Bleach activates a redox-regulated chaperone by oxidative protein unfolding

Maintaining a Healthy Proteome during Oxidative Stress

Some of the most challenging stress conditions that organisms encounter during their lifetime involve the transient accumulation of reactive oxygen and chlorine species. Extremely reactive to amino acid side chains, these oxidants cause widespread protein unfolding and aggregation. It is therefore not surprising that cells draw on a variety of different strategies to counteract the damage and maintain a healthy proteome. Orchestrated largely by direct changes in the thiol oxidation status of key proteins, the response strategies involve all layers of protein protection. Reprogramming of basic biological functions helps decrease nascent protein synthesis and restore redox homeostasis. Mobilization of oxidative stress-activated chaperones and production of stress-resistant non-proteinaceous chaperones prevent irreversible protein aggregation. Finally, redox-controlled increase in proteasome activity removes any irreversibly damaged proteins. Together, these systems pave the way to restore protein homeostasis and enable organisms to survive stress conditions that are inevitable when living an aerobic lifestyle.

Gene Expression Profiling of Endoplasmic Reticulum Stress in Hepatitis C Virus-Containing Cells Treated with an Inhibitor of Protein Disulfide Isomerases

Protein disulfide isomerases (PDIs) catalyze disulfide bond formation between protein cysteine residues during protein folding in the endoplasmic reticulum (ER) lumen and are essential for maintaining ER homoeostasis. The life cycle of the hepatitis C virus (HCV) is closely associated with the ER. Synthesis and maturation of HCV proteins occur in the ER membrane and are mediated by multiple host cell factors that include also PDI. Here, we present a study investigating the effect of PDI inhibition on Huh7 human hepatoma cells harboring an HCV subgenomic replicon using the abscisic acid-derived PDI inhibitor origamicin. Transcriptional profiling shows that origamicin changed the expression levels of genes involved in the oxidative and ER stress responses and the unfolded protein response, as indicated by the upregulation of antioxidant enzymes and chaperone proteins, the downregulation of cell-cycle proteins, and induction of apoptosis-associated genes. Our data suggest that origamicin negatively impacts HCV replication by causing an imbalance in cellular homoeostasis and induction of stress responses. These insights suggest that inhibition of PDIs by low-molecular-weight inhibitors could be a promising approach to the discovery of novel antiviral compounds.

The AGXX® Antimicrobial Coating Causes a Thiol-Specific Oxidative Stress Response and Protein S-bacillithiolation in Staphylococcus aureus

Multidrug-resistant pathogens, such as methicillin-resistant Staphylococcus aureus (MRSA) pose an increasing health burden and demand alternative antimicrobials to treat bacterial infections. The surface coating AGXX® is a novel broad-spectrum antimicrobial composed of two transition metals, silver and ruthenium that can be electroplated on various surfaces, such as medical devices and implants. AGXX® has been shown to kill nosocomial and waterborne pathogens by production of reactive oxygen species (ROS), but the effect of AGXX® on the bacterial redox balance has not been demonstrated. Since treatment options for MRSA infections are limited, ROS-producing agents are attractive alternatives to combat multi-resistant strains. In this work, we used RNA-seq transcriptomics, redox biosensor measurements and phenotype analyses to study the mode of action of AGXX® microparticles in S. aureus USA300. Using growth and survival assays, the growth-inhibitory amount of AGXX® microparticles was determined as 5 μg/ml. In the RNA-seq transcriptome, AGXX® caused a strong thiol-specific oxidative stress response and protein damage as revealed by the induction of the PerR, HypR, QsrR, MhqR, CstR, CtsR, and HrcA regulons. The derepression of the Fur, Zur, and CsoR regulons indicates that AGXX® also interferes with the metal ion homeostasis inducing Fe²⁺- and Zn²⁺-starvation responses as well as export systems for toxic Ag⁺ ions. The induction of the SigB and GraRS regulons reveals also cell wall and general stress responses. AGXX® stress was further shown to cause protein S-bacillithiolation, protein aggregation and an oxidative shift in the bacillithiol (BSH) redox potential. In phenotype assays, BSH and the HypR-controlled disulfide reductase MerA were required for protection against ROS produced under AGXX® stress in S. aureus. Altogether, our study revealed a strong thiol-reactive mode of action of AGXX® in S. aureus USA300 resulting in an increased BSH redox potential and protein S-bacillithiolation.

Detection of Escherichia coli bacteria by impact electrochemistry

We report the redox mediated detection of Escherichia coli bacteria at carbon microelectrodes, using the impact electrochemistry technique. By employing N,N,N',N'-tetramethyl-para-phenylene-diamine (TMPD) as redox mediator a concentration dependency for bacteria impacts was observed, whereby its impact frequency is shown to be in good agreement with theoretically predicted values.

Oxygen exposure deprives antimonate-reducing capability of a methane fed biofilm

This work is aiming at achieving antimonate (Sb(V)) bio-reduction in a methane (CH₄) based membrane biofilm reactor (MBfR), and elucidating the effect of oxygen (O₂) on the performance of the biofilm. Scanning electron microscope (SEM), energy dispersive X-ray (EDS) and X-ray photoelectron spectroscopy (XPS) confirm Sb₂O₃ precipitates were the main product formed from Sb(V) reduction in the CH₄-fed biofilm. Illumina sequencing shows Thermomonas may be responsible for Sb(V) reduction. Moreover, we found 8 mg/L of O₂ in the influent irreversibly inhibited Sb(V) reduction. Metagenomic prediction by Reconstruction of Unobserved State (PICRUSt) shows that the biofilm lacked efficient defense system to the oxidative stress, leading to the great suppress of key biological metabolisms such as TCA cycle, glycolysis and DNA replication, as well as potential Sb(V) reductases, by O₂. However, methanotrophs Methylomonas and Methylosinus were enriched in the biofilm with O₂ intrusion, in accordance with the enhanced abundance of genes encoding aerobic CH₄ oxidation. These insights evoke the theoretical guidance of microbial remediation using CH₄ as the electron donor towards Sb(V) contamination, and will give us a strong reference with regard to wastewater disposal.

