOCl. However, there was no relationship between the initial BSH levels and the dose of HOCl required for bacterial killing. In contrast to the HOCl systems, only 50% of total BSH was oxidized when neutrophils killed the majority of phagocytosed bacteria. Oxidation of BSH was decreased upon inhibition of myeloperoxidase, implicating HOCl in phagosomal BSH oxidation. A BSH-deficient S. aureus USA300 mutant was slightly more susceptible to treatment with either HOCl or ammonia chloramine, or to killing within neutrophil phagosomes. Collectively, our data show that myeloperoxidase-derived oxidants react with S. aureus inside neutrophil phagosomes, leading to partial BSH oxidation, and contribute to bacterial killing. However, BSH offers only limited protection against the neutrophil's multifaceted killing mechanisms.

The Catalase KatA Contributes to Microaerophilic H2O2 Priming to Acquire an Improved Oxidative Stress Resistance in Staphylococcus aureus

Staphylococcus aureus has to cope with oxidative stress during infections. In this study, S. aureus was found to be resistant to 100 mM H₂O₂ during aerobic growth. While KatA was essential for this high aerobic H₂O₂ resistance, the peroxiredoxin AhpC contributed to detoxification of 0.4 mM H₂O₂ in the absence of KatA. In addition, the peroxiredoxins AhpC, Tpx and Bcp were found to be required for detoxification of cumene hydroperoxide (CHP). The high H₂O₂ tolerance of aerobic S. aureus cells was associated with priming by endogenous H₂O₂ levels, which was supported by an oxidative shift of the bacillithiol redox potential to −291 mV compared to −310 mV in microaerophilic cells. In contrast, S. aureus could be primed by sub-lethal doses of 100 µM H₂O₂ during microaerophilic growth to acquire an improved resistance towards the otherwise lethal triggering stimulus of 10 mM H₂O₂. This microaerophilic priming was dependent on increased KatA activity, whereas aerobic cells showed constitutive high KatA activity. Thus, KatA contributes to the high H₂O₂ resistance of aerobic cells and to microaerophilic H₂O₂ priming in order to survive the subsequent lethal triggering doses of H₂O₂, allowing the adaptation of S. aureus under infections to different oxygen environments.

Staphylococcus aureus Small RNAs Possess Dephospho-CoA 5′-Caps, but No CoAlation Marks

Novel features of coenzyme A (CoA) and its precursor, 3′-dephospho-CoA (dpCoA), recently became evident. dpCoA was found to attach to 5′-ends of small ribonucleic acids (dpCoA-RNAs) in two bacterial species (Escherichia coli and Streptomyces venezuelae). Furthermore, CoA serves, in addition to its well-established coenzymatic roles, as a ubiquitous posttranslational protein modification (‘CoAlation’), thought to prevent the irreversible oxidation of cysteines. Here, we first identified and quantified dpCoA-RNAs in the small RNA fraction of the human pathogen Staphylococcus aureus, using a newly developed enzymatic assay. We found that the amount of dpCoA caps was similar to that of the other two bacteria. We furthermore tested the hypothesis that, in the environment of a cell, the free thiol of the dpCoA-RNAs, as well as other sulfur-containing RNA modifications, may be oxidized by disulfide bond formation, e.g., with CoA. While we could not find evidence for such an ‘RNA CoAlation’, we observed that CoA disulfide reductase, the enzyme responsible for reducing CoA homodisulfides in S. aureus, did efficiently reduce several synthetic dpCoA-RNA disulfides to dpCoA-RNAs in vitro. This activity may imply a role in reversing RNA CoAlation.

