Genetic and redox determinants of nitric oxide cytotoxicity in a Salmonella typhimurium model

The sulfur assimilation pathway mitigates redox stress from acidic pH in Salmonella Typhi H58

Salmonella enterica serovar Typhi is the causative agent of typhoid fever, a human-restricted systemic infection. The rapidly disseminating multi-drug resistant H58 subclade is endemic in Africa, Asia, and Oceania, yet little is known regarding its intracellular behavior and virulence properties. It was of interest to understand the H58 response to host cell infection in terms of its response to acid stress and subsequent changes in gene regulation. We compared the H58 response in vitro and during infection of THP-1 human macrophages with the well-characterized response of Salmonella Typhimurium, which causes gastroenteritis. In S. Typhimurium infections, bacteria reside in an acidic intracellular vacuole and become acidified, driving the expression of pathogenicity island 2 genes (SPI-2). SPI-2 virulence factors modify the vacuole and enable bacterial replication. In response to acid stress, the sulfur assimilation pathway is highly upregulated and unique to H58. Replacing the Typhi cysK promoter with the Typhimurium promoter resulted in a cysK gene that was upregulated by acid stress in the H58 background, highlighting the differential regulation between the two serovars. In acidic conditions, H58 experienced much greater redox stress compared with S. Typhimurium, and the sulfur assimilation pathway was required to mitigate the redox stress. Higher redox stress modified the transcriptional regulator SsrB, resulting in diminished secretion of the SPI-2 virulence factor SifA. Our results highlight significant differences between S. Typhi H58 and S. Typhimurium and emphasize the importance of studying S. Typhi strains directly to understand their unique behavior during pathogenesis.

IMPORTANCE

In this study, we examined the clinically relevant, multi-drug resistant Salmonella Typhi strain H58, which is rapidly disseminating across Southeast Asia, Africa, and Oceania. It has heretofore been uncharacterized in terms of its gene regulation. Using human THP-1 macrophages, we discovered that S. Typhi strongly activates the sulfur utilization pathway in response to acid stress encountered in the vacuole once Typhi is inside host cells. Our novel findings were that S. Typhi experiences substantially higher redox stress compared with Typhimurium, and it requires the sulfur utilization pathway to mitigate this stress. This pathway is not upregulated in Typhimurium and represents a divergence in the response of these two serovars. We emphasize that S. Typhimurium is not a reasonable model for understanding H58, a serovar that is seriously impacting human health.

The antimicrobial effects of nitric oxide: A narrative review

Nitric oxide (NO) is a versatile endogenous molecule with multiple physiological roles, including neurotransmission, vasodilation, and immune regulation. As part of the immune response, NO exerts antimicrobial effects by producing reactive nitrogen species (RNS). These RNS combat pathogens via mechanisms such as DNA deamination, S-nitrosylation of thiol groups, and lipid peroxidation, leading to disruptions in microbial cell membranes and vital protein functions. Due to these broad actions, NO targets many pathogens, including bacteria, fungi, and viruses, with minimal risk of resistance development. Given its potent antimicrobial properties, the therapeutic potential of exogenous NO has been recently studied. Various preparations, such as NO donors, inhaled gaseous NO, and topical preparations, have shown promising results in preclinical and clinical settings. This literature review examines the antimicrobial effects of exogenous NO reported in in vitro studies, animal models, and human clinical trials. We provide an overview of the mechanisms by which NO exerts its antimicrobial activity, highlighting its efficacy against diverse pathogens. By presenting the current findings, we aim to contribute to the growing body of evidence supporting the use of NO as a versatile antimicrobial agent in clinical practice.

The human gut microbiome in health and disease: time for a new chapter?

The gut microbiome, composed of the colonic microbiota and their host environment, is important for many aspects of human health. A gut microbiome imbalance (gut dysbiosis) is associated with major causes of human morbidity and mortality. Despite the central part our gut microbiome plays in health and disease, mechanisms that maintain homeostasis and properties that demarcate dysbiosis remain largely undefined. Here we discuss that sorting taxa into meaningful ecological units reveals that the availability of respiratory electron acceptors, such as oxygen, in the host environment has a dominant influence on gut microbiome health. During homeostasis, host functions that limit the diffusion of oxygen into the colonic lumen shelter a microbial community dominated by primary fermenters from atmospheric oxygen. In turn, primary fermenters break down unabsorbed nutrients into fermentation products that support host nutrition. This symbiotic relationship is disrupted when host functions that limit the luminal availability of host-derived electron acceptors become weakened. The resulting changes in the host environment drive alterations in the microbiota composition, which feature an elevated abundance of facultatively anaerobic microbes. Thus, the part of the gut microbiome that becomes imbalanced during dysbiosis is the host environment, whereas changes in the microbiota composition are secondary to this underlying cause. This shift in our understanding of dysbiosis provides a novel starting point for therapeutic strategies to restore microbiome health. Such strategies can either target the microbes through metabolism-based editing or strengthen the host functions that control their environment.

Molecular mechanism of the NOS/NOX regulation of antibacterial activity in Eriocheir sinensis

To elucidate the role of nitric oxide synthase (NOS), which produces the free radical nitric oxide (NO), and nicotinamide adenine dinucleotide phosphate oxidase (NOX), which produces the superoxide anion (O2-), in the innate immunity of Eriocheir sinensis, the full lengths of the NOS and NOX genes were cloned via rapid amplification of the cDNA ends and then expressed in the prokaryotic form to obtain the recombinant proteins, NOS-HIS and NOX-HIS. Through bacterial binding and stimulation experiments, the molecular mechanisms of NOS and NOX in the innate immunity of E. sinensis were explored. Based on the results, NOS and NOX were 5900 bp and 4504 bp long, respectively, and were evolutionarily conserved. Quantitative real-time PCR revealed that NOS and NOX were expressed in all studied tissues, and both were expressed in the highest amounts in hemocytes. NOS-HIS and NOX-HIS could bind to bacteria with different binding powers; their binding ability to gram-positive bacteria was higher than that of binding to gram-negative bacteria. After stimulation with Aeromonas hydrophila, NOS expression was significantly up-regulated at 3, 6, and 48 h, and NOX expression was significantly down-regulated at 3, 12, 24, and 48 h. After bacterial stimulation, the NOS enzyme activity in the serum of E. sinensis was also significantly up-regulated at 6 and 48 h, and the NOX enzyme activity was significantly down-regulated at 12 and 48 h, aligning with the gene expression trend. Moreover, the related free radical molecules, NO, O2-, and H2O2, tended to decrease after bacterial stimulation. Overall, the gene expression and enzyme activity of NOS and NOX had been changed respectively, and the contents of a series of free radical molecules (NO, O2- and H2O2) were induced in E. sinensis after bacterial stimulation, which then exert antibacterial immunity.

Gut dysbiosis: Ecological causes and causative effects on human disease

The gut microbiota plays a role in many human diseases, but high-throughput sequence analysis does not provide a straightforward path for defining healthy microbial communities. Therefore, understanding mechanisms that drive compositional changes during disease (gut dysbiosis) continues to be a central goal in microbiome research. Insights from the microbial pathogenesis field show that an ecological cause for gut dysbiosis is an increased availability of host-derived respiratory electron acceptors, which are dominant drivers of microbial community composition. Similar changes in the host environment also drive gut dysbiosis in several chronic human illnesses, and a better understanding of the underlying mechanisms informs approaches to causatively link compositional changes in the gut microbiota to an exacerbation of symptoms. The emerging picture suggests that homeostasis is maintained by host functions that control the availability of resources governing microbial growth. Defining dysbiosis as a weakening of these host functions directs attention to the underlying cause and identifies potential targets for therapeutic intervention.