Rhodobacter sphaeroides methionine sulfoxide reductase P reduces R- and S-diastereomers of methionine sulfoxide from a broad-spectrum of protein substrates

Methionine (Met) is prone to oxidation and can be converted to Met sulfoxide (MetO), which exists as R- and S-diastereomers. MetO can be reduced back to Met by the ubiquitous methionine sulfoxide reductase (Msr) enzymes. Canonical MsrA and MsrB were shown to be absolutely stereospecific for the reduction of S-diastereomer and R-diastereomer, respectively. Recently, a new enzymatic system, MsrQ/MsrP which is conserved in all gram-negative bacteria, was identified as a key actor for the reduction of oxidized periplasmic proteins. The haem-binding membrane protein MsrQ transmits reducing power from the electron transport chains to the molybdoenzyme MsrP, which acts as a protein-MetO reductase. The MsrQ/MsrP function was well established genetically, but the identity and biochemical properties of MsrP substrates remain unknown. In this work, using the purified MsrP enzyme from the photosynthetic bacteria Rhodobacter sphaeroides as a model, we show that it can reduce a broad spectrum of protein substrates. The most efficiently reduced MetO is found in clusters, in amino acid sequences devoid of threonine and proline on the C-terminal side. Moreover, R. sphaeroides MsrP lacks stereospecificity as it can reduce both R- and S-diastereomers of MetO, similarly to its Escherichia coli homolog, and preferentially acts on unfolded oxidized proteins. Overall, these results provide important insights into the function of a bacterial envelop protecting system, which should help understand how bacteria cope in harmful environments.

Clarifying Indeterminate Results on the Rabies Direct Fluorescent Antibody Test Using Real-Time Reverse Transcriptase Polymerase Chain Reaction

OBJECTIVES

Each year, rabies virus infection results in the death of more than 50 000 persons worldwide. In the United States, the Centers for Disease Control and Prevention (CDC) reported 23 human rabies cases from May 1, 2008, through October 1, 2017. Although rabies testing in the United States is highly reliable, some specimens submitted to rabies laboratories do not have adequate tissues or may be substantially decomposed. In these instances, the specimen may be considered unsatisfactory for testing or produce indeterminate results using the gold standard direct fluorescent antibody test. The objective of this study was to evaluate the number of unsatisfactory samples or samples with indeterminate results that were positive for rabies virus after additional testing using real-time reverse transcriptase polymerase chain reaction (RT-PCR).

METHODS

In 2016, we retested all unsatisfactory specimens or specimens with indeterminate results using real-time RT-PCR. We further typed any sample that was real-time RT-PCR positive to identify the infecting rabies virus variant.

RESULTS

Of 210 retested unsatisfactory specimens or specimens with indeterminate results, 9 (4.3%) were positive for rabies. In each case, the animal was infected with a homologous rabies virus variant.

CONCLUSION

These results confirm the recommendation by CDC and state public health laboratories that indeterminate results should be considered positive and justify the prompt treatment of exposed persons through an animal that is suspected to have rabies.

The Chaperone and Redox Properties of CnoX Chaperedoxins Are Tailored to the Proteostatic Needs of Bacterial Species

How proteins are protected from stress-induced aggregation is a crucial question in biology and a long-standing mystery. While a long series of landmark studies have provided important contributions to our current understanding of the proteostasis network, key fundamental questions remain unsolved. In this study, we show that the intrinsic features of the chaperedoxin CnoX, a folding factor that combines chaperone and redox protective function, have been tailored during evolution to fit to the specific needs of their host. Whereas Escherichia coli CnoX needs to be activated by bleach, a powerful oxidant produced by our immune system, its counterpart in Caulobacter crescentus, a bacterium living in bleach-free environments, is a constitutive chaperone. In addition, the redox properties of E. coli and C. crescentus CnoX also differ to best contribute to their respective cellular redox homeostasis. This work demonstrates how proteins from the same family have evolved to meet the needs of their hosts.

Hypochlorous acid (bleach), an oxidizing compound produced by neutrophils, turns the Escherichia coli chaperedoxin CnoX into a powerful holdase protecting its substrates from bleach-induced aggregation. CnoX is well conserved in bacteria, even in non-infectious species unlikely to encounter this oxidant, muddying the role of CnoX in these organisms. Here, we found that CnoX in the non-pathogenic aquatic bacterium Caulobacter crescentus functions as a holdase that efficiently protects 50 proteins from heat-induced aggregation. Remarkably, the chaperone activity of Caulobacter CnoX is constitutive. Like E. coli CnoX, Caulobacter CnoX transfers its substrates to DnaK/J/GrpE and GroEL/ES for refolding, indicating conservation of cooperation with GroEL/ES. Interestingly, Caulobacter CnoX exhibits thioredoxin oxidoreductase activity, by which it controls the redox state of 90 proteins. This function, which E. coli CnoX lacks, is likely welcome in a bacterium poorly equipped with antioxidant defenses. Thus, the redox and chaperone properties of CnoX chaperedoxins were fine-tuned during evolution to adapt these proteins to the specific needs of each species.

A novel chlorination-induced ribonuclease YabJ from Staphylococcus aureus

The characteristic fold of a protein is the decisive factor for its biological function. However, small structural changes to amino acids can also affect their function, for example in the case of post-translational modification (PTM). Many different types of PTMs are known, but for some, including chlorination, studies elucidating their importance are limited. A recent study revealed that the YjgF/YER057c/UK114 family (YjgF family) member RidA from Escherichia coli shows chaperone activity after chlorination. Thus, to identify the functional and structural differences of RidA upon chlorination, we studied an RidA homolog from Staphylococcus aureus: YabJ. The overall structure of S. aureus YabJ was similar to other members of the YjgF family, showing deep pockets on its surface, and the residues composing the pockets were well conserved. S. aureus YabJ was highly stable after chlorination, and the chlorinated state is reversible by treatment with DTT. However, it shows no chaperone activity after chlorination. Instead, YabJ from S. aureus shows chlorination-induced ribonuclease activity, and the activity is diminished after subsequent reduction. Even though the yabJ genes from Staphylococcus and Bacillus are clustered with regulators that are expected to code nucleic acid-interacting proteins, the nucleic acid-related activity of bacterial RidA has not been identified before. From our study, we revealed the structure and function of S. aureus YabJ as a novel chlorination-activated ribonuclease. The present study will contribute to an in-depth understanding of chlorination as a PTM.

