The Staphylococcus aureus superantigen SElX is a bifunctional toxin that inhibits neutrophil function

The RIG-I-like receptor signaling pathway triggered by Staphylococcus aureus promotes breast cancer metastasis

Host microbes are increasingly recognized as key components in various types of cancer, although their exact impact remains unclear. This study investigated the functional significance of Staphylococcus aureus (S. aureus) in breast cancer tumorigenesis and progression. We found that S. aureus invasion resulted in a compromised DNA damage response process, as evidenced by the absence of G1-phase arrest and apoptosis in breast cells in the background of double strand breaks production and the activation of the ataxia-telangiectasia mutated (ATM)-p53 signaling pathway. The high-throughput mRNA sequencing, bioinformatics analysis and pharmacological studies revealed that S. aureus facilitates breast cell metastasis through the innate immune pathway, particularly in cancer cells. During metastasis, S. aureus initially induced the expression of RIG-I-like receptors (RIG-I in normal breast cells, RIG-I and MDA5 in breast cancer cells), which in turn activated NF-κB p65 expression. We further showed that NF-κB p65 activated the CCL5-CCR5 pathway, contributing to breast cell metastasis. Our study provides novel evidence that the innate immune system, triggered by bacterial infection, plays a role in bacterial-driven cancer metastasis.

Novel insights into the immune response to bacterial T cell superantigens

Bacterial T cell superantigens (SAgs) are a family of microbial exotoxins that function to activate large numbers of T cells simultaneously. SAgs activate T cells by direct binding and crosslinking of the lateral regions of MHC class II molecules on antigen-presenting cells with T cell receptors (TCRs) on T cells; these interactions alter the normal TCR-peptide-MHC class II architecture to activate T cells in a manner that is independent of the antigen specificity of the TCR. SAgs have well-recognized, central roles in human diseases such as toxic shock syndrome and scarlet fever through their quantitative effects on the T cell response; in addition, numerous other consequences of SAg-driven T cell activation are now being recognized, including direct roles in the pathogenesis of endocarditis, bloodstream infections, skin disease and pharyngitis. In this Review, we summarize the expanding family of bacterial SAgs and how these toxins can engage highly diverse adaptive immune receptors. We highlight recent findings regarding how SAg-driven manipulation of the adaptive immune response may operate in multiple human diseases, as well as contributing to the biology and life cycle of SAg-producing bacterial pathogens.

History of S. aureus Skin Infection Significantly Associates with History of Eczema Herpeticum in Patients with Atopic Dermatitis

INTRODUCTION

Patients with atopic dermatitis (AD) are uniquely susceptible to a number of serious viral skin complications, including eczema herpeticum (EH), caused by herpes simplex virus. This study explored the associations between biomarkers of epithelial barrier dysfunction, type 2 immunity, Staphylococcus aureus infection, and S. aureus-specific immunoglobulin responses in a cohort of AD subjects with and without a history of EH (EH+ and EH-, respectively).

METHODS

A total of 112 subjects with AD (56 EH+, 56 EH-), matched by age and AD severity, were selected from a registry of over 3000 AD subjects. Logistic regression was used to test the association between history of S. aureus skin infection and history of EH, while controlling for a number of confounders.

RESULTS

Compared to those without a history of S. aureus skin infection, subjects with a history of S. aureus skin infection were found to have more than sixfold increased odds of having a history of EH (6.60, 95% confidence interval [CI]: 2.00-21.83), after adjusting for history of other viral skin infections (molluscum contagiosum virus, human papillomavirus), serum total IgE, and IgG against the S. aureus virulence factor SElX.

CONCLUSIONS

These findings indicate an important relationship between S. aureus skin infections and EH.

Local, systemic and immunologic safety comparison between xenogeneic equine umbilical cord mesenchymal stem cells, allogeneic canine adipose mesenchymal stem cells and placebo: a randomized controlled trial