Thiol Reductases in Deinococcus Bacteria and Roles in Stress Tolerance

Deinococcus species possess remarkable tolerance to extreme environmental conditions that generate oxidative damage to macromolecules. Among enzymes fulfilling key functions in metabolism regulation and stress responses, thiol reductases (TRs) harbour catalytic cysteines modulating the redox status of Cys and Met in partner proteins. We present here a detailed description of Deinococcus TRs regarding gene occurrence, sequence features, and physiological functions that remain poorly characterised in this genus. Two NADPH-dependent thiol-based systems are present in Deinococcus. One involves thioredoxins, disulfide reductases providing electrons to protein partners involved notably in peroxide scavenging or in preserving protein redox status. The other is based on bacillithiol, a low-molecular-weight redox molecule, and bacilliredoxin, which together protect Cys residues against overoxidation. Deinococcus species possess various types of thiol peroxidases whose electron supply depends either on NADPH via thioredoxins or on NADH via lipoylated proteins. Recent data gained on deletion mutants confirmed the importance of TRs in Deinococcus tolerance to oxidative treatments, but additional investigations are needed to delineate the redox network in which they operate, and their precise physiological roles. The large palette of Deinococcus TR representatives very likely constitutes an asset for the maintenance of redox homeostasis in harsh stress conditions.

Suppression of Thiol-Dependent Antioxidant System and Stress Response in Methicillin-Resistant Staphylococcus aureus by Docosanol: Explication Through Proteome Investigation

The present study was aimed to investigate the effect of docosanol on the protein expression profile of methicillin-resistant Staphylococcus aureus (MRSA). Thus, two-dimensional gel electrophoresis coupled with MALDI-TOF MS technique was utilized to identify the differentially regulated proteins in the presence of docosanol. A total of 947 protein spots were identified from the intracellular proteome of both control and docosanol treated samples among which 40 spots were differentially regulated with a fold change greater than 1.0. Prominently, the thiol-dependent antioxidant system and stress response proteins are downregulated in MRSA, which are critical for survival during oxidative stress. In particular, docosanol downregulated the expression of Tpx, AhpC, BshC, BrxA, and YceI with a fold change of 1.4 (p = 0.02), 1.4 (p = 0.01), 1.6 (p = 0.002), 4.9 (p = 0.02), and 1.4 (p = 0.02), respectively. In addition, docosanol reduced the expression of proteins involved in purine metabolic pathways, biofilm growth cycle, and virulence factor production. Altogether, these findings suggest that docosanol could efficiently target the antioxidant pathway by reducing the expression of bacillithiol and stress-associated proteins.

Thioredoxin reductase from Bacillus cereus exhibits distinct reduction and NADPH-binding properties

Low‐molecular‐weight (low M_r) thioredoxin reductases (TrxRs) are homodimeric NADPH‐dependent dithiol flavoenzymes that reduce thioredoxins (Trxs) or Trx‐like proteins involved in the activation networks of enzymes, such as the bacterial class Ib ribonucleotide reductase (RNR). During the last few decades, TrxR‐like ferredoxin/flavodoxin NADP⁺ oxidoreductases (FNRs) have been discovered and characterized in several types of bacteria, including those not encoding the canonical plant‐type FNR. In Bacillus cereus, a TrxR‐like FNR has been shown to reduce the flavodoxin‐like protein NrdI in the activation of class Ib RNR. However, some species only encode TrxR and lack the homologous TrxR‐like FNR. Due to the structural similarity between TrxRs and TrxR‐like FNRs, as well as variations in their occurrence in different microorganisms, we hypothesized that low M_r TrxR may be able to replace TrxR‐like FNR in, for example, the reduction of NrdI. In this study, characterization of TrxR from B. cereus has revealed a weak FNR activity toward NrdI reduction. Additionally, the crystal structure shows that only one out of two binding sites of the B. cereus TrxR homodimer is occupied with NADPH, indicating a possible asymmetric co‐substrate binding in TrxR.