Chlorate Specifically Targets Oxidant-Starved, Antibiotic-Tolerant Populations of Pseudomonas aeruginosa Biofilms

The anaerobic growth and survival of bacteria are often correlated with physiological tolerance to conventional antibiotics, motivating the development of novel strategies targeting pathogens in anoxic environments. A key challenge is to identify drug targets that are specific to this metabolic state. Chlorate is a nontoxic compound that can be reduced to toxic chlorite by a widespread enzyme of anaerobic metabolism. We tested the antibacterial properties of chlorate against Pseudomonas aeruginosa, a pathogen that can inhabit hypoxic or anoxic microenvironments, including those that arise in human infection. Chlorate and the antibiotic tobramycin kill distinct metabolic populations in P. aeruginosa biofilms, where chlorate targets anaerobic cells that tolerate tobramycin. Chlorate is particularly effective against P. aeruginosalasR mutants, which are frequently isolated from human infections and more resistant to some antibiotics. This work suggests that chlorate may hold potential as an anaerobic prodrug.

Nitrate respiration is a widespread mode of anaerobic energy generation used by many bacterial pathogens, and the respiratory nitrate reductase, Nar, has long been known to reduce chlorate to the toxic oxidizing agent chlorite. Here, we demonstrate the antibacterial activity of chlorate against Pseudomonas aeruginosa, a representative pathogen that can inhabit hypoxic or anoxic host microenvironments during infection. Aerobically grown P. aeruginosa cells are tobramycin sensitive but chlorate tolerant. In the absence of oxygen or an alternative electron acceptor, cells are tobramycin tolerant but chlorate sensitive via Nar-dependent reduction. The fact that chlorite, the product of chlorate reduction, is not detected in culture supernatants suggests that it may react rapidly and be retained intracellularly. Tobramycin and chlorate target distinct populations within metabolically stratified aggregate biofilms; tobramycin kills cells on the oxic periphery, whereas chlorate kills hypoxic and anoxic cells in the interior. In a matrix populated by multiple aggregates, tobramycin-mediated death of surface aggregates enables deeper oxygen penetration into the matrix, benefiting select aggregate populations by increasing survival and removing chlorate sensitivity. Finally, lasR mutants, which commonly arise in P. aeruginosa infections and are known to withstand conventional antibiotic treatment, are hypersensitive to chlorate. A lasR mutant shows a propensity to respire nitrate and reduce chlorate more rapidly than the wild type does, consistent with its heightened chlorate sensitivity. These findings illustrate chlorate’s potential to selectively target oxidant-starved pathogens, including physiological states and genotypes of P. aeruginosa that represent antibiotic-tolerant populations during infections.

Redox-Sensing Under Hypochlorite Stress and Infection Conditions by the Rrf2-Family Repressor HypR in Staphylococcus aureus

AIMS

Staphylococcus aureus is a major human pathogen and has to cope with reactive oxygen and chlorine species (ROS, RCS) during infections, which requires efficient protection mechanisms to avoid destruction. Here, we have investigated the changes in the RNA-seq transcriptome by the strong oxidant sodium hypochlorite (NaOCl) in S. aureus USA300 to identify novel redox-sensing mechanisms that provide protection under infection conditions.

RESULTS

NaOCl stress caused an oxidative stress response in S. aureus as indicated by the induction of the PerR, QsrR, HrcA, and SigmaB regulons in the RNA-seq transcriptome. The hypR-merA (USA300HOU_0588-87) operon was most strongly upregulated under NaOCl stress, which encodes for the Rrf2-family regulator HypR and the pyridine nucleotide disulfide reductase MerA. We have characterized HypR as a novel redox-sensitive repressor that controls MerA expression and directly senses and responds to NaOCl and diamide stress via a thiol-based mechanism in S. aureus. Mutational analysis identified Cys33 and the conserved Cys99 as essential for NaOCl sensing, while Cys99 is also important for repressor activity of HypR in vivo. The redox-sensing mechanism of HypR involves Cys33-Cys99 intersubunit disulfide formation by NaOCl stress both in vitro and in vivo. Moreover, the HypR-controlled flavin disulfide reductase MerA was shown to protect S. aureus against NaOCl stress and increased survival in J774A.1 macrophage infection assays. Conclusion and Innovation: Here, we identified a new member of the widespread Rrf2 family as redox sensor of NaOCl stress in S. aureus that uses a thiol/disulfide switch to regulate defense mechanisms against the oxidative burst under infections in S. aureus. Antioxid. Redox Signal. 29, 615-636.

Strong increase in the autofluorescence of cells signals struggle for survival

Prokaryotic and eukaryotic cells exhibit an intrinsic natural fluorescence due to the presence of fluorescent cellular structural components and metabolites. Therefore, cellular autofluorescence (AF) is expected to vary with the metabolic states of cells. We examined how exposure to the different stressors changes the AF of Escherichia coli cells. We observed that bactericidal treatments increased green cellular AF, and that de novo protein synthesis was required for the observed AF increase. Excitation and emission spectra and increased expression of the genes from the flavin biosynthesis pathway, strongly suggested that flavins are major contributors to the increased AF. An increased expression of genes encoding diverse flavoproteins which are involved in energy production and ROS detoxification, indicates a cellular strategy to cope with severe stresses. An observed increase in AF under stress is an evolutionary conserved phenomenon as it occurs not only in cells from different bacterial species, but also in yeast and human cells.

Method for Dynamically Detecting Secretions from Single Cells Using a Nanopore

Secreted proteins mediate cell-to-cell communications. Thus, eavesdropping on the secretome could reveal the cellular phenotype, but it is challenging to detect the proteins because they are secreted only in minute amounts and then diluted in blood plasma or contaminated by cell culture medium or the lysate. In this pilot study, it is demonstrated that secretions from single cancer cells can be detected and dynamically analyzed through measurements of blockades in the electrolytic current due to single molecules translocating through a nanopore in a thin inorganic membrane. It is established that the distribution of blockades can be used to differentiate three different cancer cell lines (U937, MDA-MB-231, and MCF-7) in real time and quickly (<20 s). Importantly, the distinctive blockades associated with the chemokine CCL5, a prognostic factor for disease progression in breast cancer, along with other low-mass biomarkers of breast cancer (PI3, TIMP1, and MMP1) were identified in the context of the secretome of these three cell types, tracked with time, and used to provide information on the cellular phenotype.