Mesenchymal stem cells are multipotent cells with a wide range of therapeutic applications, including, among others, tissue regeneration. This work aims to test the safety (EUC-MSC) of intra-articular administration of equine umbilical cord mesenchymal stem cells in young healthy dogs under field conditions following single and repeated administration. This was compared with the safety profile of allogenic canine adipose derived mesenchymal stem cells (CAD-MSC) and placebo in order to define the safety of xenogeneic use of mesenchymal stem cells when administered intra-articular. Twenty-four police working dogs were randomized in three groups in a proportion 1:1:1. EUC-MSCs and CAD-MSCs were obtained from healthy donors and were manufactured following company SOPs and under GMP and GMP-like conditions, respectively, and compliant all necessary controls to ensure the quality of the treatment. The safety of the treatment was evaluated locally, systemically and immunologically. For this purpose, an orthopedic examination and Glasgow test for the assessment of pain in the infiltrated joint, blood tests, clinical examination and analysis of the humoral and cellular response to treatment were performed. No adverse events were detected following single and repeated MSC administration despite both equine and canine MSC generate antibody titres in the dogs. The intra-articular administration of equine umbilical cord mesenchymal stem cells in dogs has demonstrated to be safe.

Staphylococcus aureus host interactions and adaptation

Invasive Staphylococcus aureus infections are common, causing high mortality, compounded by the propensity of the bacterium to develop drug resistance. S. aureus is an excellent case study of the potential for a bacterium to be commensal, colonizing, latent or disease-causing; these states defined by the interplay between S. aureus and host. This interplay is multidimensional and evolving, exemplified by the spread of S. aureus between humans and other animal reservoirs and the lack of success in vaccine development. In this Review, we examine recent advances in understanding the S. aureus-host interactions that lead to infections. We revisit the primary role of neutrophils in controlling infection, summarizing the discovery of new immune evasion molecules and the discovery of new functions ascribed to well-known virulence factors. We explore the intriguing intersection of bacterial and host metabolism, where crosstalk in both directions can influence immune responses and infection outcomes. This Review also assesses the surprising genomic plasticity of S. aureus, its dualism as a multi-mammalian species commensal and opportunistic pathogen and our developing understanding of the roles of other bacteria in shaping S. aureus colonization.

Enterotoxin Gene Cluster and selX Are Associated with Atopic Dermatitis Severity-A Cross-Sectional Molecular Study of Staphylococcus aureus Superantigens

Staphylococcus aureus superantigens (SAgs) have been reported to aggravate atopic dermatitis. However, comprehensive analyses of these molecules in multiple microniches are lacking. The present study involved 50 adult patients with active atopic dermatitis. S. aureus was isolated from the lesional skin, nonlesional skin, and anterior nares. Multiplex-PCR was performed to identify genes encoding (1) selX (core genome); (2) seg, selI, selM, selN, selO, selU (enterotoxin gene cluster, EGC); and (3) sea, seb, sec, sed, see, tstH (classic SAgs encoded on other mobile genetic elements). The results were correlated to clinical parameters of the study group. selx and EGC were the most prevalent in all microniches. The number of SAg-encoding genes correlated between the anterior nares and nonlesional skin, and between the nonlesional and lesional skin. On lesional skin, the total number of SAg genes correlated with disease severity (total and objective SCORAD, intensity, erythema, edema/papulation, lichenification and dryness). Linear regression revealed that AD severity was predicted only by selx and EGC. This study revealed that selX and EGC are associated with atopic dermatitis severity. Anterior nares and nonlesional skin could be reservoirs of SAg-positive S. aureus. Restoring the physiological microbiome could reduce the SAg burden and alleviate syndromes of atopic dermatitis.

Superantigens, a Paradox of the Immune Response

Staphylococcal enterotoxins are a wide family of bacterial exotoxins with the capacity to activate as much as 20% of the host T cells, which is why they were called superantigens. Superantigens (SAgs) can cause multiple diseases in humans and cattle, ranging from mild to life-threatening infections. Almost all S. aureus isolates encode at least one of these toxins, though there is no complete knowledge about how their production is triggered. One of the main problems with the available evidence for these toxins is that most studies have been conducted with a few superantigens; however, the resulting characteristics are attributed to the whole group. Although these toxins share homology and a two-domain structure organization, the similarity ratio varies from 20 to 89% among different SAgs, implying wide heterogeneity. Furthermore, every attempt to structurally classify these proteins has failed to answer differential biological functionalities. Taking these concerns into account, it might not be appropriate to extrapolate all the information that is currently available to every staphylococcal SAg. Here, we aimed to gather the available information about all staphylococcal SAgs, considering their functions and pathogenicity, their ability to interact with the immune system as well as their capacity to be used as immunotherapeutic agents, resembling the two faces of Dr. Jekyll and Mr. Hyde.