Atypical Bacilliredoxin AbxC Plays a Role in Responding to Oxidative Stress in Radiation-Resistant Bacterium Deinococcus radiodurans

Deinococcus radiodurans is a robust bacterium with extraordinary resistance to ionizing radiation and reactive oxygen species (ROS). D. radiodurans produces an antioxidant thiol compound called bacillithiol (BSH), but BSH-related enzymes have not been investigated. The D. radiodurans mutant lacking bshA (dr_1555), the first gene of the BSH biosynthetic pathway, was devoid of BSH and sensitive to hydrogen peroxide (H₂O₂) compared to the wild-type D. radiodurans strain. Three bacilliredoxin (Brx) proteins, BrxA, B, and C, have been identified in BSH-producing bacteria, such as Bacillus. D. radiodurans possesses DR_1832, a putative homolog of BrxC. However, because DR_1832 contains a novel signature motif (TCHKT) and a C-terminal region similar to the colicin-like immunity domain, we named it AbxC (atypical BrxC). The deletion of abxC also sensitized cells to H₂O₂. AbxC exhibited peroxidase activity in vitro, which was linked to nicotinamide adenine dinucleotide phosphate (NADPH) oxidation via the BSH disulfide reductase DR_2623 (DrBdr). AbxC proteins were present mainly as dimers after exposure to H₂O₂ in vitro, and the oxidized dimers were resolved to monomers by the reaction coupled with BSH as an electron donor, in which DrBdr transported reducing equivalents from NADPH to AbxC through BSH recycling. We identified 25 D. radiodurans proteins that potentially interact with AbxC using AbxC-affinity chromatography. Most of them are associated with cellular metabolisms, such as glycolysis and amino acid biosynthesis, and stress response. Interestingly, AbxC could bind to the proposed peroxide-sensing transcription regulator, DrOxyR. These results suggest that AbxC may be involved in the H₂O₂ signaling mechanism mediated by DrOxyR.

The Bacillus subtilis monothiol bacilliredoxin BrxC (YtxJ) and the Bdr (YpdA) disulfide reductase reduce S-bacillithiolated proteins

The bacterial cytosol is generally a reducing environment with protein cysteine residues maintained in their thiol form. The low molecular weight thiol bacillithiol (BSH) serves as a general thiol reductant, analogous to glutathione, in a wide range of bacterial species. Proteins modified by disulfide bond formation with BSH (S-bacillithiolation) are reduced by the action of bacilliredoxins, BrxA and BrxB. Here, the YtxJ protein is identified as a monothiol bacilliredoxin, renamed BrxC, and is implicated in BSH removal from oxidized cytosolic proteins, including the glyceraldehyde 3-phosphate dehydrogenases GapA and GapB. BrxC can also debacillithiolate the mixed disulfide form of the bacilliredoxin BrxB. Bdr is a thioredoxin reductase-like flavoprotein with bacillithiol-disulfide (BSSB) reductase activity. Here, Bdr is shown to additionally function as a bacilliredoxin reductase. Bdr and BrxB function cooperatively to debacillithiolate OhrR, a transcription factor regulated by S-bacillithiolation on its sole cysteine residue. Collectively, these results expand our understanding of the BSH redox network comprised of three bacilliredoxins and a BSSB reductase that serve to counter the widespread protein S-bacillithiolation that results from conditions of disulfide stress.

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Bacillithiol is the major low molecular weight thiol in Bacillus subtilis.

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Oxidative stress leads to protein S-bacillithiolation.

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BrxC functions as a monothiol class bacilliredoxin.

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The Bdr bacillithiol disulfide reductase is also a bacilliredoxin.

Overview of structurally homologous flavoprotein oxidoreductases containing the low Mr thioredoxin reductase-like fold - A functionally diverse group