The Triple Crown: NO, CO, and H2S in cancer cell biology

Nitric oxide (NO), carbon monoxide (CO), and hydrogen sulfide (H2S) are three endogenously produced gases with important functions in the vasculature, immune defense, and inflammation. It is increasingly apparent that, far from working in isolation, these three exert many effects by modulating each other's activity. Each gas is produced by three enzymes, which have some tissue specificities and can also be non-enzymatically produced by redox reactions of various substrates. Both NO and CO share similar properties, such as activating soluble guanylate cyclase (sGC) to increase cyclic guanosine monophosphate (cGMP) levels. At the same time, H2S both inhibits phosphodiesterase 5A (PDE5A), an enzyme that metabolizes sGC and exerts redox regulation on sGC. The role of NO, CO, and H2S in the setting of cancer has been quite perplexing, as there is evidence for both tumor-promoting and pro-inflammatory effects and anti-tumor and anti-inflammatory activities. Each gasotransmitter has been found to have dual effects on different aspects of cancer biology, including cancer cell proliferation and apoptosis, invasion and metastasis, angiogenesis, and immunomodulation. These seemingly contradictory actions may relate to each gas having a dual effect dependent on its local flux. In this review, we discuss the major roles of NO, CO, and H2S in the context of cancer, with an effort to highlight the dual nature of each gas in different events occurring during cancer progression.

Nitric oxide is a host cue for Salmonella Typhimurium systemic infection in mice

Nitric oxide (NO) is produced as an innate immune response against microbial infections. Salmonella Typhimurium (S. Typhimurium), the major causative pathogen of human gastroenteritis, induces more severe systemic disease in mice. However, host factors contributing to the difference in species-related virulence are unknown. Here, we report that host NO production promotes S. Typhimurium replication in mouse macrophages at the early infection stage by activating Salmonella pathogenicity island-2 (SPI-2). The NO signaling-induced SPI-2 activation is mediated by Fnr and PhoP/Q two-component system. NO significantly induced fnr transcription, while Fnr directly activated phoP/Q transcription. Mouse infection assays revealed a NO-dependent increase in bacterial burden in systemic organs during the initial days of infection, indicating an early contribution of host NO to virulence. This study reveals a host signaling-mediated virulence activation pathway in S. Typhimurium that contributes significantly to its systemic infection in mice, providing further insights into Salmonella pathogenesis and host-pathogen interaction.

Roles and current applications of S-nitrosoglutathione in anti-infective biomaterials

Bacterial infections can compromise the physical and biological functionalities of humans and pose a huge economical and psychological burden on infected patients. Nitric oxide (NO) is a broad-spectrum antimicrobial agent, whose mechanism of action is not affected by bacterial resistance. S-nitrosoglutathione (GSNO), an endogenous donor and carrier of NO, has gained increasing attention because of its potent antibacterial activity and efficient biocompatibility. Significant breakthroughs have been made in the application of GSNO in biomaterials. This review is based on the existing evidence that comprehensively summarizes the progress of antimicrobial GSNO applications focusing on their anti-infective performance, underlying antibacterial mechanisms, and application in anti-infective biomaterials. We provide an accurate overview of the roles and applications of GSNO in antibacterial biomaterials and shed new light on the avenues for future studies.

Addressing a future pandemic: how can non-biological complex drugs prepare us for antimicrobial resistance threats?

Loss of effective antibiotics through antimicrobial resistance (AMR) is one of the greatest threats to human health. By 2050, the annual death rate resulting from AMR infections is predicted to have climbed from 1.27 million per annum in 2019, up to 10 million per annum. It is therefore imperative to preserve the effectiveness of both existing and future antibiotics, such that they continue to save lives. One way to conserve the use of existing antibiotics and build further contingency against resistant strains is to develop alternatives. Non-biological complex drugs (NBCDs) are an emerging class of therapeutics that show multi-mechanistic antimicrobial activity and hold great promise as next generation antimicrobial agents. We critically outline the focal advancements for each key material class, including antimicrobial polymer materials, carbon nanomaterials, and inorganic nanomaterials, and highlight the potential for the development of antimicrobial resistance against each class. Finally, we outline remaining challenges for their clinical translation, including the need for specific regulatory pathways to be established in order to allow for more efficient clinical approval and adoption of these new technologies.

The microbiome and gut homeostasis

Changes in the composition of the gut microbiota are associated with many human diseases. So far, however, we have failed to define homeostasis or dysbiosis by the presence or absence of specific microbial species. The composition and function of the adult gut microbiota is governed by diet and host factors that regulate and direct microbial growth. The host delivers oxygen and nitrate to the lumen of the small intestine, which selects for bacteria that use respiration for energy production. In the colon, by contrast, the host limits the availability of oxygen and nitrate, which results in a bacterial community that specializes in fermentation for growth. Although diet influences microbiota composition, a poor diet weakens host control mechanisms that regulate the microbiota. Hence, quantifying host parameters that control microbial growth could help define homeostasis or dysbiosis and could offer alternative strategies to remediate dysbiosis.

Nitric Oxide Inhibition of Rickettsia rickettsii

Rickettsia rickettsii, the causative agent of Rocky Mountain spotted fever, is an enzootic, obligate, intracellular bacterial pathogen. Nitric oxide (NO) synthesized by the inducible NO synthase (iNOS) is a potent antimicrobial component of innate immunity and has been implicated in the control of virulent Rickettsia spp. in diverse cell types. In this study, we examined the antibacterial role of NO on R. rickettsii. Our results indicate that NO challenge dramatically reduces R. rickettsii adhesion through the disruption of bacterial energetics. Additionally, NO-treated R. rickettsii cells were unable to synthesize protein or replicate in permissive cells. Activated, NO-producing macrophages restricted R. rickettsii infections, but inhibition of iNOS ablated the inhibition of bacterial growth. These data indicate that NO is a potent antirickettsial effector of innate immunity that targets energy generation in these pathogenic bacteria to prevent growth and subversion of infected host cells.

The longitudinal and cross-sectional heterogeneity of the intestinal microbiota

A central goal of microbiome research is to understand the factors that balance gut-associated microbial communities, thereby creating longitudinal and cross-sectional heterogeneity in their composition and density. Whereas the diet dictates taxa dominance, microbial communities are linked intimately to host physiology through digestive and absorptive functions that generate longitudinal heterogeneity in nutrient availability. Additionally, the host differentially controls the access to electron acceptors along the longitudinal axis of the intestine to drive the development of microbial communities that are dominated by facultatively anaerobic bacteria in the small intestine or obligately anaerobic bacteria in the large intestine. By secreting mucus and antimicrobials, the host further constructs microhabitats that generate cross-sectional heterogeneity in the colonic microbiota composition. Here we will review how understanding the host factors involved in generating longitudinal and cross-sectional microbiota heterogeneity helps define physiological states that are characteristic of or appropriate to a homeostatic microbiome.