Molecular docking and proteomics reveals the synergistic antibacterial mechanism of theaflavin with β-lactam antibiotics against MRSA

Recurrent epidemics of methicillin-resistant Staphylococcus aureus (S. aureus) (MRSA) have illustrated that the effectiveness of antibiotics in clinical application is rapidly fading. A feasible approach is to combine natural products with existing antibiotics to achieve an antibacterial effect. In this molecular docking study, we found that theaflavin (TF) preferentially binds the allosteric site of penicillin-binding protein 2a (PBP2a), inducing the PBP2a active site to open, which is convenient for β-lactam antibiotics to treat MRSA infection, instead of directly exerting antibacterial activity at the active site. Subsequent TMT-labeled proteomics analysis showed that TF treatment did not significantly change the landscape of the S. aureus USA300 proteome. Checkerboard dilution tests and kill curve assays were performed to validate the synergistic effect of TF and ceftiofur, and the fractional inhibitory concentration index (FICI) was 0.1875. The antibacterial effect of TF combined with ceftiofur was better than that of single-drug treatment in vitro. In addition, TF effectively enhanced the activity of ceftiofur in a mouse model of MRSA-induced pneumonia. Our findings provide a potential therapeutic strategy to combine existing antibiotics with natural products to resolve the prevalent infections of multidrug-resistant pathogens.

First Genome-Based Characterisation and Staphylococcal Enterotoxin Production Ability of Methicillin-Susceptible and Methicillin-Resistant Staphylococcus aureus Strains Isolated from Ready-to-Eat Foods in Algiers (Algeria)

Staphylococcus aureus is a pathogenic microorganism of humans and animals, able to cause foodborne intoxication due to the production of staphylococcal enterotoxins (SEs) and to resist antibiotic treatment as in the case of methicillin-resistant S. aureus (MRSA). In this study, we performed a genomic characterisation of 12 genetically diverse S. aureus strains isolated from ready-to-eat foods in Algiers (Algeria). Moreover, their ability to produce some classical and new staphylococcal enterotoxins (SEs) was investigated. The 12 S. aureus strains resulted to belong to nine known sequence types (STs) and to the novel ST7199 and ST7200. Furthermore, S. aureus SA46 was assigned to the European clone MRSA-ST80-SCCmec-IV. The 12 strains showed a wide endowment of se and sel (staphylococcal enterotoxin-like toxin) genes (sea, seb, sed, seg, seh, sei, selj, sek, sem, sen, seo, seq, ser, selu2, selw, selx, sey, sel30; ψent1-ψent2), including variants and pseudogenes, and harboured the enterotoxin gene cluster (egc) types 1 and 5. Additionally, they produced various amounts of SEA (64.54-345.02 ng/mL), SEB (2871.28-14739.17 ng/mL), SED (322.70-398.94 ng/mL), SEH (not detectable-239.48 ng/mL), and SER (36,720.10-63,176.06 ng/mL) depending on their genotypes. The genetic determinants related to their phenotypic resistance to β-lactams (blaZ, mecA), ofloxacin (gyrA-S84L), erythromycin (ermB), lincomycin (lmrS), kanamycin (aph(3')-III, ant(6)-I), and tetracyclin (tet(L), tet(38)) were also detected. A plethora of virulence-related genes, including major virulence genes such as the tst gene, determinant for the toxic shock syndrome toxin-1, and the lukF-PV and lukS-PV genes, encoding the panton-valentine leukocidin (PVL), were present in the S. aureus strains, highlighting their pathogenic potential. Furthermore, a phylogenomic reconstruction including worldwide foodborne S. aureus showed a clear clustering based on ST and geographical origin rather than the source of isolation.

Staphylococcus lugdunensis Uses the Agr Regulatory System to Resist Killing by Host Innate Immune Effectors

Coagulase-negative staphylococci (CoNS) are frequently commensal bacteria that rarely cause disease in mammals. Staphylococcus lugdunensis is an exceptional CoNS that causes disease in humans similar to virulent Staphylococcus aureus, but the factors that enhance the virulence of this bacterium remain ill defined. Here, we used random transposon insertion mutagenesis to identify the agr quorum sensing system as a regulator of hemolysins in S. lugdunensis. Using RNA sequencing (RNA-seq), we revealed that agr regulates dozens of genes, including hemolytic S. lugdunensis synergistic hemolysins (SLUSH) peptides and the protease lugdulysin. A murine bacteremia model was used to show that mice infected systemically with wild-type S. lugdunensis do not show overt signs of disease despite there being high numbers of bacteria in the livers and kidneys of mice. Moreover, proliferation of the agr mutant in these organs was no different from that of the wild-type strain, leaving the role of the SLUSH peptides and the metalloprotease lugdulysin in pathogenesis still unclear. Nonetheless, the tropism of S. lugdunensis for humans led us to investigate the role of virulence factors in other ways. We show that agr-regulated effectors, but not SLUSH or lugdulysin alone, are important for S. lugdunensis survival in whole human blood. Moreover, we demonstrate that Agr contributes to survival of S. lugdunensis during encounters with murine and primary human macrophages. These findings demonstrate that, in S. lugdunensis, Agr regulates expression of virulence factors and is required for resistance to host innate antimicrobial defenses. This study therefore provides insight into strategies that this Staphylococcus species uses to cause disease.