Structural studies show that enzymes have a limited number of unique folds, although structurally related enzymes have evolved to perform a large variety of functions. In this review, we have focused on enzymes containing the low molecular weight thioredoxin reductase (low M_r TrxR) fold. This fold consists of two domains, both containing a three-layer ββα sandwich Rossmann-like fold, serving as flavin adenine dinucleotide (FAD) and, in most cases, pyridine nucleotide (NAD(P)H) binding-domains. Based on a search of the Protein Data Bank for all published structures containing the low M_r TrxR-like fold, we here present a comprehensive overview of enzymes with this structural architecture. These range from TrxR-like ferredoxin/flavodoxin NAD(P)⁺ oxidoreductases, through glutathione reductase, to NADH peroxidase. Some enzymes are solely composed of the low M_r TrxR-like fold, while others contain one or two additional domains. In this review, we give a detailed description of selected enzymes containing only the low M_r TrxR-like fold, however, catalyzing a diversity of chemical reactions. Our overview of this structurally similar, yet functionally distinct group of flavoprotein oxidoreductases highlights the fascinating and increasing number of studies describing the diversity among these enzymes, especially during the last decade(s).

The Crystal Structures of Bacillithiol Disulfide Reductase Bdr (YpdA) Provide Structural and Functional Insight into a New Type of FAD-Containing NADPH-Dependent Oxidoreductase

Low G+C Gram-positive Firmicutes, such as the clinically important pathogens Staphylococcus aureus and Bacillus cereus, use the low-molecular weight thiol bacillithiol (BSH) as a defense mechanism to buffer the intracellular redox environment and counteract oxidative stress encountered by human neutrophils during infections. The protein YpdA has recently been shown to function as an essential NADPH-dependent reductase of oxidized bacillithiol disulfide (BSSB) resulting from stress responses and is crucial for maintaining the reduced pool of BSH and cellular redox balance. In this work, we present the first crystallographic structures of YpdAs, namely, those from S. aureus and B. cereus. Our analyses reveal a uniquely organized biological tetramer; however, the structure of the monomeric subunit is highly similar to those of other flavoprotein disulfide reductases. The absence of a redox active cysteine in the vicinity of the FAD isoalloxazine ring implies a new direct disulfide reduction mechanism, which is backed by the presence of a potentially gated channel, serving as a putative binding site for BSSB in the proximity of the FAD cofactor. We also report enzymatic activities for both YpdAs, which along with the structures presented in this work provide important structural and functional insight into a new class of FAD-containing NADPH-dependent oxidoreductases, related to the emerging fight against pathogenic bacteria.

The Writers, Readers, and Erasers in Redox Regulation of GAPDH

Glyceraldehyde 3–phosphate dehydrogenase (GAPDH) is a key glycolytic enzyme, which is crucial for the breakdown of glucose to provide cellular energy. Over the past decade, GAPDH has been reported to be one of the most prominent cellular targets of post-translational modifications (PTMs), which divert GAPDH toward different non-glycolytic functions. Hence, it is termed a moonlighting protein. During metabolic and oxidative stress, GAPDH is a target of different oxidative PTMs (oxPTM), e.g., sulfenylation, S-thiolation, nitrosylation, and sulfhydration. These modifications alter the enzyme’s conformation, subcellular localization, and regulatory interactions with downstream partners, which impact its glycolytic and non-glycolytic functions. In this review, we discuss the redox regulation of GAPDH by different redox writers, which introduce the oxPTM code on GAPDH to instruct a redox response; the GAPDH readers, which decipher the oxPTM code through regulatory interactions and coordinate cellular response via the formation of multi-enzyme signaling complexes; and the redox erasers, which are the reducing systems that regenerate the GAPDH catalytic activity. Human pathologies associated with the oxidation-induced dysregulation of GAPDH are also discussed, featuring the importance of the redox regulation of GAPDH in neurodegeneration and metabolic disorders.

