Staphylococcus aureus SaeR/S-regulated factors reduce human neutrophil reactive oxygen species production

Staphylococcus aureus SaeRS impairs macrophage immune functions through bacterial clumps formation in the early stage of infection

The Staphylococcus aureus (S. aureus) SaeRS two-component system (TCS) regulates over 20 virulence factors. While its impact on chronic infection has been thoroughly discussed, its role in the early stage of infection remains elusive. Since macrophages serve as the primary immune defenders at the onset of infection, this study investigates the influence of SaeRS on macrophage functions and elucidates the underlying mechanisms. Macrophage expression of inflammatory and chemotactic factors, phagocytosis, and bactericidal activity against S. aureus were assessed, along with the evaluation of cellular oxidative stress. SaeRS was found to impair macrophage function. Mechanistically, SaeRS inhibited NF-κB pathway activation via toll-like receptor 2 (TLR2). Its immune-modulating effect could partially be explained by the strengthened biofilm formation. More importantly, we found SaeRS compromised macrophage immune functions at early infection stages even prior to biofilm formation. These early immune evasion effects were dependent on bacterial clumping as cytokine secretion, phagocytosis, and bactericidal activity were repaired when clumping was inhibited. We speculate that the bacterial clumping-mediated antigen mask is responsible for SaeRS-mediated immune evasion at the early infection stage. In vivo, ΔsaeRS infection was cleared earlier, accompanied by early pro-inflammatory cytokines production, and increased tissue oxidative stress. Subsequently, macrophages transitioned to an anti-inflammatory state, thereby promoting tissue repair. In summary, our findings underscore the critical role of the SaeRS TCS in S. aureus pathogenicity, particularly during early infection, which is likely initiated by SaeRS-mediated bacterial clumping.

Assessing the Effects of Surgical Irrigation Solutions on Human Neutrophil Interactions with Nascent Staphylococcus aureus Biofilms

Staphylococcus aureus (S. aureus) is the leading cause of surgical site infections (SSIs) and is capable of biofilm growth on implanted foreign devices. The use of surgical irrigation solutions has become a common strategy to combat bacterial contamination events that occur during surgery. Despite their antimicrobial activity, SSI rates remain consistent, suggesting that low-level contamination persists. In these cases, circulating neutrophils must traffic from the blood to contamination sites to aid in bacterial clearance. The influence of irrigation solutions on neutrophils' ability to engage with bacteria has not been explored. The effects of three commonly used irrigation solutions: Xperience (sodium lauryl sulfate), Irrisept (chlorhexidine gluconate), and Betadine® (povidone-iodine) on nascent S. aureus biofilms alone and in the presence of human neutrophils were assessed at manufactured and diluted concentrations. All three solutions, at a 10% dilution, inhibited bacterial growth as demonstrated by culture assays and confocal video microscopy of bacterial aggregate formation. The effects of 10% dilutions of each of these solutions on neutrophil membrane integrity (by flow cytometry and propidium iodide staining) and motility (by confocal video microscopy of neutrophil track length) were investigated with differing outcomes for each irrigation solution. At this concentration only Irrisept preserved neutrophil membrane integrity and motility. Together, this study examines an overlooked aspect of surgical irrigation solutions by investigating their impact on innate immunity and highlights the feasibility of formulations wherein solution effectiveness is complemented by neutrophil function to reduce risks of infection.

Diverse bacteria elicit distinct neutrophil responses in a physiologically relevant model of infection

An efficient neutrophil response is critical for fighting bacterial infections, which remain a significant global health concern; therefore, modulating neutrophil function could be an effective therapeutic approach. While we have a general understanding of how neutrophils respond to bacteria, how neutrophil function differs in response to diverse bacterial infections remains unclear. Here, we use a microfluidic infection-on-a-chip device to investigate the neutrophil response to four bacterial species: Pseudomonas aeruginosa, Salmonella enterica, Listeria monocytogenes, and Staphylococcus aureus. We find enhanced neutrophil extravasation to L. monocytogenes, a limited overall response to S. aureus, and identify IL-6 as universally important for neutrophil extravasation. Furthermore, we demonstrate a higher percentage of neutrophils generate reactive oxygen species (ROS) when combating gram-negative bacteria versus gram-positive bacteria. For all bacterial species, we found the percentage of neutrophils producing ROS increased following extravasation through an endothelium, underscoring the importance of studying neutrophil function in physiologically relevant models.

