Role of nitric oxide-detoxifying enzymes in the virulence of Pseudomonas aeruginosa against the silkworm, Bombyx mori

Defenses of multidrug resistant pathogens against reactive nitrogen species produced in infected hosts

Bacterial pathogens have sophisticated systems that allow them to survive in hosts in which innate immunity is the frontline of defense. One of the substances produced by infected hosts is nitric oxide (NO) that together with its derived species leads to the so-called nitrosative stress, which has antimicrobial properties. In this review, we summarize the current knowledge on targets and protective systems that bacteria have to survive host-generated nitrosative stress. We focus on bacterial pathogens that pose serious health concerns due to the growing increase in resistance to currently available antimicrobials. We describe the role of nitrosative stress as a weapon for pathogen eradication, the detoxification enzymes, protein/DNA repair systems and metabolic strategies that contribute to limiting NO damage and ultimately allow survival of the pathogen in the host. Additionally, this systematization highlights the lack of available data for some of the most important human pathogens, a gap that urgently needs to be addressed.

Identification and Functional Analysis of GTP Cyclohydrolase II in Candida glabrata in Response to Nitrosative Stress

Reactive nitrogen species (RNS) are signal molecules involved in various biological events; however, excess levels of RNS cause nitrosative stress, leading to cell death and/or cellular dysfunction. During the process of infection, pathogens are exposed to nitrosative stress induced by host-derived RNS. Therefore, the nitrosative stress resistance mechanisms of pathogenic microorganisms are important for their infection and pathogenicity, and could be promising targets for antibiotics. Previously, we demonstrated that the RIB1 gene encoding GTP cyclohydrolase II (GCH2), which catalyzes the first step of the riboflavin biosynthesis pathway, is important for nitrosative stress resistance in the yeast Saccharomyces cerevisiae. Here, we identified and characterized the RIB1 gene in the opportunistic pathogenic yeast Candida glabrata. Our genetic and biochemical analyses indicated that the open reading frame of CAGL0F04279g functions as RIB1 in C. glabrata (CgRIB1). Subsequently, we analyzed the effect of CgRIB1 on nitrosative stress resistance by a growth test in the presence of RNS. Overexpression or deletion of CgRIB1 increased or decreased the nitrosative stress resistance of C. glabrata, respectively, indicating that GCH2 confers nitrosative stress resistance on yeast cells. Moreover, we showed that the proliferation of C. glabrata in cultures of macrophage-like cells required the GCH2-dependent nitrosative stress detoxifying mechanism. Additionally, an infection assay using silkworms as model host organisms indicated that CgRIB1 is indispensable for the virulence of C. glabrata. Our findings suggest that the GCH2-dependent nitrosative stress detoxifying mechanism is a promising target for the development of novel antibiotics.

Nitric-oxide-driven oxygen release in anoxic Pseudomonas aeruginosa

Denitrification supports anoxic growth of Pseudomonas aeruginosa in infections. Moreover, denitrification may provide oxygen (O₂) resulting from dismutation of the denitrification intermediate nitric oxide (NO) as seen in Methylomirabilis oxyfera. To examine the prevalence of NO dismutation we studied O₂ release by P. aeruginosa in airtight vials. P. aeruginosa rapidly depleted O₂ but NO supplementation generated peaks of O₂ at the onset of anoxia, and we demonstrate a direct role of NO in the O₂ release. However, we were not able to detect genetic evidence for putative NO dismutases.

The supply of endogenous O₂ at the onset of anoxia could play an adaptive role when P. aeruginosa enters anaerobiosis. Furthermore, O₂ generation by NO dismutation may be more widespread than indicated by the reports on the distribution of homologues genes. In general, NO dismutation may allow removal of nitrate by denitrification without release of the very potent greenhouse gas, nitrous oxide.