The alarmone (p)ppGpp confers tolerance to oxidative stress during the stationary phase by maintenance of redox and iron homeostasis in Staphylococcus aureus

Slow growing stationary phase bacteria are often tolerant to multiple stressors and antimicrobials. Here, we show that the pathogen Staphylococcus aureus develops a non-specific tolerance towards oxidative stress during the stationary phase, which is mediated by the nucleotide second messenger (p)ppGpp. The (p)ppGpp0 mutant was highly susceptible to HOCl stress during the stationary phase. Transcriptome analysis of the (p)ppGpp0 mutant revealed an increased expression of the PerR, SigB, QsrR, CtsR and HrcA regulons during the stationary phase, indicating an oxidative stress response. The (p)ppGpp0 mutant showed a slight oxidative shift in the bacillithiol (BSH) redox potential (EBSH) and an impaired H2O2 detoxification due to higher endogenous ROS levels. The increased ROS levels in the (p)ppGpp0 mutant were shown to be caused by higher respiratory chain activity and elevated total and free iron levels. Consistent with these results, N-acetyl cysteine and the iron-chelator dipyridyl improved the growth and survival of the (p)ppGpp0 mutant under oxidative stress. Elevated free iron levels caused 8 to 31-fold increased transcription of Fe-storage proteins ferritin (ftnA) and miniferritin (dps) in the (p)ppGpp0 mutant, while Fur-regulated uptake systems for iron, heme or siderophores (efeOBU, isdABCDEFG, sirABC and sstADBCD) were repressed. Finally, the susceptibility of the (p)ppGpp0 mutant towards the bactericidal action of the antibiotics ciprofloxacin and tetracycline was abrogated with N-acetyl cysteine and dipyridyl. Taken together, (p)ppGpp confers tolerance to ROS and antibiotics by down-regulation of respiratory chain activity and free iron levels, lowering ROS formation to ensure redox homeostasis in S. aureus.

Thiol-based redox switches in the major pathogen Staphylococcus aureus

Staphylococcus aureus is a major human pathogen, which encounters reactive oxygen, nitrogen, chlorine, electrophile and sulfur species (ROS, RNS, RCS, RES and RSS) by the host immune system, during cellular metabolism or antibiotics treatments. To defend against redox active species and antibiotics, S. aureus is equipped with redox sensing regulators that often use thiol switches to control the expression of specific detoxification pathways. In addition, the maintenance of the redox balance is crucial for survival of S. aureus under redox stress during infections, which is accomplished by the low molecular weight (LMW) thiol bacillithiol (BSH) and the associated bacilliredoxin (Brx)/BSH/bacillithiol disulfide reductase (YpdA)/NADPH pathway. Here, we present an overview of thiol-based redox sensors, its associated enzymatic detoxification systems and BSH-related regulatory mechanisms in S. aureus, which are important for the defense under redox stress conditions. Application of the novel Brx-roGFP2 biosensor provides new insights on the impact of these systems on the BSH redox potential. These thiol switches of S. aureus function in protection against redox active desinfectants and antimicrobials, including HOCl, the AGXX® antimicrobial surface coating, allicin from garlic and the naphthoquinone lapachol. Thus, thiol switches could be novel drug targets for the development of alternative redox-based therapies to combat multi-drug resistant S. aureus isolates.

In Vivo Imaging with Genetically Encoded Redox Biosensors

Redox reactions are of high fundamental and practical interest since they are involved in both normal physiology and the pathogenesis of various diseases. However, this area of research has always been a relatively problematic field in the context of analytical approaches, mostly because of the unstable nature of the compounds that are measured. Genetically encoded sensors allow for the registration of highly reactive molecules in real-time mode and, therefore, they began a new era in redox biology. Their strongest points manifest most brightly in in vivo experiments and pave the way for the non-invasive investigation of biochemical pathways that proceed in organisms from different systematic groups. In the first part of the review, we briefly describe the redox sensors that were used in vivo as well as summarize the model systems to which they were applied. Next, we thoroughly discuss the biological results obtained in these studies in regard to animals, plants, as well as unicellular eukaryotes and prokaryotes. We hope that this work reflects the amazing power of this technology and can serve as a useful guide for biologists and chemists who work in the field of redox processes.