Nitric oxide for the prevention and treatment of viral, bacterial, protozoal and fungal infections

Although the antimicrobial potential of nitric oxide (NO) is widely published, it is little used clinically. NO is a key signalling molecule modulating vascular, neuronal, inflammatory and immune responses. Endogenous antimicrobial activity is largely mediated by high local NO concentrations produced by cellular inducible nitric oxide synthase, and by derivative reactive nitrogen oxide species including peroxynitrite and S-nitrosothiols. NO may be taken as dietary substrate (inorganic nitrate, L-arginine), and therapeutically as gaseous NO, and transdermal, sublingual, oral, intranasal and intravenous nitrite or nitrate. Numerous preclinical studies have demonstrated that NO has generic static and cidal activities against viruses (including β-coronaviruses such as SARS-CoV-2), bacteria, protozoa and fungi/yeasts  in vitro. Therapeutic effects have been seen in animal models  in vivo, and phase II trials have demonstrated that NO donors can reduce microbial infection. Nevertheless, excess NO, as occurs in septic shock, is associated with increased morbidity and mortality. In view of the dose-dependent positive and negative effects of NO, safety and efficacy trials of NO and its donors are needed for assessing their role in the prevention and treatment of infections. Trials should test dietary inorganic nitrate for pre- or post-exposure prophylaxis and gaseous NO or oral, topical or intravenous nitrite and nitrate for treatment of mild-to-severe infections, including due to SARS-CoV-2 (COVID-19). This review summarises the evidence base from  in vitro, in vivo and early phase clinical studies of NO activity in viral, bacterial, protozoal and fungal infections.

Nitric oxide for the prevention and treatment of viral, bacterial, protozoal and fungal infections

Although the antimicrobial potential of nitric oxide (NO) is widely published, it is little used clinically. NO is a key signalling molecule modulating vascular, neuronal, inflammatory and immune responses. Endogenous antimicrobial activity is largely mediated by high local NO concentrations produced by cellular inducible nitric oxide synthase, and by derivative reactive nitrogen oxide species including peroxynitrite and S-nitrosothiols. NO may be taken as dietary substrate (inorganic nitrate, L-arginine), and therapeutically as gaseous NO, and transdermal, sublingual, oral, intranasal and intravenous nitrite or nitrate. Numerous preclinical studies have demonstrated that NO has generic static and cidal activities against viruses (including β-coronaviruses such as SARS-CoV-2), bacteria, protozoa and fungi/yeasts  in vitro. Therapeutic effects have been seen in animal models  in vivo, and phase II trials have demonstrated that NO donors can reduce microbial infection. Nevertheless, excess NO, as occurs in septic shock, is associated with increased morbidity and mortality. In view of the dose-dependent positive and negative effects of NO, safety and efficacy trials of NO and its donors are needed for assessing their role in the prevention and treatment of infections. Trials should test dietary inorganic nitrate for pre- or post-exposure prophylaxis and gaseous NO or oral, topical or intravenous nitrite and nitrate for treatment of mild-to-severe infections, including due to SARS-CoV-2 (COVID-19). This review summarises the evidence base from  in vitro, in vivo and early phase clinical studies of NO activity in viral, bacterial, protozoal and fungal infections.

Salmonella versus the Microbiome

A balanced gut microbiota contributes to health, but the mechanisms maintaining homeostasis remain elusive. Microbiota assembly during infancy is governed by competition between species and by environmental factors, termed habitat filters, that determine the range of successful traits within the microbial community. These habitat filters include the diet, host-derived resources, and microbiota-derived metabolites, such as short-chain fatty acids. Once the microbiota has matured, competition and habitat filtering prevent engraftment of new microbes, thereby providing protection against opportunistic infections. Competition with endogenous Enterobacterales, habitat filtering by short-chain fatty acids, and a host-derived habitat filter, epithelial hypoxia, also contribute to colonization resistance against Salmonella serovars. However, at a high challenge dose, these frank pathogens can overcome colonization resistance by using their virulence factors to trigger intestinal inflammation. In turn, inflammation increases the luminal availability of host-derived resources, such as oxygen, nitrate, tetrathionate, and lactate, thereby creating a state of abnormal habitat filtering that enables the pathogen to overcome growth inhibition by short-chain fatty acids. Thus, studying the process of ecosystem invasion by Salmonella serovars clarifies that colonization resistance can become weakened by disrupting host-mediated habitat filtering. This insight is relevant for understanding how inflammation triggers dysbiosis linked to noncommunicable diseases, conditions in which endogenous Enterobacterales expand in the fecal microbiota using some of the same growth-limiting resources required by Salmonella serovars for ecosystem invasion. In essence, ecosystem invasion by Salmonella serovars suggests that homeostasis and dysbiosis simply represent states where competition and habitat filtering are normal or abnormal, respectively.

Targeting Evolutionary Conserved Oxidative Stress and Immunometabolic Pathways for the Treatment of Respiratory Infectious Diseases

Significance: Up until recently, metabolism has scarcely been referenced in terms of immunology. However, emerging evidence has shown that immune cells undergo an adaptation of metabolic processes, known as the metabolic switch. This switch is key to the activation, and sustained inflammatory phenotype in immune cells, which includes the production of cytokines and reactive oxygen species (ROS) that underpin infectious diseases, respiratory and cardiovascular disease, neurodegenerative disease, as well as cancer. Recent Advances: There is a burgeoning body of evidence that immunometabolism and redox biology drive infectious diseases. For example, influenza A virus (IAV) utilizes endogenous ROS production via NADPH oxidase (NOX)2-containing NOXs and mitochondria to circumvent antiviral responses. These evolutionary conserved processes are promoted by glycolysis, the pentose phosphate pathway, and the tricarboxylic acid (TCA) cycle that drive inflammation. Such metabolic products involve succinate, which stimulates inflammation through ROS-dependent stabilization of hypoxia-inducible factor-1α, promoting interleukin-1β production by the inflammasome. In addition, itaconate has recently gained significant attention for its role as an anti-inflammatory and antioxidant metabolite of the TCA cycle. Critical Issues: The molecular mechanisms by which immunometabolism and ROS promote viral and bacterial pathology are largely unknown. This review will provide an overview of the current paradigms with an emphasis on the roles of immunometabolism and ROS in the context of IAV infection and secondary complications due to bacterial infection such as Streptococcus pneumoniae. Future Directions: Molecular targets based on metabolic cell processes and ROS generation may provide novel and effective therapeutic strategies for IAV and associated bacterial superinfections.

Nitric oxide disrupts bacterial cytokinesis by poisoning purine metabolism

Cytostasis is the most salient manifestation of the potent antimicrobial activity of nitric oxide (NO), yet the mechanism by which NO disrupts bacterial cell division is unknown. Here, we show that in respiring Escherichia coli, Salmonella, and Bacillus subtilis, NO arrests the first step in division, namely, the GTP-dependent assembly of the bacterial tubulin homolog FtsZ into a cytokinetic ring. FtsZ assembly fails in respiring cells because NO inactivates inosine 5'-monophosphate dehydrogenase in de novo purine nucleotide biosynthesis and quinol oxidases in the electron transport chain, leading to drastic depletion of nucleoside triphosphates, including the GTP needed for the polymerization of FtsZ. Despite inhibiting respiration and dissipating proton motive force, NO does not destroy Z ring formation and only modestly decreases nucleoside triphosphates in glycolytic cells, which obtain much of their ATP by substrate-level phosphorylation and overexpress inosine 5'-monophosphate dehydrogenase. Purine metabolism dictates the susceptibility of early morphogenic steps in cytokinesis to NO toxicity.