Sodium Persulfate and Potassium Permanganate Inhibit Methanogens and Methanogenesis in Stored Liquid Dairy Manure

Stored liquid dairy manure is a hotspot for methane (CH) emission, thus effective mitigation strategies are required. We assessed sodium persulfate (NaSO), potassium permanganate (KMnO), and sodium hypochlorite (NaOCl) for impacts on the abundance of microbial communities and CH production in liquid dairy manure. Liquid dairy manure treated with different rates (1, 3, 6, and 9 g or mL L slurry) of these chemicals or their combinations were incubated under anoxic conditions at 22.5 ± 1.3°C for 120 d. Untreated and sodium 2-bromoethanesulfonate (BES)-treated manures were included as negative and positive controls, respectively, whereas sulfuric acid (HSO)-treated manure was used as a reference. Quantitative real-time polymerase chain reaction was used to quantify the abundances of bacteria and methanogens on Days 0, 60, and 120. Headspace CH/CO ratios were used as a proxy to determine CH production. Unlike bacterial abundance, methanogen abundance and CH/CO ratios varied with treatments. Addition of 1 to 9 g L slurry of NaSO and KMnO reduced methanogen abundance (up to ∼28%) and peak CH/CO ratios (up to 92-fold). Except at the lowest rate, chemical combinations also reduced the abundance of methanogens (up to ∼17%) and CH/CO ratios (up to ninefold), although no impacts were observed when 3% NaOCl was used alone. With slurry acidification, the ratios reduced up to twofold, whereas methanogen abundance was unaffected. Results suggest that NaSO and KMnO may offer alternative options to reduce CH emission from stored liquid dairy manure, but this warrants further assessment at larger scales for environmental impacts and characteristics of the treated manure.

Emerging fields in chaperone proteins: A French workshop

The "Bioénergétique et Ingénierie des Protéines (BIP)" laboratory, CNRS (France), organized its first French workshop on molecular chaperone proteins and protein folding in November 2017. The goal of this workshop was to gather scientists working in France on chaperone proteins and protein folding. This initiative was a great success with excellent talks and fruitful discussions. The highlights were on the description of unexpected functions and post-translational regulation of known molecular chaperones (such as Hsp90, Hsp33, SecB, GroEL) and on state-of-the-art methods to tackle questions related to this theme, including Cryo-electron microscopy, Nuclear Magnetic Resonance (NMR), Electron Paramagnetic Resonance (EPR), simulation and modeling. We expect to organize a second workshop in two years that will include more scientists working in France in the chaperone field.

Chemical Warfare at the Microorganismal Level: A Closer Look at the Superoxide Dismutase Enzymes of Pathogens

Superoxide anion radical is generated as a natural byproduct of aerobic metabolism but is also produced as part of the oxidative burst of the innate immune response design to kill pathogens. In living systems, superoxide is largely managed through superoxide dismutases (SODs), families of metalloenzymes that use Fe, Mn, Ni, or Cu cofactors to catalyze the disproportionation of superoxide to oxygen and hydrogen peroxide. Given the bursts of superoxide faced by microbial pathogens, it comes as no surprise that SOD enzymes play important roles in microbial survival and virulence. Interestingly, microbial SOD enzymes not only detoxify host superoxide but also may participate in signaling pathways that involve reactive oxygen species derived from the microbe itself, particularly in the case of eukaryotic pathogens. In this Review, we will discuss the chemistry of superoxide radicals and the role of diverse SOD metalloenzymes in bacterial, fungal, and protozoan pathogens. We will highlight the unique features of microbial SOD enzymes that have evolved to accommodate the harsh lifestyle at the host-pathogen interface. Lastly, we will discuss key non-SOD superoxide scavengers that specific pathogens employ for defense against host superoxide.

Defining Hsp33's Redox-regulated Chaperone Activity and Mapping Conformational Changes on Hsp33 Using Hydrogen-deuterium Exchange Mass Spectrometry

Living organisms regularly need to cope with fluctuating environments during their life cycle, including changes in temperature, pH, the accumulation of reactive oxygen species, and more. These fluctuations can lead to a widespread protein unfolding, aggregation, and cell death. Therefore, cells have evolved a dynamic and stress-specific network of molecular chaperones, which maintain a "healthy" proteome during stress conditions. ATP-independent chaperones constitute one major class of molecular chaperones, which serve as first-line defense molecules, protecting against protein aggregation in a stress-dependent manner. One feature these chaperones have in common is their ability to utilize structural plasticity for their stress-specific activation, recognition, and release of the misfolded client. In this paper, we focus on the functional and structural analysis of one such intrinsically disordered chaperone, the bacterial redox-regulated Hsp33, which protects proteins against aggregation during oxidative stress. Here, we present a toolbox of diverse techniques for studying redox-regulated chaperone activity, as well as for mapping conformational changes of the chaperone, underlying its activity. Specifically, we describe a workflow which includes the preparation of fully reduced and fully oxidized proteins, followed by an analysis of the chaperone anti-aggregation activity in vitro using light-scattering, focusing on the degree of the anti-aggregation activity and its kinetics. To overcome frequent outliers accumulated during aggregation assays, we describe the usage of Kfits, a novel graphical tool which allows easy processing of kinetic measurements. This tool can be easily applied to other types of kinetic measurements for removing outliers and fitting kinetic parameters. To correlate the function with the protein structure, we describe the setup and workflow of a structural mass spectrometry technique, hydrogen-deuterium exchange mass spectrometry, that allows the mapping of conformational changes on the chaperone and substrate during different stages of Hsp33 activity. The same methodology can be applied to other protein-protein and protein-ligand interactions.

CnoX Is a Chaperedoxin: A Holdase that Protects Its Substrates from Irreversible Oxidation

Bleach (HOCl) is a powerful oxidant that kills bacteria in part by causing protein aggregation. It inactivates ATP-dependent chaperones, rendering cellular proteins mostly dependent on holdases. Here we identified Escherichia coli CnoX (YbbN) as a folding factor that, when activated by bleach via chlorination, functions as an efficient holdase, protecting the substrates of the major folding systems GroEL/ES and DnaK/J/GrpE. Remarkably, CnoX uniquely combines this function with the ability to prevent the irreversible oxidation of its substrates. This dual activity makes CnoX the founding member of a family of proteins, the "chaperedoxins." Because CnoX displays a thioredoxin fold and a tetratricopeptide (TPR) domain, two structural motifs conserved in all organisms, this investigation sets the stage for the discovery of additional chaperedoxins in bacteria and eukaryotes that could cooperate with proteins from both the Hsp60 and Hsp70 families.

Chloride-accelerated Cu-Fenton chemistry for biofilm removal

Biofilms present challenges to numerous industries. Herein, a simple approach was developed based on chloride-accelerated Fenton chemistry, where copper oxide nanoparticles facilitate efficient generation of reactive chlorine species for biofilm removal.