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Pseudomonas aeruginosa was found to release O₂ at the onset of anoxia

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Peaks of O₂ were amplified in a nitric oxide reductase (NOR) mutant

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The O₂ release was mediated by nitric oxide (NO)

Interplay between enterohaemorrhagic Escherichia coli and nitric oxide during the infectious process

Enterohaemorrhagic Escherichia coli (EHEC) are bacterial pathogens responsible for life-threatening diseases in humans such as bloody diarrhoea and the hemolytic and uremic syndrome. To date, no specific therapy is available and treatments remain essentially symptomatic. In recent years, we demonstrated in vitro that nitric oxide (NO), a major mediator of the intestinal immune response, strongly represses the synthesis of the two cardinal virulence factors in EHEC, namely Shiga toxins (Stx) and the type III secretion system, suggesting NO has a great potential to protect against EHEC infection. In this study, we investigated the interplay between NO and EHEC in vivo using mouse models of infection. Using a NO-sensing reporter strain, we determined that EHEC sense NO in the gut of infected mice. Treatment of infected mice with a specific NOS inhibitor increased EHEC adhesion to the colonic mucosa but unexpectedly decreased Stx activity in the gastrointestinal tract, protecting mice from renal failure. Taken together, our data indicate that NO can have both beneficial and detrimental consequences on the outcome of an EHEC infection, and underline the importance of in vivo studies to increase our knowledge in host–pathogen interactions.

Role of the Nitric Oxide Reductase NorVW in the Survival and Virulence of Enterohaemorrhagic Escherichia coli during Infection

Enterohaemorrhagic Escherichia coli (EHEC) are bacterial pathogens responsible for life-threatening diseases in humans, such as hemolytic and uremic syndrome. It has been previously demonstrated that the interplay between EHEC and nitric oxide (NO), a mediator of the host immune innate response, is critical for infection outcome, since NO affects both Shiga toxin (Stx) production and adhesion to enterocytes. In this study, we investigated the role of the NO reductase NorVW in the virulence and fitness of two EHEC strains in a murine model of infection. We determined that the deletion of norVW in the strain O91:H21 B2F1 has no impact on its virulence, whereas it reduces the ability of the strain O157:H7 620 to persist in the mouse gut and to produce Stx. We also revealed that the fitness defect of strain 620 ΔnorVW is strongly attenuated when mice are treated with an NO synthase inhibitor. Altogether, these results demonstrate that the NO reductase NorVW participates in EHEC resistance against NO produced by the host and promotes virulence through the modulation of Stx synthesis. The contribution of NorVW in the EHEC infectious process is, however, strain-dependent and suggests that the EHEC response to nitrosative stress is complex and multifactorial.

Validation of the anti-infective potential of a polyherbal 'Panchvalkal' preparation, and elucidation of the molecular basis underlining its efficacy against Pseudomonas aeruginosa

BACKGROUND

A Panchvalkal formulation (Pentaphyte P-5®) mentioned in ancient texts of Indian traditional medicine was investigated for its anti-infective potential against Pseudomonas aeruginosa.

METHODS

Effect of the test formulation on bacterial growth and pigment production was evaluated by broth dilution assay. In vivo efficacy was evaluated using Caenorhabditis elegans as the model host. Whole transcriptome approach was taken to study the effect of test formulation on bacterial gene expression.

RESULTS

This formulation in vitro was found to be capable of affecting quorum sensing (QS)-regulated traits (pyocyanin, pyoverdine, biofilm) of Pseudomonas aeruginosa. In combination with antibiotics, it enhanced susceptibility of the test bacterium to antibiotics like cephalexin and tetracycline. Effect of Panchvalkal formulation (PF) on QS-regulated traits of P. aeruginosa was not reversed even after repeated exposure of the bacterium to PF. In vivo efficacy of PF was demonstrated employing Caenorhabditis elegans as the model host, wherein PF-treated bacteria were able to kill lesser worms than their extract-unexposed counterparts. Whole transcriptome study revealed that approximately 14% of the P. aeruginosa genome was expressed differently under the influence of PF.

CONCLUSIONS

Major mechanisms through which Panchvalkal seems to exert its anti-virulence effect are generation of nitrosative and oxidative stress, and disturbing iron and molybdenum homeostasis, besides interfering with QS machinery. This study is a good demonstration of the therapeutic utility of the 'polyherbalism' concept, so common in ayurved. It also demonstrates utility of the modern 'omics' tools for validating the traditional medicine i.e. ayuromics.