SEC is an antiangiogenic virulence factor that promotes Staphylococcus aureus endocarditis independent of superantigen activity

The superantigen staphylococcal enterotoxin C (SEC) is critical for Staphylococcus aureus infective endocarditis (SAIE) in rabbits. Superantigenicity, its hallmark function, was proposed to be a major underlying mechanism driving SAIE but was not directly tested. With the use of S. aureus MW2 expressing SEC toxoids, we show that superantigenicity does not sufficiently account for vegetation growth, myocardial inflammation, and acute kidney injury in the rabbit model of native valve SAIE. These results highlight the critical contribution of an alternative function of superantigens to SAIE. In support of this, we provide evidence that SEC exerts antiangiogenic effects by inhibiting branching microvessel formation in an ex vivo rabbit aortic ring model and by inhibiting endothelial cell expression of one of the most potent mediators of angiogenesis, VEGF-A. SEC's ability to interfere with tissue revascularization and remodeling after injury serves as a mechanism to promote SAIE and its life-threatening systemic pathologies.

Multiple Domains of Staphylococcal Superantigen-like Protein 11 (SSL11) Contribute to Neutrophil Inhibition

Staphylococcus aureus is an opportunistic pathogen producing many immune evasion molecules targeting various components of the host immune defense, including the Staphylococcal superantigen-like protein (SSL 1-14) family. Despite sharing similar structures with the powerful superantigens (SAgs), which cause massive T cell activation, SSLs interfere with a wide range of innate immune defenses. SSLs are divided into two subgroups, SSLs that contain a conserved carbohydrate Sialyl Lewis X [Neu5Acα2-3Galβ1-4(Fucα1-3) GlcNAcβ, SLeX] binding site and SSLs that lack the SLeX binding site. SSL2-6 and SSL11 possess the SLeX binding site. Our previous studies showed that SSL11 arrests cell motility by inducing cell adhesion in differentiated HL60 (dHL60) cells, while SSL7 did not bind dHL60 cells. SSL7-based chimeras were engineered by exchanging the SSL7 sequence with the corresponding SSL11 sequence and assaying for a gain of SSL11 function, namely, the induction of cell spreading and motility arrest. In addition to the SLeX-binding site, we observed that three beta-strands β6, β7, and β9 and the N-terminal residues, Y16 and Y17, transitioned SSL7 to gain SSL11 activities. These studies define the structure-function properties of SSL11 that may allow SSL11 to inhibit S. aureus clearance by the host innate immune system, allowing S. aureus to maintain a carrier state in humans, an understudied aspect of S. aureus pathogenesis.

Superantigens promote Staphylococcus aureus bloodstream infection by eliciting pathogenic interferon-gamma production

Since their discovery over 30 y ago, it has become clear that the superantigens (SAgs) are important virulence factors produced during severe Staphylococcus aureus–mediated disease including bacteremia. However, until the current study, it was unclear how these toxins manipulated the immune system to promote infection. Here, we have demonstrated that the SAgs can target a critical immune signaling molecule (interferon gamma), inducing overproduction that promotes bacterial survival by subverting the ability of macrophages to be able to kill the pathogen. This highlights SAg activity as a critical target for antistaphylococcal therapy to mitigate the impact of severe S. aureus disease.