Flavone inhibits Staphylococcus aureus virulence via inhibiting the sae two component system

The rising prevalence of antibiotic resistance in Staphylococcus aureus calls for the development of innovative antimicrobial agents targeting novel pathways. S. aureus generates various virulence factors that compromise host defense mechanisms. Flavone, a core structure of flavonoids, has been shown to diminish the production of staphyloxanthin and alpha-hemolysin. Nonetheless, the influence of flavone on the majority of other virulence factors in S. aureus and its underlying molecular mechanism remain elusive. In this study, we examined the impact of flavone on the transcriptional profile of S. aureus using transcriptome sequencing. Our findings revealed that flavone substantially downregulated the expression of over 30 virulence factors implicated in immune evasion by the pathogen. Gene set enrichment analysis of the fold change-ranked gene list in relation to the Sae regulon indicated a robust association between flavone-induced downregulation and membership in the Sae regulon. Through the analysis of Sae target promoter-gfp fusion expression patterns, we observed a dose-dependent inhibition of Sae target promoter activity by flavone. Moreover, we discovered that flavone protected human neutrophils from S. aureus-mediated killing. Flavone also decreased the expression of alpha-hemolysin and other hemolytic toxins, resulting in a reduction in S. aureus' hemolytic capacity. Additionally, our data suggested that the inhibitory effect of flavone on the Sae system operates independently of its capacity to lower staphyloxanthin levels. In conclusion, our study proposes that flavone exhibits a broad inhibitory action on multiple virulence factors of S. aureus by targeting the Sae system, consequently diminishing the bacterium's pathogenicity.

Inside the phagosome: A bacterial perspective

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

Network analysis for identifying potential anti-virulence targets from whole transcriptome of Pseudomonas aeruginosa and Staphylococcus aureus exposed to certain anti-pathogenic polyherbal formulations

Introduction

Antimicrobial resistance (AMR) is a serious global threat. Identification of novel antibacterial targets is urgently warranted to help antimicrobial drug discovery programs. This study attempted identification of potential targets in two important pathogens Pseudomonas aeruginosa and Staphylococcus aureus.

Methods

Transcriptomes of P. aeruginosa and S. aureus exposed to two different quorum-modulatory polyherbal formulations were subjected to network analysis to identify the most highly networked differentially expressed genes (hubs) as potential anti-virulence targets.

Results

Genes associated with denitrification and sulfur metabolism emerged as the most important targets in P. aeruginosa. Increased buildup of nitrite (NO₂) in P. aeruginosa culture exposed to the polyherbal formulation Panchvalkal was confirmed through in vitro assay too. Generation of nitrosative stress and inducing sulfur starvation seemed to be effective anti-pathogenic strategies against this notorious gram-negative pathogen. Important targets identified in S. aureus were the transcriptional regulator sarA, immunoglobulin-binding protein Sbi, serine protease SplA, the saeR/S response regulator system, and gamma-hemolysin components hlgB and hlgC.

Conclusion

Further validation of the potential targets identified in this study is warranted through appropriate in vitro and in vivo assays in model hosts. Such validated targets can prove vital to many antibacterial drug discovery programs globally.

Network analysis for identifying potential anti-virulence targets from whole transcriptome of Pseudomonas aeruginosa and Staphylococcus aureus exposed to certain anti-pathogenic polyherbal formulations

Introduction

Antimicrobial resistance (AMR) is a serious global threat. Identification of novel antibacterial targets is urgently warranted to help antimicrobial drug discovery programs. This study attempted identification of potential targets in two important pathogens Pseudomonas aeruginosa and Staphylococcus aureus.

Methods

Transcriptomes of P. aeruginosa and S. aureus exposed to two different quorum-modulatory polyherbal formulations were subjected to network analysis to identify the most highly networked differentially expressed genes (hubs) as potential anti-virulence targets.