Redox Aspects of Chaperones in Cardiac Function

Molecular chaperones are stress proteins that allow the correct folding or unfolding as well as the assembly or disassembly of macromolecular cellular components. Changes in expression and post-translational modifications of chaperones have been linked to a number of age- and stress-related diseases including cancer, neurodegeneration, and cardiovascular diseases. Redox sensible post-translational modifications, such as S-nitrosylation, glutathionylation and phosphorylation of chaperone proteins have been reported. Redox-dependent regulation of chaperones is likely to be a phenomenon involved in metabolic processes and may represent an adaptive response to several stress conditions, especially within mitochondria, where it impacts cellular bioenergetics. These post-translational modifications might underlie the mechanisms leading to cardioprotection by conditioning maneuvers as well as to ischemia/reperfusion injury. In this review, we discuss this topic and focus on two important aspects of redox-regulated chaperones, namely redox regulation of mitochondrial chaperone function and cardiac protection against ischemia/reperfusion injury.

A photocaged fluorescent probe for imaging hypochlorous acid in lysosomes

By combining the advantages of the photocaging technology and traditional analyte-responsive fluorescent probes, we designed and synthesized the first photocaged lysosomal-targeted fluorescent HOCl probe (PL-HA), which is capable of remote light-controlled intracellular target recognition of HOCl in lysosomes.

Rationalizing and advancing the 3-MPBA SERS sandwich assay for rapid detection of bacteria in environmental and food matrices

Bacterial foodborne illness continues to be a pressing issue in our food supply. Rapid detection methods are needed for perishable foods due to their short shelf lives and significant contribution to foodborne illness. Previously, a sensitive and reliable surface-enhanced Raman spectroscopy (SERS) sandwich assay based on 3-mercaptophenylboronic acid (3-MBPA) as a capturer and indicator molecule was developed for rapid bacteria detection. In this study, we explored the advantages and constraints of this assay over the conventional aerobic plate count (APC) method and further developed methods for detection in real environmental and food matrices. The SERS sandwich assay was able to detect environmental bacteria in pond water and on spinach leaves at higher levels than the APC method. In addition, the SERS assay appeared to have higher sensitivity to quantify bacteria in the stationary phase. On the other hand, the APC method was more sensitive to cell viability. Finally, a method to detect bacteria in a challenging high-sugar juice matrix was developed to enhance bacteria capture. This study advanced the SERS technique for real applications in environment and food matrices.

A Role of Metastable Regions and Their Connectivity in the Inactivation of a Redox-Regulated Chaperone and Its Inter-Chaperone Crosstalk

AIMS

A recently discovered group of conditionally disordered chaperones share a very unique feature; they need to lose structure to become active as chaperones. This activation mechanism makes these chaperones particularly suited to respond to protein-unfolding stress conditions, such as oxidative unfolding. However, the role of this disorder in stress-related activation, chaperone function, and the crosstalk with other chaperone systems is not yet clear. Here, we focus on one of the members of the conditionally disordered chaperones, a thiol-redox switch of the bacterial proteostasis system, Hsp33.

RESULTS

By modifying the Hsp33's sequence, we reveal that the metastable region has evolved to abolish redox-dependent chaperone activity, rather than enhance binding affinity for client proteins. The intrinsically disordered region of Hsp33 serves as an anchor for the reduced, inactive state of Hsp33, and it dramatically affects the crosstalk with the synergetic chaperone system, DnaK/J. Using mass spectrometry, we describe the role that the metastable region plays in determining client specificity during normal and oxidative stress conditions in the cell. Innovation and Conclusion: We uncover a new role of protein plasticity in Hsp33's inactivation, client specificity, crosstalk with the synergistic chaperone system DnaK/J, and oxidative stress-specific interactions in bacteria. Our results also suggest that Hsp33 might serve as a member of the house-keeping proteostasis machinery, tasked with maintaining a "healthy" proteome during normal conditions, and that this function does not depend on the metastable linker region. Antioxid. Redox Signal. 27, 1252-1267.

Effect of Sterilants on Amplification and Detection of Target DNA from Bacillus cereus Spores

To conceal criminal activity of a bioterrorist or agroterrorist, the site of pathogen generation is often treated with sterilants to kill the organisms and remove evidence. As dead organisms cannot be analyzed by culture, this study examined whether DNA from sterilant-treated Bacillus cereus spores was viable for amplification. The spores were exposed to five common sterilants: bleach, Sterilox®, oxidizer foam (L-Gel), a peroxyacid (Actril®), and formaldehyde vapor. The spores were inoculated on typical surfaces found in offices and laboratories to test for environmental effects. It was found that the surface influenced the efficiency of recovery of the organisms. The DNA isolated from the recovered spores was successfully detected using RT-qPCR for all treatments except for formaldehyde, by amplifying the phosphatidylinositol phospholipase C and sphingomyelinase genes. The results demonstrated that evidence from sites treated with sterilants can still provide information on the uncultured organism, using DNA amplification.

From structure to redox: The diverse functional roles of disulfides and implications in disease

This review provides a comprehensive overview of the functional roles of disulfide bonds and their relevance to human disease. The critical roles of disulfide bonds in protein structure stabilization and redox regulation of protein activity are addressed. Disulfide bonds are essential to the structural stability of many proteins within the secretory pathway and can exist as intramolecular or inter-domain disulfides. The proper formation of these bonds often relies on folding chaperones and oxidases such as members of the protein disulfide isomerase (PDI) family. Many of the PDI family members catalyze disulfide-bond formation, reduction, and isomerization through redox-active disulfides and perturbed PDI activity is characteristic of carcinomas and neurodegenerative diseases.  In addition to catalytic function in oxidoreductases, redox-active disulfides are also found on a diverse array of cellular proteins and act to regulate protein activity and localization in response to oxidative changes in the local environment. These redox-active disulfides are either dynamic intramolecular protein disulfides or mixed disulfides with small-molecule thiols generating glutathionylation and cysteinylation adducts. The oxidation and reduction of redox-active disulfides are mediated by cellular reactive oxygen species and activity of reductases, such as glutaredoxin and thioredoxin.  Dysregulation of cellular redox conditions and resulting changes in mixed disulfide formation are directly linked to diseases such as cardiovascular disease and Parkinson's disease.