Salmonella and Reactive Oxygen Species: A Love-Hate Relationship

Salmonella enterica represents an enterobacterial species including numerous serovars that cause infections at, or initiated at, the intestinal epithelium. Many serovars also act as facultative intracellular pathogens with a tropism for phagocytic cells. These bacteria not only survive in phagocytes but also undergo de facto replication therein. Phagocytes, through the activities of phagocyte NADPH-dependent oxidase and inducible nitric oxide synthase, are very proficient in converting molecular oxygen to reactive oxygen (ROS) and nitrogen species (RNS). These compounds represent highly efficient effectors of the innate immune defense. Salmonella is by no means resistant to these effectors, which may stand in contrast to the host niches chosen. To cope with this paradox, these bacteria rely on an array of detoxification and repair systems. Combination these systems allows for a high enough tolerance to ROS and RNS to enable establishment of infection. In addition, salmonella possesses protein factors that have the potential to dampen the infection-associated inflammation, which evidently results in a reduced exposure to ROS and RNS. This review attempts to summarize the activities and strategies by which salmonella tries to cope with ROS and RNS and how the bacterium can make use of these innate defense factors.

Arsenite suppresses NO production evoked by lipopolysaccharide and poly(I:C) via the suppression of interferon-β expression in RAW264.7 cells

Immunological functions are disturbed in humans who have been chronically exposed to arsenic via contaminated groundwater. Little is known about the specific mechanisms underlying the impairment of immunological defense system caused by arsenic. The activation of macrophage cells upon infection with bacteria and viruses plays important roles in the defense against these pathogens. Here we show that exposure to arsenite (As(III)) suppresses nitric oxide (NO) production in murine RAW264.7 macrophage cells stimulated with lipopolysaccharide (LPS) and poly(I:C), the compounds mimicking bacterial and viral infection, respectively. As(III) suppressed the LPS- or poly(I:C)-evoked induction of inducible NO synthase (iNOS) without affecting the transactivation of NF-κB. As the interferon (IFN)-β/STAT1 pathway is also involved in the induction of iNOS in addition to NF-κB, we examined the effects of As(III) on the expression and secretion of IFN-β, the expression of the components of IFN-α/β receptor, the phosphorylation of STAT1, and the levels of cytokines involved in STAT1 activation. The results showed that the expression and secretion of IFN-β were specifically suppressed by As(III) treatment in RAW264.7 cells stimulated with LPS or poly(I:C). These results suggest that As(III) suppresses the expression and secretion of IFN-β, leading to the reduced STAT1 activation and consequently the reduced iNOS induction in macrophage cells. Our data suggest an important role of the arsenic-induced suppression of IFN-β on the disturbances in immunological defense against both bacteria and viruses.

Zinc-dependent substrate-level phosphorylation powers Salmonella growth under nitrosative stress of the innate host response

The metabolic processes that enable the replication of intracellular Salmonella under nitrosative stress conditions engendered in the innate response of macrophages are poorly understood. A screen of Salmonella transposon mutants identified the ABC-type high-affinity zinc uptake system ZnuABC as a critical determinant of the adaptation of Salmonella to the nitrosative stress generated by the enzymatic activity of inducible nitric oxide (NO) synthase of mononuclear phagocytic cells. NO limits the virulence of a znuB mutant in an acute murine model of salmonellosis. The ZnuABC transporter is crucial for the glycolytic function of fructose bisphosphate aldolase, thereby fueling growth of Salmonella during nitrosative stress produced in the innate response of macrophages. Our investigations demonstrate that glycolysis mediates resistance of Salmonella to the antimicrobial activity of NO produced in an acute model of infection. The ATP synthesized by substrate-level phosphorylation at the payoff phase of glycolysis and acetate fermentation powers the replication of Salmonella experiencing high levels of nitrosative stress. In contrast, despite its high potential for ATP synthesis, oxidative phosphorylation is a major target of inhibition by NO and contributes little to the antinitrosative defenses of intracellular Salmonella. Our investigations have uncovered a previously unsuspected conjunction between zinc homeostasis, glucose metabolism and cellular energetics in the adaptation of intracellular Salmonella to the reactive nitrogen species synthesized in the innate host response.

Microbial pathogens are exposed to multiple antimicrobial defenses during their associations with host cells. Nitric oxide generated in the innate response exerts widespread antimicrobial activity against a variety of pathogenic microorganisms. Nitric oxide has high affinity for metal groups of terminal cytochromes of the respiratory chain, and thus nitrosative stress exerts extreme deleterious actions against the cellular energetics that rely on oxidative phosphorylation. Intracellular Salmonella have resolved this dilemma by satisfying a significant portion of their energetic demands via substrate level phosphorylation in the payoff phase of glycolysis and acetate fermentation. A high affinity zinc uptake system promotes antinitrosative defense of intracellular Salmonella by in great part supporting the enzymatic activity of an essential enzyme in the preparatory phase of glycolysis. Our research provides novel insights into the metabolic and energetic adaptations that allow a bacterial pathogen to thrive in the midst of the innate host response of vertebrate cells.

Iron and innate antimicrobial immunity-Depriving the pathogen, defending the host

The acute-phase response is triggered by the presence of infectious agents and danger signals which indicate hazards for the integrity of the mammalian body. One central feature of this response is the sequestration of iron into storage compartments including macrophages. This limits the availability of this essential nutrient for circulating pathogens, a host defence strategy known as 'nutritional immunity'. Iron metabolism and the immune response are intimately linked. In infections, the availability of iron affects both the efficacy of antimicrobial immune pathways and pathogen proliferation. However, host strategies to withhold iron from microbes vary according to the localization of pathogens: Infections with extracellular bacteria such as Staphylococcus aureus, Streptococcus, Klebsiella or Yersinia stimulate the expression of the iron-regulatory hormone hepcidin which targets the cellular iron-exporter ferroportin-1 causing its internalization and blockade of iron egress from absorptive enterocytes in the duodenum and iron-recycling macrophages. This mechanism disrupts both routes of iron delivery to the circulation, contributes to iron sequestration in the mononuclear phagocyte system and mediates the hypoferraemia of the acute phase response subsequently resulting in the development of anaemia of inflammation. When intracellular microbes are present, other strategies of microbial iron withdrawal are needed. For instance, in macrophages harbouring intracellular pathogens such as Chlamydia, Mycobacterium tuberculosis, Listeria monocytogenes or Salmonella Typhimurium, ferroportin-1-mediated iron export is turned on for the removal of iron from infected cells. This also leads to reduced iron availability for intra-macrophage pathogens which inhibits their growth and in parallel strengthens anti-microbial effector pathways of macrophages including the formation of inducible nitric oxide synthase and tumour necrosis factor. Iron plays a key role in infectious diseases both as modulator of the innate immune response and as nutrient for microbes. We need to gain a more comprehensive understanding of how the body can differentially respond to infection by extra- or intracellular pathogens. This knowledge may allow us to modulate mammalian iron homeostasis pharmaceutically and to target iron-acquisition systems of pathogens, thus enabling us to treat infections with novel strategies that act independent of established antimicrobials.

Salmonella enterica serovar Typhimurium has three transketolase enzymes contributing to the pentose phosphate pathway

The genus Salmonella is responsible for many illnesses in humans and other vertebrate animals. We report here that Salmonella enterica serovar Typhimurium harbors three transketolases that support the non-oxidative branch of the pentose phosphate pathway. BLAST analysis identified two genes, STM14_2885 and STM14_2886, that together encode a putative transketolase (TktC) with 46-47% similarity to the known TktA and TktB isoforms. Assessing the mRNA and protein expression for each of the three transketolases, we determined that all are expressed in WT cells and regulated to varying extents by the alternative sigma factor RpoS. Enzyme assays with lysates from WT and transketolase-knockout strains established that TktA is responsible for >88% of the transketolase activity in WT cells. We purified recombinant forms of each isoenzyme to assess the kinetics for canonical transketolase reactions. TktA and TktB had comparable values for V_max (539-1362 μm NADH consumed/s), K_m (80-739 μm), and catalytic efficiency (1.02 × 10⁸-1.06 × 10⁹ m^-1/s) for each substrate tested. The recombinant form of TktC had lower K_m values (23-120 μm), whereas the V_max (7.8-16 μm NADH consumed/s) and catalytic efficiency (5.58 × 10⁶ to 6.07 × 10⁸ m^-1/s) were 10-100-fold lower. Using a murine model of Salmonella infection, we showed that a strain lacking all three transketolases is avirulent in C57BL/6 mice. These data provide evidence that S Typhimurium possesses three transketolases that contribute to pathogenesis.