Results

Genes associated with denitrification and sulfur metabolism emerged as the most important targets in P. aeruginosa. Increased buildup of nitrite (NO2) in P. aeruginosa culture exposed to the polyherbal formulation Panchvalkal was confirmed through in vitro assay too. Generation of nitrosative stress and inducing sulfur starvation seemed to be effective anti-pathogenic strategies against this notorious gram-negative pathogen. Important targets identified in S. aureus were the transcriptional regulator sarA, immunoglobulin-binding protein Sbi, serine protease SplA, the saeR/S response regulator system, and gamma-hemolysin components hlgB and hlgC.

Conclusion

Further validation of the potential targets identified in this study is warranted through appropriate in vitro and in vivo assays in model hosts. Such validated targets can prove vital to many antibacterial drug discovery programs globally.

CF-Seq, an accessible web application for rapid re-analysis of cystic fibrosis pathogen RNA sequencing studies

Researchers studying cystic fibrosis (CF) pathogens have produced numerous RNA-seq datasets which are available in the gene expression omnibus (GEO). Although these studies are publicly available, substantial computational expertise and manual effort are required to compare similar studies, visualize gene expression patterns within studies, and use published data to generate new experimental hypotheses. Furthermore, it is difficult to filter available studies by domain-relevant attributes such as strain, treatment, or media, or for a researcher to assess how a specific gene responds to various experimental conditions across studies. To reduce these barriers to data re-analysis, we have developed an R Shiny application called CF-Seq, which works with a compendium of 128 studies and 1,322 individual samples from 13 clinically relevant CF pathogens. The application allows users to filter studies by experimental factors and to view complex differential gene expression analyses at the click of a button. Here we present a series of use cases that demonstrate the application is a useful and efficient tool for new hypothesis generation. (CF-Seq: http://scangeo.dartmouth.edu/CFSeq/ ).

Regulation of neutrophil myeloperoxidase inhibitor SPIN by the small RNA Teg49 in Staphylococcus aureus

Teg49 is a Staphylococcus aureus trans-acting regulatory sRNA derived from cleavage of the sarA P3 transcript. We showed by RNA-Seq here that the 5' trident-like structure in Teg49 regulates transcriptionally (direct and indirect) 22 genes distinct from sarA. Among these, Teg49 was noted to repress spn, encoding a 102 residue preprotein which yields the mature 73 residue peptide which inhibits the catalytic activity of myeloperoxidase in human neutrophils. Teg49 was found to regulate spn mRNA post-transcriptionally in strain SH1000 through 9-nt base-pairing between hairpin loop 2 of Teg49 and an exposed bulge of the spn mRNA. Mutations of the Teg49 binding site disrupted the repression of spn, leading to reduced degradation, and increased half-life of spn mRNA in the Teg49 mutant. The spn-Teg49 interaction was also confirmed with a synonymous spn mutation to yield enhanced spn expression in the mutant vs. the parent. The Teg49 mutant with increased spn expression exhibited enhanced resistance to MPO activity in vitro. Killing assays with human neutrophils showed that the Teg49 mutant was more resistant to killing after phagocytosis. Altogether, this study shows that Teg49 in S. aureus has a distinct and important regulatory profile whereby this sRNA modulates resistance to myeloperoxidase-mediated killing by human neutrophils.

Interspecies Regulation Between Staphylococcus caprae and Staphylococcus aureus Colonized on Healed Skin After Injury