Stress-Activated Chaperones: A First Line of Defense

Proteins are constantly challenged by environmental stress conditions that threaten their structure and function. Especially problematic are oxidative, acid, and severe heat stress which induce very rapid and widespread protein unfolding and generate conditions that make canonical chaperones and/or transcriptional responses inadequate to protect the proteome. We review here recent advances in identifying and characterizing stress-activated chaperones which are inactive under non-stress conditions but become potent chaperones under specific protein-unfolding stress conditions. We discuss the post-translational mechanisms by which these chaperones sense stress, and consider the role that intrinsic disorder plays in their regulation and function. We examine their physiological roles under both non-stress and stress conditions, their integration into the cellular proteostasis network, and their potential as novel therapeutic targets.

Pseudomonas aeruginosa defense systems against microbicidal oxidants

The most abundant oxidants controlling bacterial colonization on mucosal barrier epithelia are hypochlorous acid (HOCl), hypobromous acid (HOBr) and hypothiocyanous acid (HOSCN). All three oxidants are highly antimicrobial but little is known about their relative efficacies, their respective cellular targets, or what specific responses they elicit in bacteria. To address these important questions, we directly tested the individual oxidants on the virulent Pseudomonas aeruginosa strain PA14. We discovered that HOCl and HOBr work almost interchangeably, impacting non-growing bacterial cultures more significantly than actively growing bacteria, and eliciting similar stress responses, including the heat shock response. HOSCN treatment is distinctly different, affecting primarily actively growing PA14 and evoking stress responses suggestive of membrane damage. What all three oxidants have in common, however, is their ability to cause substantial protein aggregation. This effect became particularly obvious in strains lacking polyphosphate, a newly recognized chemical chaperone. Treatment of PA14 with the FDA-approved anti-inflammatory drug mesalamine, which has recently been shown to attenuate polyP production in a wide range of bacteria, effectively decreased the resistance of PA14 toward all three oxidants, suggesting that we have discovered a novel, targetable defense system in P. aeruginosa.

Suppression of NF-κB signal pathway by NLRC3-like protein in stony coral Acropora aculeus under heat stress

Heat stress is the most common factor for coral bleaching, which has increased both in frequency and severity due to global warming. In the present study, the stony coral Acropora aculeus was subjected to acute heat stress and entire transcriptomes were sequenced via the next generation sequencing platform. Four paired-end libraries were constructed and sequenced in two groups, including a control and a heat stress group. A total of 120,319,751 paired-end reads with lengths of 2 × 100 bp were assembled and 55,021 coral-derived genes were obtained. After read mapping and abundance estimation, 9110 differentially expressed genes were obtained in the comparison between the control and heat stress group, including 4465 significantly upregulated and 4645 significantly downregulated genes. Twenty-three GO terms in the Biological Process category were overrepresented for significantly upregulated genes, and divided into six groups according to their relationship. These three groups were related to the NF-κB signal pathway, and the remaining three groups were relevant for pathogen response, immunocyte activation and protein ubiquitination. Forty-three common genes were found in four GO terms, which were directly related to the NF-κB signal pathway. These included 2 NACHT, LRR, PYD domains-containing protein, 5 nucleotide-binding oligomerization domain-containing protein, 29 NLRC3-like protein, 4 NLRC5-like protein, and 3 uncharacterized protein. For significantly downregulated genes, 27 overrepresented GO terms were found in the Biological Process category, which were relevant to protein ubiquitination and ATP metabolism. Our results indicate that heat stress suppressed the immune response level via the NLRC3-like protein, the fine-tuning of protein turnover activity, and ATP metabolism. This might disrupt the balance of coral-zooxanthellae symbiosis and result in the bleaching of the coral A. aculeus.

Oxidation of phosphorothioate DNA modifications leads to lethal genomic instability

Genomic modification with sulfur as phosphorothioate (PT) is widespread among prokaryotes, including human pathogens. Apart from its physiological functions, the redox and nucleophilic properties of PT sulfur suggest effects on bacterial fitness in stressful environments. Here we show that PTs are dynamic and labile DNA modifications that cause genomic instability during oxidative stress. Using coupled isotopic labeling-mass spectrometry, we observed sulfur replacement in PTs at a rate of ~2%/h in unstressed Escherichia coli and Salmonella enterica. While PT levels were unaffected by exposure to hydrogen peroxide (H₂O₂) or hypochlorous acid (HOCl), PT turnover increased to 3.8–10%/h for HOCl and was unchanged for H₂O₂, consistent with repair of HOCl-induced sulfur damage. PT-dependent HOCl sensitivity extended to cytotoxicity and DNA strand-breaks, which occurred at orders-of-magnitude lower doses of HOCl than H₂O₂. The genotoxicity of HOCl in PT-containing bacteria suggests reduced fitness in competition with HOCl-producing organisms and during human infections.

Effect of ozonation on anaerobic digestion sludge activity and viability

The effect of ozonation of anaerobic digested sludge on methane production was studied as a means of increasing the capacity of municipal anaerobic digesters. Ozone doses ranging from 0 to 192 mg O₃/g sludge COD were evaluated in batch tests with a bench scale ozonation unit. Ozonation initially, and temporarily, reduced biomass viability and acetoclastic methanogenic activity, resulting in an initial lag phase ranging from 0.8 to 10 days. Following this lag phase, ozonation enhanced methane production with an optimal methane yield attained at 86 mg O₃/g COD. Under these conditions, the yield of methane and the rate of its formation were 52% and 95% higher, respectively, than those factors measured without ozonation. A required optimal ozone dose could be feasible to improve the anaerobic digestion performance by increasing the methane production potential with a minimum impact on microbial activity; thus, an optimal ozone dose would enable an increase in the capacity of anaerobic digesters.

Protective efficacy of six immunogenic recombinant proteins of Vibrio anguillarum and evaluation them as vaccine candidate for flounder (Paralichthys olivaceus)

Vibrio anguillarum is a severe bacterium that causes terminal haemorrhagic septicaemia in freshwater and marine fish. Virulence-associated proteins play an important role in bacterial pathogenicity and could be applied for immunoprophylaxis. In this study, six antigenic proteins from V. anguillarum were selected and the immune protective efficacy of their recombinant proteins was investigated. VirA, CheR, FlaC, OmpK, OmpR and Hsp33 were recombinantly produced and the reactions of recombinant proteins to flounder-anti-V. anguillarum antibodies (fV-ab) were detected, respectively. Then the recombinant proteins were injected to fish, after immunization, the percentages of surface membrane immunoglobulin-positive (sIg+) cell in lymphocytes, total antibodies, antibodies against V. anguillarum, antibodies against recombinant proteins and relative percent survival (RPS) were analyzed, respectively. The results showed that all the recombinant proteins could react to fV-ab, proliferate sIg + cells in lymphocytes and induce production of total antibodies, specific antibodies against V. anguillarum or the recombinant proteins; the RPS of rVirA, rCheR, rFlaC, rOmpK, rOmpR and rHsp33 against V. anguillarum was 70.27%, 27.03%, 16.22%, 62.16%, 45.95% and 81.08%, respectively. The results revealed that rHsp33, rVirA and rOmpK have good protections against V. anguillarum and could be vaccine candidates against V. anguillarum.