Staphylococcus aureus is a foremost bacterial pathogen responsible for a vast array of human diseases. Staphylococcal superantigens (SAgs) constitute a family of exotoxins from S. aureus that bind directly to major histocompatibility complex (MHC) class II and T cell receptors to drive extensive T cell activation and cytokine release. Although these toxins have been implicated in serious disease, including toxic shock syndrome, the specific pathological mechanisms remain unclear. Herein, we aimed to elucidate how SAgs contribute to pathogenesis during bloodstream infections and utilized transgenic mice encoding human MHC class II to render mice susceptible to SAg activity. We demonstrate that SAgs contribute to S. aureus bacteremia by massively increasing bacterial burden in the liver, and this was mediated by CD4⁺ T cells that produced interferon gamma (IFN-γ) to high levels in a SAg-dependent manner. Bacterial burdens were reduced by blocking IFN-γ, phenocopying SAg-deletion mutant strains, and inhibiting a proinflammatory response. Infection kinetics and flow cytometry analyses suggested that this was a macrophage-driven mechanism, which was confirmed through macrophage-depletion experiments. Experiments in human cells demonstrated that excessive IFN-γ allowed S. aureus to replicate efficiently within macrophages. This indicates that SAgs promote bacterial survival by manipulating the immune response to inhibit effective clearing of S. aureus. Altogether, this work implicates SAg toxins as critical therapeutic targets for preventing persistent or severe S. aureus disease.

Pathogenicity and virulence of Staphylococcus aureus

Staphylococcus aureus is one of the most frequent worldwide causes of morbidity and mortality due to an infectious agent. This pathogen can cause a wide variety of diseases, ranging from moderately severe skin infections to fatal pneumonia and sepsis. Treatment of S. aureus infections is complicated by antibiotic resistance and a working vaccine is not available. There has been ongoing and increasing interest in the extraordinarily high number of toxins and other virulence determinants that S. aureus produces and how they impact disease. In this review, we will give an overview of how S. aureus initiates and maintains infection and discuss the main determinants involved. A more in-depth understanding of the function and contribution of S. aureus virulence determinants to S. aureus infection will enable us to develop anti-virulence strategies to counteract the lack of an anti-S. aureus vaccine and the ever-increasing shortage of working antibiotics against this important pathogen.

Superantigen Recognition and Interactions: Functions, Mechanisms and Applications

Superantigens are unconventional antigens which recognise immune receptors outside their usual recognition sites e.g. complementary determining regions (CDRs), to elicit a response within the target cell. T-cell superantigens crosslink T-cell receptors and MHC Class II molecules on antigen-presenting cells, leading to lymphocyte recruitment, induction of cytokine storms and T-cell anergy or apoptosis among many other effects. B-cell superantigens, on the other hand, bind immunoglobulins on B-cells, affecting opsonisation, IgG-mediated phagocytosis, and driving apoptosis. Here, through a review of the structural basis for recognition of immune receptors by superantigens, we show that their binding interfaces share specific physicochemical characteristics when compared with other protein-protein interaction complexes. Given that antibody-binding superantigens have been exploited extensively in industrial antibody purification, these observations could facilitate further protein engineering to optimize the use of superantigens in this and other areas of biotechnology.

Staphylococcus aureus Arsenal To Conquer the Lower Respiratory Tract

Staphylococcus aureus is both a commensal and a pathogenic bacterium for humans. Its ability to induce severe infections is based on a wide range of virulence factors. S. aureus community-acquired pneumonia (SA-CAP) is rare and severe, and the contribution of certain virulence factors in this disease has been recognized over the past 2 decades. First, the factors involved in metabolism adaptation are crucial for S. aureus survival in the lower respiratory tract, and toxins and enzymes are required for it to cross the pulmonary epithelial barrier. S. aureus subsequently faces host defense mechanisms, including the epithelial barrier, but most importantly the immune system. Here, again, S. aureus uses myriad virulence factors to successfully escape from the host's defenses and takes advantage of them. The impact of S. aureus virulence, combined with the collateral damage caused by an overwhelming immune response, leads to severe tissue damage and adverse clinical outcomes. In this review, we summarize step by step all of the S. aureus factors implicated in CAP and described to date, and we provide an outlook for future research.