Staphylococcus spp. colonize commensally on the human skin. Some commensal coagulase-negative staphylococci and Staphylococcus aureus are also involved in nosocomial infections. Bacteria were collected from skin healed from pressure injury (PI). After the collection time points, some patients suffered from recurrent PI (RPI). This study analyzed the characteristics of Staphylococcus spp. on healed skin before recurrence between healed skin that suffered from RPI within 6 weeks (RPI group) and healed skin that did not suffer within the duration (non-RPI group) by Staphylococcus spp.-specific sequencing. Of the seven patients in the RPI group, two were dominated by S. aureus and four by Staphylococcus caprae, coagulase-negative human commensal staphylococci in the RPI group. Using mouse models, both S. caprae and S. aureus, but not Staphylococcus epidermidis, colonized on skin healed from injury at significantly higher rates than normal skin. Although subcutaneous injection of S. caprae did not induce lesion formation, the bacterium exhibited high hemolytic activity on human red blood cells. Lesion formation by subcutaneous injection of S. aureus was significantly suppressed in the presence of S. caprae. The hemolytic activity of rabbit blood cells of S. aureus was suppressed by S. caprae, whereas the hemolytic activity of S. caprae was dramatically suppressed by S. aureus. Data indicated that each of the two Staphylococcus spp. suppresses the pathogenicity of the other and that the imbalance between the two is associated with RPI.

Deficiency of GPI Glycan Modification by Ethanolamine Phosphate Results in Increased Adhesion and Immune Resistance of Aspergillus fumigatus

Glycosylphosphatidylinositol (GPI)-anchored proteins play important roles in maintaining the function of the cell wall and participating in pathogenic processes. The addition and removal of phosphoethanolamine (EtN-P) on the second mannose residue in the GPI anchor are vital for maturation and sorting of GPI-anchored proteins. Previously, we have shown that deletion of the gpi7, the gene that encodes an EtN-P transferase responsible for the addition of EtN-P to the second mannose residue of the GPI anchor, leads to the mislocalization of GPI-anchored proteins, abnormal polarity, reduced conidiation, and fast germination in Aspergillus fumigatus. In this report, the adherence and virulence of the A. fumigatus gpi7 deletion mutant were further investigated. The germinating conidia of the mutant exhibited an increased adhesion and a higher exposure of cell wall polysaccharides. Although the virulence was not affected, an increased adherence and a stronger inflammation response of the mutant were documented in an immunocompromised mouse model. An in vitro assay confirmed that the Δgpi7 mutant induced a stronger immune response and was more resistant to killing. Our findings, for the first time, demonstrate that in A. fumigatus, GPI anchoring is required for proper organization of the conidial cell wall. The lack of Gpi7 leads to fast germination, stronger immune response, and resistance to macrophage killing.

The LiaFSR transcriptome reveals an interconnected regulatory network in group A Streptococcus

The mechanisms by which bacteria sense the host environment and alter gene expression are poorly understood. LiaFSR is a gene regulatory system unique to Gram-positive bacteria including group A Streptococcus (GAS) and responds to cell envelope stress. We previously showed that LiaF acts as an inhibitor to LiaFSR activation in GAS. To better understand gene regulation associated with LiaFSR activation, we performed RNA-sequencing on isogenic deletion mutants fixed in a LiaFSR "always on" (ΔliaF) or "always off" (ΔliaR) state. Transcriptome analyses of ΔliaF and ΔliaR in GAS showed near perfect inverse correlation including the gene encoding the global transcriptional regulator SpxA2. In addition, mutant transcriptomes included genes encoding multiple virulence factors and showed substantial overlap with the CovRS regulon. Chromatin immunoprecipitation quantitative PCR demonstrated direct spxA2 gene regulation following activation of the response regulator, LiaR. High SpxA2 levels as a result of LiaFSR activation were directly correlated with increased CovR-regulated virulence gene transcription. Further, consistent with known virulence gene repression by phosphorylated CovR, elevated SpxA2 levels were inversely correlated with CovR phosphorylation. Despite increased transcription of several virulence factors, ΔliaF (high SpxA2) exhibited a paradoxical virulence phenotype in both in vivo mouse and ex vivo human blood models of disease. Likewise, despite decreased virulence factor transcription in ΔliaR (low SpxA2), increased virulence was observed in an in vivo mouse model of disease - a phenotype attributable, in part, to known SpxA2-associated speB transcription. Our findings provide evidence of a critical role of LiaFSR in sensing the host environment and suggest a potential mechanism for gene regulatory system crosstalk shared by many Gram-positive pathogens.