Harnessing host ROS-generating machinery for the robust genome replication of a plant RNA virus

As sessile organisms, plants have to accommodate to rapid changes in their surrounding environment. Reactive oxygen species (ROS) act as signaling molecules to transduce biotic and abiotic stimuli into plant stress adaptations. It is established that a respiratory burst oxidase homolog B of Nicotiana benthamiana (NbRBOHB) produces ROS in response to microbe-associated molecular patterns to inhibit pathogen infection. Plant viruses are also known as causative agents of ROS induction in infected plants; however, the function of ROS in plant-virus interactions remains obscure. Here, we show that the replication of red clover necrotic mosaic virus (RCNMV), a plant positive-strand RNA [(+)RNA] virus, requires NbRBOHB-mediated ROS production. The RCNMV replication protein p27 plays a pivotal role in this process, redirecting the subcellular localization of NbRBOHB and a subgroup II calcium-dependent protein kinase of N. benthamiana (NbCDPKiso2) from the plasma membrane to the p27-containing intracellular aggregate structures. p27 also induces an intracellular ROS burst in an RBOH-dependent manner. NbCDPKiso2 was shown to be an activator of the p27-triggered ROS accumulations and to be required for RCNMV replication. Importantly, this RBOH-derived ROS is essential for robust viral RNA replication. The need for RBOH-derived ROS was demonstrated for the replication of another (+)RNA virus, brome mosaic virus, suggesting that this characteristic is true for plant (+)RNA viruses. Collectively, our findings revealed a hitherto unknown viral strategy whereby the host ROS-generating machinery is diverted for robust viral RNA replication.

Review of Atopic Dermatitis and Topical Therapies

Atopic dermatitis is one of the most common skin disorders in the developed world, affecting up to 20% of children and 1% to 3% of adults. This review concisely explains the pathophysiology and epidemiology of atopic dermatitis, as well as potential challenges facing its successful treatment. Furthermore, mainstay topical treatment modalities are evaluated, such as emollients, topical corticosteroids, and topical calcineurin inhibitors. The use of topical corticosteroids and topical calcineurin inhibitors in combination is discussed, as studies have indicated encouraging results. The proactive use of topical corticosteroids and topical calcineurin inhibitors is also investigated, in order to bring attention to a new possibility in long-term management of atopic dermatitis. Last, new and upcoming topical medications are described, including Janus kinase inhibitors, phosphodiesterase-4 inhibitors, and benvitimod. Although topical corticosteroids and topical calcineurin inhibitors can be very effective in the treatment of atopic dermatitis, it is important that practitioners are aware of mechanistically unique and new treatments for patients for whom more traditional topical therapies have failed. Overall, this review article hopes to serve as a comprehensive overview of currently available topical treatments for atopic dermatitis, while shedding light on new treatments coming in the future.

Oxidative Unfolding of the Rubredoxin Domain and the Natively Disordered N-terminal Region Regulate the Catalytic Activity of Mycobacterium tuberculosis Protein Kinase G

Mycobacterium tuberculosis escapes killing in human macrophages by secreting protein kinase G (PknG). PknG intercepts host signaling to prevent fusion of the phagosome engulfing the mycobacteria with the lysosome and, thus, their degradation. The N-terminal NORS (no regulatory secondary structure) region of PknG (approximately residues 1-75) has been shown to play a role in PknG regulation by (auto)phosphorylation, whereas the following rubredoxin-like metal-binding motif (RD, residues ∼74-147) has been shown to interact tightly with the subsequent catalytic domain (approximately residues 148-420) to mediate its redox regulation. Deletions or mutations in NORS or the redox-sensitive RD significantly decrease PknG survival function. Based on combined NMR spectroscopy, in vitro kinase assay, and molecular dynamics simulation data, we provide novel insights into the regulatory roles of the N-terminal regions. The NORS region is indeed natively disordered and rather dynamic. Consistent with most earlier data, autophosphorylation occurs in our assays only when the NORS region is present and, thus, in the NORS region. Phosphorylation of it results only in local conformational changes and does not induce interactions with the subsequent RD. Although the reduced, metal-bound RD makes tight interactions with the following catalytic domain in the published crystal structures, it can also fold in its absence. Our data further suggest that oxidation-induced unfolding of the RD regulates substrate access to the catalytic domain and, thereby, PknG function under different redox conditions, e.g. when exposed to increased levels of reactive oxidative species in host macrophages.

Impact of reducing and oxidizing agents on the infectivity of Qβ phage and the overall structure of its capsid

Qβ phages infect Escherichia coli in the human gut by recognizing F-pili as receptors. Infection therefore occurs under reducing conditions induced by physiological agents (e.g. glutathione) or the intestinal bacterial flora. After excretion in the environment, phage particles are exposed to oxidizing conditions and sometimes disinfection. If inactivation does not occur, the phage may infect new hosts in the human gut through the oral route. During such a life cycle, we demonstrated that, outside the human gut, cysteines of the major protein capsid of Qβ phage form disulfide bonds. Disinfection with NaClO does not allow overoxidation to occur. Such oxidation induces inactivation rather by irreversible damage to the minor proteins. In the presence of glutathione, most disulfide bonds are reduced, which slightly increases the capacity of the phage to infect E. coli in vitro Such reduction is reversible and barely alters infectivity of the phage. Reduction of all disulfide bonds by dithiothreitol leads to complete capsid destabilization. These data provide new insights into how the phages are impacted by oxidizing-reducing conditions outside their host cell and raises the possibility of the intervention of the redox during life cycle of the phage.