Synthesis and detoxification of nitric oxide in the plant beneficial rhizobacterium Bacillus amyloliquefaciens SQR9 and its effect on biofilm formation

Nitric oxide (NO) is an important gas signal that regulates many biological processes, and due to the high nitrogen recycling activity in the rhizosphere, NO is an important signaling molecule in this region. Thus, an understanding of the effect of NO on the rhizomicrobiome, especially on plant beneficial rhizobacteria, is important for the use of these bacteria in agriculture. In this study, the effect of exogenous NO on the beneficial rhizobacterium Bacillus amyloliquefaciens SQR9 was investigated. The results showed that low concentrations of NO increased the ability of the strain SQR9 to form biofilms, while high concentrations of NO inhibited the growth of this bacterium. The SQR9 gene yflM encodes nitric oxide synthase (NOS), which is used to synthesize NO, while the gene ykvO encodes a sepiapterin reductase that is used to synthesize tetrahydrobiopterin, the coenzyme of NOS. Isothermal titration calorimetry and high-performance liquid chromatography analyses demonstrated an interaction between YkvO and NADPH. SQR9 has two hmp genes, although only one was observed to be responsible for NO detoxification through oxidization. This study revealed the effect of NO on plant beneficial rhizobacterium and assessed the ability of this strain to adapt to exogenous NO, which will help to improve the application of this strain in agricultural production.

Nitric Oxide, an Old Molecule With Noble Functions in Pseudomonas aeruginosa Biology

Pseudomonas aeruginosa, a Gram-negative bacterium, is characterized by its versatility that enables persistent survival under adverse conditions. It can grow on diverse energy sources and readily acquire resistance to antimicrobial agents. As an opportunistic human pathogen, it also causes chronic infections inside the anaerobic mucus airways of cystic fibrosis patients. As a strict respirer, P. aeruginosa can grow by anaerobic nitrate ( [Formula: see text] ) respiration. Nitric oxide (NO) produced as an intermediate during anaerobic respiration exerts many important effects on the biological characteristics of P. aeruginosa. This review provides information regarding (i) how P. aeruginosa grows by anaerobic respiration, (ii) mechanisms by which NO is produced under such growth, and (iii) bacterial adaptation to NO. We also review the clinical relevance of NO in the fitness of P. aeruginosa and the use of NO as a potential therapeutic for treating P. aeruginosa infection.

Microenvironmental characteristics and physiology of biofilms in chronic infections of CF patients are strongly affected by the host immune response

In vitro studies of Pseudomonas aeruginosa and other pathogenic bacteria in biofilm aggregates have yielded detailed insight into their potential growth modes and metabolic flexibility under exposure to gradients of substrate and electron acceptor. However, the growth pattern of P. aeruginosa in chronic lung infections of cystic fibrosis (CF) patients is very different from what is observed in vitro, for example, in biofilms grown in flow chambers. Dense in vitro biofilms of P. aeruginosa exhibit rapid O₂ depletion within <50-100 μm due to their own aerobic metabolism. In contrast, in vivo investigations show that P. aeruginosa persists in the chronically infected CF lung as relatively small cell aggregates that are surrounded by numerous PMNs, where the activity of PMNs is the major cause of O₂ depletion rendering the P. aeruginosa aggregates anoxic. High levels of nitrate and nitrite enable P. aeruginosa to persist fueled by denitrification in the PMN-surrounded biofilm aggregates. This configuration creates a potentially long-term stable ecological niche for P. aeruginosa in the CF lung, which is largely governed by slow growth and anaerobic metabolism and enables persistence and resilience of this pathogen even under the recurring aggressive antimicrobial treatments of CF patients. As similar slow growth of other CF pathogens has recently been observed in endobronchial secretions, there is now a clear need for better in vitro models that simulate such in vivo growth patterns and anoxic microenvironments in order to help unravel the efficiency of existing or new antimicrobials targeting anaerobic metabolism in P. aeruginosa and other CF pathogens. We also advocate that host immune responses such as PMN-driven O₂ depletion play a central role in the formation of anoxic microniches governing bacterial persistence in other chronic infections such as chronic wounds.