Cystic Fibrosis Sputum Impairs the Ability of Neutrophils to Kill Staphylococcus aureus

Cystic fibrosis (CF) airway disease is characterized by chronic microbial infections and infiltration of inflammatory polymorphonuclear (PMN) granulocytes. Staphylococcus aureus (S. aureus) is a major lung pathogen in CF that persists despite the presence of PMNs and has been associated with CF lung function decline. While PMNs represent the main mechanism of the immune system to kill S. aureus, it remains largely unknown why PMNs fail to eliminate S. aureus in CF. The goal of this study was to observe how the CF airway environment affects S. aureus killing by PMNs. PMNs were isolated from the blood of healthy volunteers and CF patients. Clinical isolates of S. aureus were obtained from the airways of CF patients. The results show that PMNs from healthy volunteers were able to kill all CF isolates and laboratory strains of S. aureus tested in vitro. The extent of killing varied among strains. When PMNs were pretreated with supernatants of CF sputum, S. aureus killing was significantly inhibited suggesting that the CF airway environment compromises PMN antibacterial functions. CF blood PMNs were capable of killing S. aureus. Although bacterial killing was inhibited with CF sputum, PMN binding and phagocytosis of S. aureus was not diminished. The S. aureus-induced respiratory burst and neutrophil extracellular trap release from PMNs also remained uninhibited by CF sputum. In summary, our data demonstrate that the CF airway environment limits killing of S. aureus by PMNs and provides a new in vitro experimental model to study this phenomenon and its mechanism.

The Accessory Gene saeP of the SaeR/S Two-Component Gene Regulatory System Impacts Staphylococcus aureus Virulence During Neutrophil Interaction

Staphylococcus aureus (S. aureus) causes a range of diseases ranging from superficial skin and soft-tissue infections to invasive and life-threatening conditions (Klevens et al., 2007; Kobayashi et al., 2015). S. aureus utilizes the Sae sensory system to adapt to neutrophil challenge. Although the roles of the SaeR response regulator and its cognate sensor kinase SaeS have been demonstrated to be critical for surviving neutrophil interaction and for causing infection, the roles for the accessory proteins SaeP and SaeQ remain incompletely defined. To characterize the functional role of these proteins during innate immune interaction, we generated isogenic deletion mutants lacking these accessory genes in USA300 (USA300ΔsaeP and USA300ΔsaeQ). S. aureus survival was increased following phagocytosis of USA300ΔsaeP compared to USA300 by neutrophils. Additionally, secreted extracellular proteins produced by USA300ΔsaeP cells caused significantly more plasma membrane damage to human neutrophils than extracellular proteins produced by USA300 cells. Deletion of saeQ resulted in a similar phenotype, but effects did not reach significance during neutrophil interaction. The enhanced cytotoxicity of USA300ΔsaeP cells toward human neutrophils correlated with an increased expression of bi-component leukocidins known to target these immune cells. A saeP and saeQ double mutant (USA300ΔsaePQ) showed a significant increase in survival following neutrophil phagocytosis that was comparable to the USA300ΔsaeP single mutant and increased the virulence of USA300 during murine bacteremia. These data provide evidence that SaeP modulates the Sae-mediated response of S. aureus against human neutrophils and suggest that saeP and saeQ together impact pathogenesis in vivo.

Staphylococcus aureus SaeR/S-Regulated Factors Decrease Monocyte-Derived Tumor Necrosis Factor-α to Reduce Neutrophil Bactericidal Activity

Background

The ability of Staphylococcus aureus to evade killing by human neutrophils significantly contributes to disease progression. In this study, we characterize an influential role for the S. aureus SaeR/S 2-component gene regulatory system in suppressing monocyte production of tumor necrosis factor alpha (TNF-α) to subsequently influence human neutrophil priming.

Methods

Using flow cytometry and TNF-α specific enzyme-linked immunosorbent assays we identify the primary cellular source of TNF-α in human blood and in purified peripheral blood mononuclear cells (PBMCs) during interaction with USA300 and an isogenic saeR/S deletion mutant (USA300∆saeR/S). Assays with conditioned media from USA300 and USA300∆saeR/S exposed PBMCs were used to investigate priming on neutrophil bactericidal activity.