Heme-independent Redox Sensing by the Heme-Nitric Oxide/Oxygen-binding Protein (H-NOX) from Vibrio cholerae

Heme nitric oxide/oxygen (H-NOX)-binding proteins act as nitric oxide (NO) sensors among various bacterial species. In several cases, they act to mediate communal behavior such as biofilm formation, quorum sensing, and motility by influencing the activity of downstream signaling proteins such as histidine kinases (HisKa) in a NO-dependent manner. An H-NOX/HisKa regulatory circuit was recently identified in Vibrio cholerae, and the H-NOX protein has been spectroscopically characterized. However, the influence of the H-NOX protein on HisKa autophosphorylation has not been evaluated. This process may be important for persistence and pathogenicity in this organism. Here, we have expressed and purified the V. cholerae HisKa (HnoK) and H-NOX in its heme-bound (holo) and heme-free (apo) forms. Autophosphorylation assays of HnoK in the presence of H-NOX show that the holoprotein in the Fe(II)-NO and Fe(III) forms is a potent inhibitor of HnoK. Activity of the Fe(III) form and aerobic instability of the Fe(II) form suggested that Vibrio cholerae H-NOX may act as a sensor of the redox state as well as NO. Remarkably, the apoprotein also showed robust HnoK inhibition that was dependent on the oxidation of cysteine residues to form disulfide bonds at a highly conserved zinc site. The importance of cysteine in this process was confirmed by mutagenesis, which also showed that holo Fe(III), but not Fe(II)-NO, H-NOX relied heavily upon cysteine for activation. These results highlight a heme-independent mechanism for activation of V. cholerae H-NOX that implicates this protein as a dual redox/NO sensor.

Near diffusion-controlled reaction of a Zn(Cys)4 zinc finger with hypochlorous acid

Reaction rate constants of HOCl with zinc-bound cysteines are determined, demonstrating that zinc fingers are potent targets for HOCl and may serve as HOCl sensors.

Hypochlorous acid (HOCl) is one of the strongest oxidants produced in mammals to kill invading microorganisms. The bacterial response to HOCl involves proteins that are able to sense HOCl using methionine, free cysteines or zinc-bound cysteines of zinc finger sites. Although the reactivity of methionine or free cysteine with HOCl is well documented at the molecular level, this is not the case for zinc-bound cysteines. We present here a study that aims at filling this gap. Using a model peptide of the Zn(Cys)₄ zinc finger site of the chaperone Hsp33, a protein involved in the defence against HOCl in bacteria, we show that HOCl oxidation of this model leads to the formation of two disulfides. A detailed mechanistic and kinetic study of this reaction, relying on stopped-flow measurements and competitive oxidation with methionine, reveals very high rate constants: the absolute second-order rate constants for the reaction of the model zinc finger with HOCl and its conjugated base ClO^– are (9.3 ± 0.8) × 10⁸ M^–1 s^–1 and (1.2 ± 0.2) × 10⁴ M^–1 s^–1, the former approaching the diffusion limit. Revised values of the second-order rate constants for the reaction of methionine with HOCl and ClO^– were also determined to be (5.5 ± 0.8) × 10⁸ M^–1 s^–1 and (7 ± 5) × 10² M^–1 s^–1, respectively. At physiological pH, the zinc finger site reacts faster with HOCl than methionine and glutathione or cysteine. This study demonstrates that zinc fingers are potent targets for HOCl and confirms that they may serve as HOCl sensors as proposed for Hsp33.

Reactive Oxygen Species and Neutrophil Function

Neutrophils are essential for killing bacteria and other microorganisms, and they also have a significant role in regulating the inflammatory response. Stimulated neutrophils activate their NADPH oxidase (NOX2) to generate large amounts of superoxide, which acts as a precursor of hydrogen peroxide and other reactive oxygen species that are generated by their heme enzyme myeloperoxidase. When neutrophils engulf bacteria they enclose them in small vesicles (phagosomes) into which superoxide is released by activated NOX2 on the internalized neutrophil membrane. The superoxide dismutates to hydrogen peroxide, which is used by myeloperoxidase to generate other oxidants, including the highly microbicidal species hypochlorous acid. NOX activation occurs at other sites in the cell, where it is considered to have a regulatory function. Neutrophils also release oxidants, which can modify extracellular targets and affect the function of neighboring cells. We discuss the identity and chemical properties of the specific oxidants produced by neutrophils in different situations, and what is known about oxidative mechanisms of microbial killing, inflammatory tissue damage, and signaling.

Protein unfolding as a switch from self-recognition to high-affinity client binding

Stress-specific activation of the chaperone Hsp33 requires the unfolding of a central linker region. This activation mechanism suggests an intriguing functional relationship between the chaperone's own partial unfolding and its ability to bind other partially folded client proteins. However, identifying where Hsp33 binds its clients has remained a major gap in our understanding of Hsp33's working mechanism. By using site-specific Fluorine-19 nuclear magnetic resonance experiments guided by in vivo crosslinking studies, we now reveal that the partial unfolding of Hsp33's linker region facilitates client binding to an amphipathic docking surface on Hsp33. Furthermore, our results provide experimental evidence for the direct involvement of conditionally disordered regions in unfolded protein binding. The observed structural similarities between Hsp33's own metastable linker region and client proteins present a possible model for how Hsp33 uses protein unfolding as a switch from self-recognition to high-affinity client binding.

Under stress conditions the molecular chaperone Hsp33 is activated to process unfolded proteins. Here, the authors use in vivo and in vitro crosslinking and ¹⁹F-NMR to elucidate the binding site for misfolded proteins and are able to propose a model for its mechanism of action.

Microbial responses to membrane cleaning using sodium hypochlorite in membrane bioreactors: Cell integrity, key enzymes and intracellular reactive oxygen species

Sodium hypochlorite (NaClO) is a commonly used reagent for membrane cleaning in membrane bioreactors (MBRs), while it, being a kind of disinfectant (oxidant), may impair viability of microbes or even totally inactivate them upon its diffusion into mixed liquor during membrane cleaning. In this study, we systematically examine the effects of NaClO on microorganisms in terms of microbial cell integrity, metabolism behaviours (key enzymes), and intracellular reactive oxygen species (ROS) under various NaClO concentrations. Different proportions of microbial cells in activated sludge were damaged within several minutes dependent on NaClO dosages (5-50 mg/g-SS), and correspondingly organic matters were released to bulk solution. Inhibition of key enzymes involved in organic matter biodegradation, nitrification and denitrification was observed in the presence of NaClO above 1 mg/g-SS, and thus organic matter and nitrogen removal efficiencies were decreased. It was also demonstrated that intracellular ROS production was increased with the NaClO dosage higher than 1 mg/g-SS, which likely induced further damage to microbial cells.