The plant-derived naphthoquinone lapachol causes an oxidative stress response in Staphylococcus aureus

Staphylococcus aureus is a major human pathogen, which causes life-threatening systemic and chronic infections and rapidly acquires resistance to multiple antibiotics. Thus, new antimicrobial compounds are required to combat infections with drug resistant S. aureus isolates. The 2-hydroxy-3-(3-methyl-2-butenyl)-1,4-naphthoquinone lapachol was previously shown to exert antimicrobial effects. In this study, we investigated the antimicrobial mode of action of lapachol in S. aureus using RNAseq transcriptomics, redox biosensor measurements, S-bacillithiolation assays and phenotype analyses of mutants. In the RNA-seq transcriptome, lapachol caused an oxidative and quinone stress response as well as protein damage as revealed by induction of the PerR, HypR, QsrR, MhqR, CtsR and HrcA regulons. Lapachol treatment further resulted in up-regulation of the SigB and GraRS regulons, which is indicative for cell wall and general stress responses. The redox-cycling mode of action of lapachol was supported by an elevated bacillithiol (BSH) redox potential (E_BSH), higher endogenous ROS levels, a faster H₂O₂ detoxification capacity and increased thiol-oxidation of GapDH and the HypR repressor in vivo. The ROS scavenger N-acetyl cysteine and microaerophilic growth conditions improved the survival of lapachol-treated S. aureus cells. Phenotype analyses revealed an involvement of the catalase KatA and the Brx/BSH/YpdA pathway in protection against lapachol-induced ROS-formation in S. aureus. However, no evidence for irreversible protein alkylation and aggregation was found in lapachol-treated S. aureus cells. Thus, the antimicrobial mode of action of lapachol in S. aureus is mainly caused by ROS formation resulting in an oxidative stress response, an oxidative shift of the E_BSH and increased protein thiol-oxidation. As ROS-generating compound, lapachol is an attractive alternative antimicrobial to combat multi-resistant S. aureus isolates.

Oxidation of bacillithiol by myeloperoxidase-derived oxidants

Bacillithiol is a major low-molecular-weight thiol in gram-positive firmicutes including the human pathogen Staphylococcus aureus. Bacillithiol is regarded as an important defence mechanism against oxidants produced by the immune system, especially myeloperoxidase-derived hypochlorous acid (HOCl). However, it is unknown how fast BSH reacts with HOCl and what products are formed in the reaction. In the present study, we used sensitive MRM-based LC-MS methods to characterize the reaction of BSH with HOCl in cell-free solutions and in S. aureus. In the cell-free system, BSH formed predominantly the disulfide dimer (BSSB) at low mole ratios of HOCl and the sulfinic and sulfonic acids at higher oxidant concentrations. HOCl also promoted the formation of bacillithiol sulfonamide. In S. aureus, the oxidation pattern was similar except that a small proportion of BSH also formed mixed disulfides with protein thiols. Using competition with methionine, we determined the second-order rate constant for the reaction of HOCl with BSH to be 6 × 10⁷ M^-1s^-1, which indicated a fast, near diffusion-controlled reaction. Other reactive halogen species, including hypothiocyanous acid (HOSCN), also produced bacillithiol sulfonamide, albeit to a smaller extent than HOCl. The sulfonamide was not produced by hydrogen peroxide, which instead formed BSSB. This study helps our understanding of BSH redox biology and provides tools for gauging the exposure of BSH-producing bacteria to oxidative stress.

Redox and Thiols in Archaea

Low molecular weight (LMW) thiols have many functions in bacteria and eukarya, ranging from redox homeostasis to acting as cofactors in numerous reactions, including detoxification of xenobiotic compounds. The LMW thiol, glutathione (GSH), is found in eukaryotes and many species of bacteria. Analogues of GSH include the structurally different LMW thiols: bacillithiol, mycothiol, ergothioneine, and coenzyme A. Many advances have been made in understanding the diverse and multiple functions of GSH and GSH analogues in bacteria but much less is known about distribution and functions of GSH and its analogues in archaea, which constitute the third domain of life, occupying many niches, including those in extreme environments. Archaea are able to use many energy sources and have many unique metabolic reactions and as a result are major contributors to geochemical cycles. As LMW thiols are major players in cells, this review explores the distribution of thiols and their biochemistry in archaea.