Decontamination of Bacillus spores adhered to iron and cement-mortar drinking water infrastructure in a model system using disinfectants

Decontamination of Bacillus spores adhered to common drinking water infrastructure surfaces was evaluated using a variety of disinfectants. Corroded iron and cement-mortar lined iron represented the infrastructure surfaces, and were conditioned in a 23 m long, 15 cm diameter (75 ft long, 6 in diameter) pilot-scale drinking water distribution pipe system. Decontamination was evaluated using increased water velocity (flushing) alone at 0.5 m s^-1 (1.7 ft s^-1), as well as free chlorine (5 and 25 mg L^-1), monochloramine (25 mg L^-1), chlorine dioxide (5 and 25 mg L^-1), ozone (2.0 mg L^-1), peracetic acid 25 mg L^-1) and acidified nitrite (0.1 mol L^-1 at pH 2 and 3), all followed by flushing at 0.3 m s^-1 (1 ft s^-1). Flushing alone reduced the adhered spores by 0.5 and 2.0 log₁₀ from iron and cement-mortar, respectively. Log₁₀ reduction on corroded iron pipe wall coupons ranged from 1.0 to 2.9 at respective chlorine dioxide concentrations of 5 and 25 mg L^-1, although spores were undetectable on the iron surface during disinfection at 25 mg L^-1. Acidified nitrite (pH 2, 0.1 mol L^-1) yielded no detectable spores on the iron surface during the flushing phase after disinfection. Chlorine dioxide was the best performing disinfectant with >3.0 log₁₀ removal from cement-mortar at 5 and 25 mg L^-1. The data show that free chlorine, monochloramine, ozone and chlorine dioxide followed by flushing can reduce adhered spores by > 3.0 log₁₀ on cement-mortar.

Elongation factor P: Function and effects on bacterial fitness

The elongation phase of translation is promoted by three universal elongation factors, EF-Tu, EF-Ts, and EF-G in bacteria and their homologs in archaea and eukaryotes. Recent findings demonstrate that the translation of a subset of mRNAs requires a fourth elongation factor, EF-P in bacteria or the homologs factors a/eIF5A in other kingdoms of life. EF-P prevents the ribosome from stalling during the synthesis of proteins containing consecutive Pro residues, such as PPG, PPP, or longer Pro clusters. The efficient and coordinated synthesis of such proteins is required for bacterial growth, motility, virulence, and stress response. EF-P carries a unique post-translational modification, which contributes to its catalytic proficiency. The modification enzymes, which are lacking in higher eukaryotes, provide attractive new targets for the development of new, highly specific antimicrobials.

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.

Disinfection of Bacillus spores with acidified nitrite

Disinfecting water generated from a bioterrorism contamination event will require large amounts of disinfectant since the volume of water flushed from a drinking water distribution system or wash water collected from a contaminated outdoor area can accumulate quickly. Commonly used disinfectants may be unavailable in the necessary amounts, so evaluation of alternative disinfectants is needed. This study focuses on disinfection of Bacillus spores in water using acidified nitrite. The effect of varying pH (2 or 3), temperature (5°C or 24°C), nitrite concentration (0.01 or 0.1M), buffer (Butterfields or Phosphate Buffered Saline, PBS) and Bacillus species (B. globigii and B. anthracis Sterne) was evaluated. B. globigii was more resistant to disinfection under all water quality conditions. Disinfection was more effective for B. globigii and B. anthracis Sterne at 0.1M nitrite, pH 2, and 24°C. Disinfection of B. anthracis Sterne was enhanced in low ionic strength Butterfields buffer compared to PBS.

Mechanisms and targets of the modulatory action of S-nitrosoglutathione (GSNO) on inflammatory cytokines expression

A number of experimental studies has documented that S-nitrosoglutathione (GSNO), the main endogenous low-molecular-weight S-nitrosothiol, can exert modulatory effects on inflammatory processes, thus supporting its potential employment in medicine for the treatment of important disease conditions. At molecular level, GSNO effects have been shown to modulate the activity of a series of transcription factors (notably NF-κB, AP-1, CREB and others) as well as other components of signal transduction chains (e.g. IKK-β, caspase 1, calpain and others), resulting in the modulation of several cytokines and chemokines expression (TNFα, IL-1β, IFN-γ, IL-4, IL-8, RANTES, MCP-1 and others). Results reported to date are however not univocal, and a single main mechanism of action for the observed anti-inflammatory effects of GSNO has not been identified. Conflicting observations can be explained by differences among the various cell types studies as to the relative abundance of enzymes in charge of GSNO metabolism (GSNO reductase, γ-glutamyltransferase, protein disulfide isomerase and others), as well as by variables associated with the individual experimental models employed. Altogether, anti-inflammatory properties of GSNO seem however to prevail, and exploration of the therapeutic potential of GSNO and analogues appears therefore warranted.

The impact of intestinal inflammation on the nutritional environment of the gut microbiota

The intestinal epithelium is a single cell barrier separating a sterile mucosal tissue from a large microbial community dominated by obligate anaerobic bacteria, which inhabit the gut lumen. To maintain mucosal integrity, any breach in the epithelial barrier needs to be met with an inflammatory host response designed to repel microbial intruders from the tissue, protect the mucosal surface and repair injuries to the epithelium. In addition, inflammation induces mechanisms of nutritional immunity, which limit the availability of metals in the intestinal lumen, thereby imposing new selective forces on microbial growth. However, the inflammatory host response also has important side effects. A by-product of producing reactive oxygen and nitrogen species aimed at eradicating microbial intruders is the luminal generation of exogenous electron acceptors. The presence of these electron acceptors creates a new metabolic niche that is filled by facultative anaerobic bacteria. Here we review the changes in microbial nutrient utilization that accompany intestinal inflammation and the consequent changes in the composition of gut-associated microbial communities.

Antibacterial responses of retinal Müller glia: production of antimicrobial peptides, oxidative burst and phagocytosis

BACKGROUND

We have previously shown that, in response to microbial infection, activated Müller glia secrete inflammatory cytokines/chemokines and exhibit antimicrobial properties. The aim of this study is to understand the mechanisms and the key components involved in this response.

METHODS

Immortalized human retinal Müller glia (MIO-M1 cells) were challenged with Staphylococcus (S) aureus, the leading cause of severe intraocular infection followed by RT2 profile PCR array analysis. The expression of human β-defensin 1 (HBD1), 2 (HBD2), 3 (HBD3), hepcidine and cathelicidin LL37 was checked by RT-PCR and quantified by Taqman qPCR. The expression of AMPs was confirmed at protein level by dot-blot analysis. The production of ROS was measured by dicholoro-dihydro-fluorescein diacetate (DCFH-DA) staining by flow cytometry as well as fluorescence microscopy. The level of nitric oxide (NO) was measured by measuring a stable metabolite, nitrite using the Griess reagent. In vitro killing assay was performed by Live/Dead BacLight staining as well as by dilution plating in suspension and adherent conditions following S. aureus infection. Phagocytosis was measured by CFU enumeration following infection.