Results

TNF-α production from monocytes was significantly reduced following challenge with USA300 compared to USA300∆saeR/S. We observed that priming of neutrophils using conditioned medium from peripheral blood mononuclear cells stimulated with USA300∆saeR/S significantly increased neutrophil bactericidal activity against USA300 relative to unprimed neutrophils and neutrophils primed with USA300 conditioned medium. The increased neutrophil bactericidal activity was associated with enhanced reactive oxygen species production that was significantly influenced by elevated TNF-α concentrations.

Conclusions

Our findings identify an immune evasion strategy used by S. aureus to impede neutrophil priming and subsequent bactericidal activity.

Staphylococcus aureus versus neutrophil: Scrutiny of ancient combat

Staphylococcus aureus (S.aureus) is a Gram-positive bacterium that causes many infections and diseases. This pathogen can cause many types of infections such as impetigo, toxic shock syndrome toxin (TSST1), pneumonia, endocarditis, and autoimmune diseases like lupus erythematosus and can infect other healthy individuals. In the pathogenic process, colonization is a main risk factor for invasive diseases. Various factors including the cell wall-associated factors and receptors of the epithelial cells facilitate adhesion and colonization of this pathogen. S. aureus has many enzymes, toxins, and strategies to evade from the immune system either by an enzyme that lyses cellular component or by hiding from the immune system via surface antigens like protein A and second immunoglobulin-binding protein (Sbi). The strategies of this bacterium can be divided into five groups: A: Inhibit neutrophil recruitment B: Inhibit phagocytosis C: Inhibit killing by ROS, D: Neutrophil killing, and E: Resistance to antimicrobial peptide. On the other hand, innate immune system via neutrophils, the most important polymorphonuclear leukocytes, fights against bacterial cells by neutrophil extracellular trap (NET). In this review, we try to explain the role of each factor in immune evasion.

Immune Evasion by Staphylococcus aureus

Staphylococcus aureus has become a serious threat to human health. In addition to having increased antibiotic resistance, the bacterium is a master at adapting to its host by evading almost every facet of the immune system, the so-called immune evasion proteins. Many of these immune evasion proteins target neutrophils, the most important immune cells in clearing S. aureus infections. The neutrophil attacks pathogens via a plethora of strategies. Therefore, it is no surprise that S. aureus has evolved numerous immune evasion strategies at almost every level imaginable. In this review we discuss step by step the aspects of neutrophil-mediated killing of S. aureus, such as neutrophil activation, migration to the site of infection, bacterial opsonization, phagocytosis, and subsequent neutrophil-mediated killing. After each section we discuss how S. aureus evasion molecules are able to resist the neutrophil attack of these different steps. To date, around 40 immune evasion molecules of S. aureus are known, but its repertoire is still expanding due to the discovery of new evasion proteins and the addition of new functions to already identified evasion proteins. Interestingly, because the different parts of neutrophil attack are redundant, the evasion molecules display redundant functions as well. Knowing how and with which proteins S. aureus is evading the immune system is important in understanding the pathophysiology of this pathogen. This knowledge is crucial for the development of therapeutic approaches that aim to clear staphylococcal infections.

Aspartic Acid Residue 51 of SaeR Is Essential for Staphylococcus aureus Virulence

Staphylococcus aureus is a common Gram-positive bacteria that is a major cause of human morbidity and mortality. The SaeR/S two-component sensory system of S. aureus is important for virulence gene transcription and pathogenesis. However, the influence of SaeR phosphorylation on virulence gene transcription is not clear. To determine the importance of potential SaeR phosphorylation sites for S. aureus virulence, we generated genomic alanine substitutions at conserved aspartic acid residues in the receiver domain of the SaeR response regulator in clinically significant S. aureus pulsed-field gel electrophoresis (PFGE) type USA300. Transcriptional analysis demonstrated a dramatic reduction in the transcript abundance of various toxins, adhesins, and immunomodulatory proteins for SaeR with an aspartic acid to alanine substitution at residue 51. These findings corresponded to a significant decrease in cytotoxicity against human erythrocytes and polymorphonuclear leukocytes, the ability to block human myeloperoxidase activity, and pathogenesis during murine soft-tissue infection. Analysis of SaeR sequences from over 8,000 draft S. aureus genomes revealed that aspartic acid residue 51 is 100% conserved. Collectively, these results demonstrate that aspartic acid residue 51 of SaeR is essential for S. aureus virulence and underscore a conserved target for novel antimicrobial strategies that treat infection caused by this pathogen.