Oxidative Stress-Generating Antimicrobials, a Novel Strategy to Overcome Antibacterial Resistance

Antimicrobial resistance is becoming one of the most important human health issues. Accordingly, the research focused on finding new antibiotherapeutic strategies is again becoming a priority for governments and major funding bodies. The development of treatments based on the generation of oxidative stress with the aim to disrupt the redox defenses of bacterial pathogens is an important strategy that has gained interest in recent years. This approach is allowing the identification of antimicrobials with repurposing potential that could be part of combinatorial chemotherapies designed to treat infections caused by recalcitrant bacterial pathogens. In addition, there have been important advances in the identification of novel plant and bacterial secondary metabolites that may generate oxidative stress as part of their antibacterial mechanism of action. Here, we revised the current status of this emerging field, focusing in particular on novel oxidative stress-generating compounds with the potential to treat infections caused by intracellular bacterial pathogens.

Method for measurement of bacillithiol redox potential changes using the Brx-roGFP2 redox biosensor in Staphylococcus aureus

Recent advances in the design of genetically encoded redox biosensors, such as redox-sensitive GFP (roGFP) have facilitated the real-time imaging of the intracellular redox potential in eukaryotic cells at high sensitivity and at spatiotemporal resolution. To increase the specificity of roGFP2 for the interaction with the glutathione (GSH)/ glutathione disulfide (GSSG) redox couple, roGFP2 has been fused to glutaredoxin (Grx) to construct the Grx-roGFP2 biosensor. We have previously designed the related Brx-roGFP2 redox biosensor for dynamic measurement of the bacillithiol redox potential (E_BSH) in the human pathogen Staphylococcus aureus. Here, we describe the detailed method for measurements of the oxidation degree (OxD) of the Brx-roGFP2 biosensor in S. aureus using the microplate reader. In particularly, we provide details for determination of the E_BSH changes during the growth and after oxidative stress. For future biosensor applications at the single cell level, we recommend the design of genome-encoded roGFP2 biosensors enabling stable expression and fluorescence in bacteria.•

Brx-roGFP2 is specific for measurements of the bacillithiol redox potential in Staphylococcus aureus cells

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Control samples for fully reduced and oxidized states of Brx-roGFP2 are required for calibration during OxD measurements

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Easy to measure fluorescence excitation intensities at the 405 and 488 nm excitation maxima using microplate readers

Altered redox status, DNA damage and modulation of L-tryptophan metabolism contribute to antimicrobial action of curcumin

Identification and development of newer and better antimicrobials from natural products represent ongoing research efforts by many investigators. Curcumin is a polyphenol commonly found in the plant Curcuma longa (better known as turmeric). It has been reported to possess several bioactivities including antioxidant, anti-cancer, anti-inflammatory, anti-diabetic, anti-fibrotic, and antimicrobial properties. However, little is known about the antimicrobial mode of action of curcumin, thus undermining its prospects as an alternative antimicrobial agent. In this study, we investigated the mechanism of antimicrobial action by curcumin. The mechanism of inhibition was evaluated in representatives of Gram negative (Escherichia coli) and Gram positive (Staphylococcus aureus) bacteria isolates, treated with either curcumin singly or in combination with ascorbic acid (1000 μg/mL). Results showed that curcumin has broad antimicrobial capacity. In addition, curcumin only and/or co-treatment with ascorbic acid caused lipid peroxidation in S. aureus and E. coli, and by extension led to DNA damage, indicative of oxidative stress. It is plausible that the oxidative might be related to the activation of the kynurenine pathway in S. aureus but not in E. coli. Furthermore, curcumin exposure led to elevated total antioxidant capacity (TAC) and level of total thiol, but decreased nitric oxide level in the bacteria isolates. Together, the findings suggest that oxidative stress and DNA damage might be partly responsible for the antimicrobial action of curcumin.