An integrated network analysis reveals that nitric oxide reductase prevents metabolic cycling of nitric oxide by Pseudomonas aeruginosa

Nitric oxide (NO) is a chemical weapon within the arsenal of immune cells, but is also generated endogenously by different bacteria. Pseudomonas aeruginosa are pathogens that contain an NO-generating nitrite (NO₂^-) reductase (NirS), and NO has been shown to influence their virulence. Interestingly, P. aeruginosa also contain NO dioxygenase (Fhp) and nitrate (NO₃^-) reductases, which together with NirS provide the potential for NO to be metabolically cycled (NO→NO₃^-→NO₂^-→NO). Deeper understanding of NO metabolism in P. aeruginosa will increase knowledge of its pathogenesis, and computational models have proven to be useful tools for the quantitative dissection of NO biochemical networks. Here we developed such a model for P. aeruginosa and confirmed its predictive accuracy with measurements of NO, O₂, NO₂^-, and NO₃^- in mutant cultures devoid of Fhp or NorCB (NO reductase) activity. Using the model, we assessed whether NO was metabolically cycled in aerobic P. aeruginosa cultures. Calculated fluxes indicated a bottleneck at NO₃^-, which was relieved upon O₂ depletion. As cell growth depleted dissolved O₂ levels, NO₃^- was converted to NO₂^- at near-stoichiometric levels, whereas NO₂^- consumption did not coincide with NO or NO₃^- accumulation. Assimilatory NO₂^- reductase (NirBD) or NorCB activity could have prevented NO cycling, and experiments with ΔnirB, ΔnirS, and ΔnorC showed that NorCB was responsible for loss of flux from the cycle. Collectively, this work provides a computational tool to analyze NO metabolism in P. aeruginosa, and establishes that P. aeruginosa use NorCB to prevent metabolic cycling of NO.

Reduction of PCN biosynthesis by NO in Pseudomonas aeruginosa

Pyocyanin (PCN), a virulence factor synthesized by Pseudomonas aeruginosa, plays an important role during clinical infections. There is no study of the effect of nitric oxide (NO) on PCN biosynthesis. Here, the effect of NO on PCN levels in Pseudomonas aeruginosa strain PAO1, a common reference strain, was tested. The results showed that the NO donor sodium nitroprusside (SNP) can significantly reduce PCN levels (82.5% reduction at 60 μM SNP). Furthermore, the effect of endogenous NO on PCN was tested by constructing PAO1 nor (NO reductase gene) knockout mutants. Compared to the wild-type strain, the Δnor strain had a lower PCN (86% reduction in Δnor). To examine whether the results were universal with other P. aeruginosa strains, we collected 4 clinical strains from a hospital, tested their PCN levels after SNP treatment, and obtained similar results, i.e., PCN biosynthesis was inhibited by NO. These results suggest that NO treatment may be a new strategy to inhibit PCN biosynthesis and could provide novel insights into eliminating P. aeruginosa virulence as a clinical goal.

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NO can significantly reduce PCN production in Pseudomonas aeruginosa.

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NOR knockout mutants effectively inhibit the synthesis of PCN in P. aeruginosa.

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NO can also significantly reduce PCN production in clinical strains.

Nitrate, nitrite and nitric oxide reductases: from the last universal common ancestor to modern bacterial pathogens

The electrochemical gradient that ensues from the enzymatic activity of cytochromes such as nitrate reductase, nitric oxide reductase, and quinol oxidase contributes to the bioenergetics of the bacterial cell. Reduction of nitrogen oxides by bacterial pathogens can, however, be uncoupled from proton translocation and biosynthesis of ATP or NH4(+), but still linked to quinol and NADH oxidation. Ancestral nitric oxide reductases, as well as cytochrome c oxidases and quinol bo oxidases evolved from the former, are capable of binding and detoxifying nitric oxide to nitrous oxide. The NO-metabolizing activity associated with these cytochromes can be a sizable source of antinitrosative defense in bacteria during their associations with host cells. Nitrosylation of terminal cytochromes arrests respiration, reprograms bacterial metabolism, stimulates antioxidant defenses and alters antibiotic cytotoxicity. Collectively, the bioenergetics and regulation of redox homeostasis that accompanies the utilization of nitrogen oxides and detoxification of nitric oxide by cytochromes of the electron transport chain increases fitness of many Gram-positive and -negative pathogens during their associations with invertebrate and vertebrate hosts.