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

AIMS

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

RESULTS

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

Virulence Factor Targeting of the Bacterial Pathogen Staphylococcus aureus for Vaccine and Therapeutics

BACKGROUND

Staphylococcus aureus is a major bacterial pathogen capable of causing a range of infections in humans from gastrointestinal disease, skin and soft tissue infections, to severe outcomes such as sepsis. Staphylococcal infections in humans can be frequent and recurring, with treatments becoming less effective due to the growing persistence of antibiotic resistant S. aureus strains. Due to the prevalence of antibiotic resistance, and the current limitations on antibiotic development, an active and highly promising avenue of research has been to develop strategies to specifically inhibit the activity of virulence factors produced S. aureus as an alternative means to treat disease.

OBJECTIVE

In this review we specifically highlight several major virulence factors produced by S. aureus for which recent advances in antivirulence approaches may hold promise as an alternative means to treating diseases caused by this pathogen. Strategies to inhibit virulence factors can range from small molecule inhibitors, to antibodies, to mutant and toxoid forms of the virulence proteins.

CONCLUSION

The major prevalence of antibiotic resistant strains of S. aureus combined with the lack of new antibiotic discoveries highlight the need for vigorous research into alternative strategies to combat diseases caused by this highly successful pathogen. Current efforts to develop specific antivirulence strategies, vaccine approaches, and alternative therapies for treating severe disease caused by S. aureus have the potential to stem the tide against the limitations that we face in the post-antibiotic era.

Inside job: Staphylococcus aureus host-pathogen interactions

Staphylococcus aureus is a notorious opportunistic pathogen causing a plethora of diseases. Recent research established that once phagocytosed by neutrophils and macrophages, a certain percentage of S. aureus is able to survive within these phagocytes which thereby even may contribute to dissemination of the pathogen. S. aureus further induces its uptake by otherwise non-phagocytic cells and the ensuing intracellular cytotoxicity is suggested to lead to tissue destruction, whereas bacterial persistence within cells is thought to lead to immune evasion and chronicity of infections. We here review recent work on the S. aureus host pathogen interactions with a focus on the intracellular survival of the pathogen.

Immune evasion by a staphylococcal inhibitor of myeloperoxidase

Staphylococcus aureus is highly adapted to its host and has evolved many strategies to resist opsonization and phagocytosis. Even after uptake by neutrophils, S. aureus shows resistance to killing, which suggests the presence of phagosomal immune evasion molecules. With the aid of secretome phage display, we identified a highly conserved protein that specifically binds and inhibits human myeloperoxidase (MPO), a major player in the oxidative defense of neutrophils. We have named this protein "staphylococcal peroxidase inhibitor" (SPIN). To gain insight into inhibition of MPO by SPIN, we solved the cocrystal structure of SPIN bound to a recombinant form of human MPO at 2.4-Å resolution. This structure reveals that SPIN acts as a molecular plug that prevents H2O2 substrate access to the MPO active site. In subsequent experiments, we observed that SPIN expression increases inside the neutrophil phagosome, where MPO is located, compared with outside the neutrophil. Moreover, bacteria with a deleted gene encoding SPIN showed decreased survival compared with WT bacteria after phagocytosis by neutrophils. Taken together, our results demonstrate that S. aureus secretes a unique proteinaceous MPO inhibitor to enhance survival by interfering with MPO-mediated killing.