RESULTS

PCR array data showed that, in comparison to uninfected control cells, bacterial challenge significantly (> two-fold) induced the expression of 26 genes involved in cytokine/chemokine, antimicrobials, Toll-like receptor, apoptotic, and NF-κB signaling. RT-PCR analysis showed time-dependent increased expression of HBD1, HBD2, HBD3, LL-37, and hepcidin mRNA in bacteria-challenged Müller glia. The expression of these antimicrobial molecules was also increased at the protein level in the culture supernatant, as detected by dot-blot analysis. Additionally, the bacteria-stimulated Müller glia were found to produce reactive oxygen (ROS) and reactive nitrogen (RNS) species. In vitro, killing assays revealed that Müller glia exhibited bactericidal activity against S. aureus in both adherent and suspension cultures. Furthermore, our data demonstrated that Müller glia can phagocytize and kill the bacteria in a time-dependent manner.

CONCLUSIONS

These data suggest that retinal Müller glia behave like classical innate immune cells by producing a variety of antimicrobial molecules in response to bacterial challenge, suggesting their pivotal role in retinal innate defense.

Dysbiosis in the inflamed intestine: chance favors the prepared microbe

The bacterial microbiota of the human large bowel is a complex ecosystem consisting of several hundred, mostly anaerobic, species. To maintain colonization of the gut lumen and maximize growth in the presence of nutritional competitors, highly diverse metabolic pathways have evolved, with each microbe utilizing a different "winning strategy" for nutrient acquisition and utilization. Conditions and diseases leading to intestinal inflammation are accompanied by a severe disruption the microbiota composition characterized by an expansion of facultative anaerobic Enterobacteriaceae. Here, we review evidence that the local inflammatory response creates a unique nutritional environment that is conducive to a bloom of bacterial species whose genomes encode the capability of utilizing inflammation-derived nutrients.

Analysis of the bacterial response to Ru(CO)3Cl(Glycinate) (CORM-3) and the inactivated compound identifies the role played by the ruthenium compound and reveals sulfur-containing species as a major target of CORM-3 action

AIMS

Carbon monoxide (CO)-releasing molecules (CO-RMs) are being developed with the ultimate goal of safely utilizing the therapeutic potential of CO clinically. One such application is antimicrobial activity; therefore, we aimed to characterize and compare the effects of the CO-RM, CORM-3, and its inactivated counterpart, where all labile CO has been removed, at the transcriptomic and cellular level.

RESULTS

We found that both compounds are able to penetrate the cell, but the inactive form is not inhibitory to bacterial growth under conditions where CORM-3 is. Transcriptomic analyses revealed that the bacterial response to inactivated CORM-3 (iCORM-3) is much lower than to the active compound and that a wide range of processes appear to be affected by CORM-3 and to a lesser extent iCORM-3, including energy metabolism, membrane transport, motility, and the metabolism of many sulfur-containing species, including cysteine and methionine.

INNOVATION

This work has demonstrated that both CORM-3 and its inactivated counterpart react with cellular functions to yield a complex response at the transcriptomic level. A full understanding of the actions of both compounds is vital to understand the toxic effects of CO-RMs.

CONCLUSION

This work has furthered our understanding of how CORM-3 behaves at the cellular level and identifies the responses that occur when the host is exposed to the Ru compound as well as those that result from the released CO. This is a vital step in laying the groundwork for future development of optimized CO-RMs for eventual use in antimicrobial therapy.

Streptomycin-induced inflammation enhances Escherichia coli gut colonization through nitrate respiration

Treatment with streptomycin enhances the growth of human commensal Escherichia coli isolates in the mouse intestine, suggesting that the resident microbial community (microbiota) can inhibit the growth of invading microbes, a phenomenon known as “colonization resistance.” However, the precise mechanisms by which streptomycin treatment lowers colonization resistance remain obscure. Here we show that streptomycin treatment rendered mice more susceptible to the development of chemically induced colitis, raising the possibility that the antibiotic might lower colonization resistance by changing mucosal immune responses rather than by preventing microbe-microbe interactions. Investigation of the underlying mechanism revealed a mild inflammatory infiltrate in the cecal mucosa of streptomycin-treated mice, which was accompanied by elevated expression of Nos2, the gene that encodes inducible nitric oxide synthase. In turn, this inflammatory response enhanced the luminal growth of E. coli by nitrate respiration in a Nos2-dependent fashion. These data identify low-level intestinal inflammation as one of the factors responsible for the loss of resistance to E. coli colonization after streptomycin treatment.

Our intestine is host to a complex microbial community that confers benefits by educating the immune system and providing niche protection. Perturbation of intestinal communities by streptomycin treatment lowers “colonization resistance” through unknown mechanisms. Here we show that streptomycin increases the inflammatory tone of the intestinal mucosa, thereby making the bowel more susceptible to dextran sulfate sodium treatment and boosting the Nos2-dependent growth of commensal Escherichia coli by nitrate respiration. These data point to the generation of alternative electron acceptors as a by-product of the inflammatory host response as an important factor responsible for lowering resistance to colonization by facultative anaerobic bacteria such as E. coli.

The dynamics of gut-associated microbial communities during inflammation

Our intestine is host to a large microbial community (microbiota) that educates the immune system and confers niche protection. Profiling of the gut-associated microbial community reveals a dominance of obligate anaerobic bacteria in healthy individuals. However, intestinal inflammation is associated with a disturbance of the microbiota-known as dysbiosis-that often includes an increased prevalence of facultative anaerobic bacteria. This group contains potentially harmful bacterial species, the bloom of which can further exacerbate inflammation. Here, we review the mechanisms that generate changes in the microbial community structure during inflammation. One emerging concept is that electron acceptors generated as by-products of the host inflammatory response feed facultative anaerobic bacteria selectively, thereby increasing their prevalence within the community. This new paradigm has broad implications for understanding dysbiosis during gut inflammation and identifies potential targets for intervention strategies.

S-nitrosoglutathione

BACKGROUND

S-Nitrosoglutathione (GSNO) is the S-nitrosated derivative of glutathione and is thought to be a critical mediator of the down stream signaling effects of nitric oxide (NO). GSNO has also been implicated as a contributor to various disease states.

SCOPE OF REVIEW

This review focuses on the chemical nature of GSNO, its biological activities, the evidence that it is an endogenous mediator of NO action, and implications for therapeutic use.

MAJOR CONCLUSIONS

GSNO clearly exerts its cellular actions through both NO- and S-nitrosation-dependent mechanisms; however, the chemical and biological aspects of this compound should be placed in the context of S-nitrosation as a whole.

GENERAL SIGNIFICANCE

GSNO is a central intermediate in formation and degradation of cellular S-nitrosothiols with potential therapeutic applications; thus, it remains an important molecule of study. This article is part of a Special Issue entitled Cellular functions of glutathione.

Nitrosothiols in bacterial pathogens and pathogenesis

SIGNIFICANCE

The formation and degradation of S-nitrosothiols (SNOs) are important mechanisms of post-translational protein modification and appear to be ubiquitous in biology. These processes play well-characterized roles in eukaryotic cells, including a variety of pathologies and in relation to chronic conditions. We know little of the roles of these processes in pathogenic and other bacteria.