Examination of the Staphylococcus aureus nitric oxide reductase (saNOR) reveals its contribution to modulating intracellular NO levels and cellular respiration

Staphylococcus aureus nitrosative stress resistance is due in part to flavohemoprotein (Hmp). Although hmp is present in all sequenced S. aureus genomes, 37% of analyzed strains also contain nor, encoding a predicted quinol-type nitric oxide (NO) reductase (saNOR). DAF-FM staining of NO-challenged wild-type, nor, hmp and nor hmp mutant biofilms suggested that Hmp may have a greater contribution to intracellular NO detoxification relative to saNOR. However, saNOR still had a significant impact on intracellular NO levels and complemented NO detoxification in a nor hmp mutant. When grown as NO-challenged static (low-oxygen) cultures, hmp and nor hmp mutants both experienced a delay in growth initiation, whereas the nor mutant's ability to initiate growth was comparable with the wild-type strain. However, saNOR contributed to cell respiration in this assay once growth had resumed, as determined by membrane potential and respiratory activity assays. Expression of nor was upregulated during low-oxygen growth and dependent on SrrAB, a two-component system that regulates expression of respiration and nitrosative stress resistance genes. High-level nor promoter activity was also detectable in a cell subpopulation near the biofilm substratum. These results suggest that saNOR contributes to NO-dependent respiration during nitrosative stress, possibly conferring an advantage to nor+ strains in vivo.

Fine-tuned regulation of the dissimilatory nitrite reductase gene by oxygen and nitric oxide in Pseudomonas aeruginosa

Nitrite reductase (NIR) catalyses the reduction of nitrite to nitric oxide (NO) in the denitrification pathway. In Pseudomonas aeruginosa, expression of the gene encoding NIR (nirS) is induced by NO and is under control of the NO-sensing regulator DNR (dissimilatory nitrate respiration regulator). Because DNR is under control of the oxygen-sensing regulator ANR (anaerobic regulator of arginine deiminase and nitrate reductase), nirS is expressed only under low oxygen and anaerobic conditions. Both ANR and DNR are FNR (fumarate and nitrate reductase regulator)-type regulators and recognize the consensus FNR-binding motif. The motif of the nirS promoter is thought to be recognized only by DNR, and not by ANR. Here, mutant strains expressing either ANR or DNR were constructed and used to analyse the role of ANR and DNR in the activation of nirS expression. Analysis of transcriptional activity by microarray and quantitative reverse transcription polymerase chain reaction revealed that nirS is transcribed under low oxygen conditions in an ANR-dependent manner, although the expression level was 10-fold lower than that of the DNR-dependent expression. An artificial promoter containing the FNR-binding motif of the nirS promoter was also twofold upregulated by ANR. These results indicate that low-level expression of NIR in the presence of nitrite may provide NO as a trigger for the full expression of denitrification genes when oxygen is depleted.

Transcriptional regulation of bacterial virulence gene expression by molecular oxygen and nitric oxide

Molecular oxygen (O₂) and nitric oxide (NO) are diatomic gases that play major roles in infection. The host innate immune system generates reactive oxygen species and NO as bacteriocidal agents and both require O₂ for their production. Furthermore, the ability to adapt to changes in O₂ availability is crucial for many bacterial pathogens, as many niches within a host are hypoxic. Pathogenic bacteria have evolved transcriptional regulatory systems that perceive these gases and respond by reprogramming gene expression. Direct sensors possess iron-containing co-factors (iron–sulfur clusters, mononuclear iron, heme) or reactive cysteine thiols that react with O₂ and/or NO. Indirect sensors perceive the physiological effects of O₂ starvation. Thus, O₂ and NO act as environmental cues that trigger the coordinated expression of virulence genes and metabolic adaptations necessary for survival within a host. Here, the mechanisms of signal perception by key O₂- and NO-responsive bacterial transcription factors and the effects on virulence gene expression are reviewed, followed by consideration of these aspects of gene regulation in two major pathogens, Staphylococcus aureus and Mycobacterium tuberculosis.