Epic Immune Battles of History: Neutrophils vs. Staphylococcus aureus

Neutrophils are the most abundant leukocytes in human blood and the first line of defense after bacteria have breached the epithelial barriers. After migration to a site of infection, neutrophils engage and expose invading microorganisms to antimicrobial peptides and proteins, as well as reactive oxygen species, as part of their bactericidal arsenal. Ideally, neutrophils ingest bacteria to prevent damage to surrounding cells and tissues, kill invading microorganisms with antimicrobial mechanisms, undergo programmed cell death to minimize inflammation, and are cleared away by macrophages. Staphylococcus aureus (S. aureus) is a prevalent Gram-positive bacterium that is a common commensal and causes a wide range of diseases from skin infections to endocarditis. Since its discovery, S. aureus has been a formidable neutrophil foe that has challenged the efficacy of this professional assassin. Indeed, proper clearance of S. aureus by neutrophils is essential to positive infection outcome, and S. aureus has developed mechanisms to evade neutrophil killing. Herein, we will review mechanisms used by S. aureus to modulate and evade neutrophil bactericidal mechanisms including priming, activation, chemotaxis, production of reactive oxygen species, and resolution of infection. We will also highlight how S. aureus uses sensory/regulatory systems to tailor production of virulence factors specifically to the triggering signal, e.g., neutrophils and defensins. To conclude, we will provide an overview of therapeutic approaches that may potentially enhance neutrophil antimicrobial functions.

Dectin-1 Compromises Innate Responses and Host Resistance against Neospora caninum Infection

Neospora caninum is an intracellular protozoan parasite that has drawn increasing interest due to its association with worldwide repetitive bovine abortions, which cause billionaire losses to the meat and dairy industries annually. Innate immunity plays an important role in infection control, and N. caninum activates the production of inflammatory mediators through toll-like receptors, NOD-like receptors, and mitogen-activated protein kinase signaling pathways. Advances in the knowledge of initial host–parasite interactions are desirable for the design of control measures against the infection, obliterating its pathogenesis. In that sense, we here aimed to describe the role of the innate C-type lectin receptor Dectin-1 during the infection by N. caninum. With that intent, we observed that the absence of Dectin-1, observed in genetically depleted (Dectin-1^−/−) mice or competitively inhibited by an inert agonist [laminarin (LAM)], rescued 50% of the mice infected with lethal doses of N. caninum. Dectin-1^−/− and LAM-treated mice also presented a reduction in the parasite load during acute and chronic phases, associated with decreased inflammatory scores in the central nervous system. Among all the cell phenotypes that migrated to the initial site of infection, dendritic cells and macrophages gained subpopulations with high Dectin-1 surface expression. The impairment of the receptor in these cells led to a decreased parasite burden, as well as augmented production of IL-12p40. We also found that Dectin-1⁺ cells produced less reactive oxygen species (ROS) at the initial site of the infection, while mice deficient in NADPH oxidase isoform 2 (NOX2^−/−) were not able to control parasite replication and produce IL-12p40, even upon LAM treatment. Interestingly, the absence of functional Dectin-1 did not alter the susceptibility of mice against closely related Toxoplasma gondii. In conclusion, the gathered data suggest that Dectin-1 is involved in the parasite-induced downmodulation of ROS, and other key molecules triggered for the control of N. caninum infection and are a promising target for future development of protocols intended for intervention against neosporosis.

Neutrophils and Immunity: From Bactericidal Action to Being Conquered

The neutrophil is the major phagocyte and the final effector cell of the innate immunity, with a primary role in the clearance of extracellular pathogens. Using the broad array of cytokines, extracellular traps, and effector molecules as the humoral arm, neutrophils play a crucial role in the host defense against pathogen infections. On the other hand, the pathogen has the capacity to overcome neutrophil-mediated host defense to establish infection causing human disease. Pathogens, such as S. aureus, have the potential to thwart neutrophil chemotaxis and phagocytosis and thereby succeed in evading killing by neutrophils. Furthermore, S. aureus surviving within neutrophils promotes neutrophil cytolysis, resulting in the release of host-derived molecules that promote local inflammation. Here, we provide a detailed overview of the mechanisms by which neutrophils kill the extracellular pathogens and how pathogens evade neutrophils degradation. This review will provide insights that might be useful for the development of novel therapies against infections caused by antibiotic resistant pathogens.