RECENT ADVANCES

It is clear, mostly from growth and transcriptional studies, that bacteria sense and respond to exogenous SNOs. These responses are phenotypically and mechanistically distinct from the responses of bacteria to nitric oxide (NO) and NO-releasing agents, as well as peroxynitrite. Small SNOs, such as S-nitrosoglutathione (GSNO), are accumulated by bacteria with the result that intracellular S-nitrosoproteins (the 'S-nitrosoproteome') are detectable. Recently, conditions for endogenous SNO formation in enterobacteria have been described.

CRITICAL ISSUES

The propensity of intracellular proteins to form SNOs is presumably constrained by the same rules of selectivity that have been discovered in eukaryotic systems, but is also influenced by uniquely bacterial NO detoxification systems, exemplified by the flavohemoglobin Hmp in enterobacteria and NO reductase of meningococci. Furthermore, the bacterial expression of such proteins impacts upon the formation of SNOs in mammalian hosts.

FUTURE DIRECTIONS

The impairment during bacterial infections of specific SNO events in the mammalian host is of considerable interest in the context of proteins involved in innate immunity and intracellular signalling. In bacteria, numerous mechanisms of S-nitrosothiol degradation have been reported (e.g., GSNO reductase); others are thought to operate, based on consideration of their mammalian counterparts. The nitrosothiols of bacteria and particularly of pathogens warrant more intensive investigation.

Low-molecular-weight thiol-dependent antioxidant and antinitrosative defences in Salmonella pathogenesis

We found herein that the intracytoplasmic pool of the low-molecular-weight (LMW) thiol glutathione (GSH) is readily oxidized in Salmonella exposed to nitric oxide (NO). The hypersusceptibility of gshA and gshB mutants lacking γ-glutamylcysteine and glutathione synthetases to NO and S-nitrosoglutathione indicates that GSH antagonizes the bacteriostatic activity of reactive nitrogen species. Metabolites of the GSH biosynthetic pathway do not affect the enzymatic activity of classical NO targets such as quinol oxidases. In contrast, LMW thiols diminish the nitrosative stress experienced by enzymes, such as glutamine oxoglutarate amidotransferase, that contain redox active cysteines. LMW thiols also preserve the transcription of Salmonella pathogenicity island 2 gene targets from the inhibitory activity of nitrogen oxides. These findings are consistent with the idea that GSH scavenges reactive nitrogen species (RNS) other than NO. Compared with the adaptive response afforded by inducible systems such as the hmp-encoded flavohaemoprotein, gshA, encoding the first step of GSH biosynthesis, is constitutively expressed in Salmonella. An acute model of salmonellosis has revealed that the antioxidant and antinitrosative properties associated with the GSH biosynthetic pathway represent a first line of Salmonella resistance against reactive oxygen and nitrogen species engendered in the context of a functional NRAMP1(R) divalent metal transporter.

S-nitrosated α-1-acid glycoprotein kills drug-resistant bacteria and aids survival in sepsis

Treating infections with exogenous NO, which shows broad-spectrum antimicrobial activity, appears to be effective. Similar to NO biosynthesis, biosynthesis of α-1-acid glycoprotein variant A (AGPa), with a reduced cysteine (Cys149), increases markedly during inflammation and infection. We hypothesized that AGPa is an S-nitrosation target in acute-phase proteins. This study aimed to determine whether S-nitrosated AGPa (SNO-AGPa) may be the first compound of this novel antibacterial class against multidrug-resistant bacteria. AGPa was incubated with RAW264.7 cells activated by lipopolysaccharide and interferon-γ. The antimicrobial effects of SNO-AGPa were determined by measuring the turbidity of the bacterial suspensions in vitro and survival in a murine sepsis model in vivo, respectively. Results indicated that endogenous NO generated by activated RAW264.7 cells caused S-nitrosation of AGPa at Cys149. SNO-AGPa strongly inhibited growth of gram-positive, gram-negative, and multidrug-resistant bacteria and was an extremely potent bacteriostatic compound (IC(50): 10(-9) to 10(-6) M). The antibacterial mechanism of SNO-AGPa involves S-transnitrosation from SNO-AGPa to bacterial cells. Treatment with SNO-AGPa, but not with AGPa, markedly reduced bacterial counts in blood and liver in a mouse sepsis model. The sialyl residues of AGPa seem to suppress the antibacterial activity, since SNO-asialo AGPa was more potent than SNO-AGPa.

Nitric oxide-dependent killing of aerobic, anaerobic and persistent Burkholderia pseudomallei

Burkholderia pseudomallei infections are fastidious to treat with conventional antibiotic therapy, often involving a combination of drugs and long-term regimes. Bacterial genetic determinants contribute to the resistance of B. pseudomallei to many classes of antibiotics. In addition, anaerobiosis and hypoxia in abscesses typical of melioidosis select for persistent populations of B. pseudomallei refractory to a broad spectrum of antibacterials. We tested the susceptibility of B. pseudomallei to the drugs hydroxyurea, spermine NONOate and DETA NONOate that release nitric oxide (NO). Our investigations indicate that B. pseudomallei are killed by NO in a concentration and time-dependent fashion. The cytoxicity of this diatomic radical against B. pseudomallei depends on both the culture medium and growth phase of the bacteria. Rapidly growing, but not stationary phase, B. pseudomallei are readily killed upon exposure to the NO donor spermine NONOate. NO also has excellent antimicrobial activity against anaerobic B. pseudomallei. In addition, persistent bacteria highly resistant to most conventional antibiotics are remarkably susceptible to NO. Sublethal concentrations of NO inhibited the enzymatic activity of [4Fe-4S]-cofactored aconitase of aerobic and anaerobic B. pseudomallei. The strong anti-B. pseudomallei activity of NO described herein merits further studies on the application of NO-based antibiotics for the treatment of melioidosis.

Role of Nitric Oxide in Salmonella Infection

Nitric oxide (NO) derivative of l-arginine is an important signaling molecule that mediates a variety of essential physiological processes including vasodilation neurotransmission, and host cell defense. Many types of cells produce NO e.g., smooth muscle cell, endothelial cell, and leukocytes. Host defense functions are known for many bacterial and parasitic infections. In the present study we estimated the levels of serum NO in cases of salmonellosis and in controls. The nitric oxide was estimated by cadmium reduction method, Griess reaction. We observed that in controls the level of NO was (22 ± 2.06) μmol/l and in cases the level was (137.49 ± 29.84) μmol/l. The level of NO was significantly higher than controls (p < 0.001). The raised level of NO could be accounted for by host response to the infection. The host rapidly expresses iNOS, which in turn produces an excess amount of NO. Its cytotoxic effect is by its reactive nitrogen oxide derivative e.g., peroxynitrite. Apart from this it also has anti apoptotic functions. In future one can do follow up study of typhoid cases by bacterial culture.