Denitrification by cystic fibrosis pathogens - Stenotrophomonas maltophilia is dormant in sputum

OBJECTIVE

Chronic Pseudomonas aeruginosa lung infection is the most severe complication for cystic fibrosis (CF) patients. Infected endobronchial mucus of CF patients contains anaerobic zones mainly due to the respiratory burst of polymorphonuclear leukocytes. We have recently demonstrated ongoing denitrification in sputum from patients infected with P. aeruginosa. Therefore we aimed to investigate, whether the pathogenicity of several known CF pathogens is correlated to their ability to perform denitrification.

METHODS

We measured denitrification with N(2)O microsensors in concert with anaerobic growth measurements by absorbance changes and colony counting in isolates from 32 CF patients chronically infected with the highly pathogenic bacteria P. aeruginosa, Achromobacter xylosoxidans, Burkholderia multivorans or the less pathogenic bacterium Stenotrophomonas maltophilia. Consumption of NO(3)(-) and NO(2)(-) was estimated by the Griess Assay. All isolates were assayed during 2 days of incubation in anaerobic LB broth with NO(3)(-) or NO(2)(-). PNA FISH staining of 16S rRNA was used to estimate the amount of ribosomes per bacterial cells and thereby the in situ growth rate of S. maltophilia in sputum.

RESULTS

Supplemental NO(3)(-) caused increased production of N(2)O by P. aeruginosa, A. xylosoxidans and B. multivorans and increased growth for all pathogens. Growth was, however, lowest for S. maltophilia. NO(3)(-) was metabolized by all pathogens, but only P. aeruginosa was able to remove NO(2)(-). S. maltophilia had limited growth in sputum as seen by the weak PNA FISH staining.

CONCLUSIONS

All four pathogens were able to grow anaerobically by NO(3)(-) reduction. Denitrification as demonstrated by N(2)O production was, however, not found in S. maltophilia isolates. The ability to perform denitrification may contribute to the pathogenicity of the infectious isolates since complete denitrification promotes faster anaerobic growth. The inability of S. maltophilia to proliferate by denitrification and therefore grow in the anaerobic CF sputum may explain its low pathogenicity in CF patients.

Nitrous oxide production in sputum from cystic fibrosis patients with chronic Pseudomonas aeruginosa lung infection

Chronic lung infection by Pseudomonas aeruginosa is the major severe complication in cystic fibrosis (CF) patients, where P. aeruginosa persists and grows in biofilms in the endobronchial mucus under hypoxic conditions. Numerous polymorphonuclear leukocytes (PMNs) surround the biofilms and create local anoxia by consuming the majority of O₂ for production of reactive oxygen species (ROS). We hypothesized that P. aeruginosa acquires energy for growth in anaerobic endobronchial mucus by denitrification, which can be demonstrated by production of nitrous oxide (N₂O), an intermediate in the denitrification pathway. We measured N₂O and O₂ with electrochemical microsensors in 8 freshly expectorated sputum samples from 7 CF patients with chronic P. aeruginosa infection. The concentrations of NO₃⁻ and NO₂⁻ in sputum were estimated by the Griess reagent. We found a maximum median concentration of 41.8 µM N₂O (range 1.4–157.9 µM N₂O). The concentration of N₂O in the sputum was higher below the oxygenated layers. In 4 samples the N₂O concentration increased during the initial 6 h of measurements before decreasing for approximately 6 h. Concomitantly, the concentration of NO₃⁻ decreased in sputum during 24 hours of incubation. We demonstrate for the first time production of N₂O in clinical material from infected human airways indicating pathogenic metabolism based on denitrification. Therefore, P. aeruginosa may acquire energy for growth by denitrification in anoxic endobronchial mucus in CF patients. Such ability for anaerobic growth may be a hitherto ignored key aspect of chronic P. aeruginosa infections that can inform new strategies for treatment and prevention.