The diversity of microbial responses to nitric oxide and agents of nitrosative stress close cousins but not identical twins

Nitric oxide and related nitrogen species (reactive nitrogen species) now occupy a central position in contemporary medicine, physiology, biochemistry, and microbiology. In particular, NO plays important antimicrobial defenses in innate immunity but microbes have evolved intricate NO-sensing and defense mechanisms that are the subjects of a vast literature. Unfortunately, the burgeoning NO literature has not always been accompanied by an understanding of the intricacies and complexities of this radical and other reactive nitrogen species so that there exists confusion and vagueness about which one or more species exert the reported biological effects. The biological chemistry of NO and derived/related molecules is complex, due to multiple species that can be generated from NO in biological milieu and numerous possible reaction targets. Moreover, the fate and disposition of NO is always a function of its biological environment, which can vary significantly even within a single cell. In this review, we consider newer aspects of the literature but, most importantly, consider the underlying chemistry and draw attention to the distinctiveness of NO and its chemical cousins, nitrosonium (NO(+)), nitroxyl (NO(-), HNO), peroxynitrite (ONOO(-)), nitrite (NO(2)(-)), and nitrogen dioxide (NO(2)). All these species are reported to be generated in biological systems from initial formation of NO (from nitrite, NO synthases, or other sources) or its provision in biological experiments (typically from NO gas, S-nitrosothiols, or NO donor compounds). The major targets of NO and nitrosative damage (metal centers, thiols, and others) are reviewed and emphasis is given to newer "-omic" methods of unraveling the complex repercussions of NO and nitrogen oxide assaults. Microbial defense mechanisms, many of which are critical for pathogenicity, include the activities of hemoglobins that enzymically detoxify NO (to nitrate) and NO reductases and repair mechanisms (e.g., those that reverse S-nitrosothiol formation). Microbial resistance to these stresses is generally inducible and many diverse transcriptional regulators are involved-some that are secondary sensors (such as Fnr) and those that are "dedicated" (such as NorR, NsrR, NssR) in that their physiological function appears to be detecting primarily NO and then regulating expression of genes that encode enzymes with NO as a substrate. Although generally harmful, evidence is accumulating that NO may have beneficial effects, as in the case of the squid-Vibrio light-organ symbiosis, where NO serves as a signal, antioxidant, and specificity determinant. Progress in this area will require a thorough understanding not only of the biology but also of the underlying chemical principles.

DksA-dependent resistance of Salmonella enterica serovar Typhimurium against the antimicrobial activity of inducible nitric oxide synthase

In coordination with the ppGpp alarmone, the RNA polymerase regulatory protein DksA controls the stringent response of eubacteria, negatively regulating transcription of translational machinery and directly activating amino acid promoters and de novo amino acid biosynthesis. Given the effects of nitric oxide (NO) on amino acid biosynthetic pathways and the intimate relationship of DksA with amino acid synthesis and transport, we tested whether DksA contributes to the resistance of Salmonella to reactive nitrogen species (RNS). Our studies show that the zinc finger predicted to position DksA in the secondary channel of the RNA polymerase is essential for the resistance of Salmonella enterica serovar Typhimurium to RNS in a murine model of systemic salmonellosis. Despite exhibiting auxotrophies for various amino acids, ΔdksA mutant Salmonella strains regain virulence in mice lacking inducible NO synthase (iNOS). DksA is also important for growth of this intracellular pathogen in the presence of NO congeners generated by iNOS during the innate response of murine macrophages. Accordingly, dksA mutant Salmonella strains are hypersusceptible to chemically generated NO, a phenotype that can be prevented by adding amino acids. The DksA-dependent antinitrosative defenses do not rely on the Hmp flavohemoprotein that detoxifies NO to NO(3)(-) and appear to operate independently of the ppGpp alarmone. Our investigations are consistent with a model by which NO produced in the innate response to Salmonella exerts considerable pressure on amino acid biosynthesis. The cytotoxicity of NO against Salmonella amino acid biosynthetic pathways is antagonized in great part by the DksA-dependent regulation of amino acid biosynthesis and transport.

Loss of elongation factor P disrupts bacterial outer membrane integrity

Elongation factor P (EF-P) is posttranslationally modified at a conserved lysyl residue by the coordinated action of two enzymes, PoxA and YjeK. We have previously established the importance of this modification in Salmonella stress resistance. Here we report that, like poxA and yjeK mutants, Salmonella strains lacking EF-P display increased susceptibility to hypoosmotic conditions, antibiotics, and detergents and enhanced resistance to the compound S-nitrosoglutathione. The susceptibility phenotypes are largely explained by the enhanced membrane permeability of the efp mutant, which exhibits increased uptake of the hydrophobic dye 1-N-phenylnaphthylamine (NPN). Analysis of the membrane proteomes of wild-type and efp mutant Salmonella strains reveals few changes, including the prominent overexpression of a single porin, KdgM, in the efp mutant outer membrane. Removal of KdgM in the efp mutant background ameliorates the detergent, antibiotic, and osmosensitivity phenotypes and restores wild-type permeability to NPN. Our data support a role for EF-P in the translational regulation of a limited number of proteins that, when perturbed, renders the cell susceptible to stress by the adventitious overexpression of an outer membrane porin.

Improved antimicrobial efficacy with nitric oxide releasing nanoparticle generated S-nitrosoglutathione

Nitric oxide (NO) plays a vital role in mammalian host defense through a variety of mechanisms. In particular, NO can oxidize to form reactive nitrogen species or interact with protein thiols and metal centers, blocking essential microbial processes. S-nitrosoglutathione (GSNO), a potent NO donor formed by the interaction of NO with intracellular glutathione (GSH), is a major factor in this pathway and is considered one of the strongest naturally occurring nitrosating agent. We previously described the broad-spectrum antimicrobial activity of a nanoparticulate platform capable of controlled and sustained release of NO (NO-np). Interestingly, in vivo efficacy of the NO-np surpassed in vitro data generated. We hypothesized that the enhanced activity was in part achieved via the interaction between the generated NO and available GSH, forming GSNO. In the current study, we investigated the efficiency of NO-np to form GSNO in the presence of GSH was evaluated, and assessed the antimicrobial activity of the formed GSNO against methicillin resistant Staphylococcus aureus (MRSA), Escherichia coli, Klebsiella pneumoniae, and Pseudomonas aeruginosa. When GSH was combined with NO-np, GSNO was rapidly produced and significant concentrations of GSNO were maintained for >24h. The GSNO generated was more effective compared to NO-np alone against all bacterial strains examined, with P. aeruginosa being the most sensitive and K. pneumoniae the most resistant. We conclude that the combination of NO-np with GSH is an effective means of generating GSNO, and presents a novel approach to potent antimicrobial therapy.

Lipid metabolism modulation by the P2X7 receptor in the immune system and during the course of infection: new insights into the old view

For decades, scientists have described numerous protein pathways and functions. Much of a protein's function depends on its interactions with different partners, and those partners can change depending on the cell type or system. The P2X7 receptor (P2X7R) is one such multifunctional protein that is related to multiple partners and signaling pathways. The relationship between P2X7R and different enzymes involved in lipid metabolism represents a relatively new field in P2X7R research. This field of research began in epithelial cells and currently includes immune and nervous cells. The P2X7R-lipid metabolism pathway is related to many biological functions of P2X7R, such as cell death and pathogen clearance, and this signaling pathway may be involved in many functions that are dependent on bioactive lipids. In the present review, we will attempt to summarize data related to the P2X7R-lipid metabolism pathway, focusing on signaling pathways and their biological relevance to the immune system and infection.

Cellular aspects of immunity to intracellular Salmonella enterica

Salmonella enterica is a frequent gastrointestinal pathogen with ability to cause diseases ranging from local gastrointestinal inflammation and diarrhea to life-threatening typhoid fever. Salmonella is an invasive, facultative intracellular pathogen that infects various cell types of the host and can survive and proliferate in different populations of immune cells. During pathogenesis, Salmonella is confronted with various lines of immune defense. To successfully colonize host organisms, the pathogen deploys a set of sophisticated mechanisms of immune evasion and direct manipulation of immune cell functions. In addition to resistance against innate immune mechanisms, including the ability to avoid killing by macrophages and dendritic cells (DCs), Salmonella interferes with antigen presentation by DCs and the formation of an efficient adaptive immune response. In this review, we describe the current understanding of Salmonella virulence factors during intracellular life and focus on the recent advances in the understanding of interference of intracellular Salmonella with cellular functions of immune cells.