Sialoglycan recognition is a common connection linking acidosis, zinc, and HMGB1 in sepsis

Blood pH is tightly maintained between 7.35 and 7.45, and acidosis (pH <7.3) indicates poor prognosis in sepsis, wherein lactic acid from anoxic tissues overwhelms the buffering capacity of blood. Poor sepsis prognosis is also associated with low zinc levels and the release of High mobility group box 1 (HMGB1) from activated and/or necrotic cells. HMGB1 added to whole blood at physiological pH did not bind leukocyte receptors, but lowering pH with lactic acid to mimic sepsis conditions allowed binding, implying the presence of natural inhibitor(s) preventing binding at normal pH. Testing micromolar concentrations of divalent cations showed that zinc supported the robust binding of sialylated glycoproteins with HMGB1. Further characterizing HMGB1 as a sialic acid-binding lectin, we found that optimal binding takes place at normal blood pH and is markedly reduced when pH is adjusted with lactic acid to levels found in sepsis. Glycan array studies confirmed the binding of HMGB1 to sialylated glycan sequences typically found on plasma glycoproteins, with binding again being dependent on zinc and normal blood pH. Thus, HMGB1-mediated hyperactivation of innate immunity in sepsis requires acidosis, and micromolar zinc concentrations are protective. We suggest that the potent inflammatory effects of HMGB1 are kept in check via sequestration by plasma sialoglycoproteins at physiological pH and triggered when pH and zinc levels fall in late stages of sepsis. Current clinical trials independently studying zinc supplementation, HMGB1 inhibition, or pH normalization may be more successful if these approaches are combined and perhaps supplemented by infusions of heavily sialylated molecules.

Toxic Shock Syndrome Toxin 1 Induces Immune Response via the Activation of NLRP3 Inflammasome

Staphylococcus aureus is a Gram-positive opportunistic pathogen which causes infections in a variety of vertebrates. Virulence factors are the main pathogenesis of S. aureus as a pathogen, which induce the host’s innate and adaptive immune responses. Toxic shock syndrome toxin 1 (TSST-1) is one of the most important virulence factors of S. aureus. However, the role of nucleotide-binding oligomerization domain-like receptor family pyrin domain containing 3 (NLRP3) in TSST-1-induced innate immune response is still unclear. Here, purified recombinant TSST-1 (rTSST-1) was prepared and used to stimulate mouse peritoneal macrophages. The results showed that under the action of adenosine-triphosphate (ATP), rTSST-1 significantly induced interleukin-1β (IL-1β) and tumor necrosis factor-α (TNF-α) production in mouse macrophages and the production was dose-dependent. In addition, rTSST-1+ATP-stimulated cytokine production in macrophage depends on the activation of toll like receptor 4 (TLR4), but not TLR2 on the cells. Furthermore, the macrophages of NLRP3^−/− mice stimulated with rTSST-1+ATP showed significantly low levels of IL-1β production compared to that of wild-type mice. These results demonstrated that TSST-1 can induce the expression of inflammatory cytokines in macrophages via the activation of the TLR4 and NLRP3 signaling pathways. Our study provides new information about the mechanism of the TSST-1-inducing host’s innate immune responses.

Population Analysis of Staphylococcus aureus Reveals a Cryptic, Highly Prevalent Superantigen SElW That Contributes to the Pathogenesis of Bacteremia

Staphylococcus aureus is an important human and animal pathogen associated with an array of diseases, including life-threatening necrotizing pneumonia and infective endocarditis. The success of S. aureus as a pathogen has been linked in part to its ability to manipulate the host immune response through the secretion of toxins and immune evasion molecules. The staphylococcal superantigens (SAgs) have been studied for decades, but their role in S. aureus pathogenesis is not well understood, and an appreciation for how SAgs manipulate the host immune response to promote infection may be crucial for the development of novel intervention strategies. Here, we characterized a widely prevalent, previously cryptic, staphylococcal SAg, SElW, that contributes to the severity of S. aureus infections caused by an important epidemic clone of S. aureus CC398. Our findings add to the understanding of staphylococcal SAg diversity and function and provide new insights into the capacity of S. aureus to cause disease.

Staphylococcal superantigens (SAgs) are a family of secreted toxins that stimulate T cell activation and are associated with an array of diseases in humans and livestock. Most SAgs produced by Staphylococcus aureus are encoded by mobile genetic elements, such as pathogenicity islands, bacteriophages, and plasmids, in a strain-dependent manner. Here, we carried out a population genomic analysis of >800 staphylococcal isolates representing the breadth of S. aureus diversity to investigate the distribution of all 26 identified SAg genes. Up to 14 SAg genes were identified per isolate with the most common gene selw (encoding a putative SAg, SElW) identified in 97% of isolates. Most isolates (62.5%) have a full-length open reading frame of selw with an alternative TTG start codon that may have precluded functional characterization of SElW to date. Here, we demonstrate that S. aureus uses the TTG start codon to translate a potent SAg SElW that induces Vβ-specific T cell proliferation, a defining feature of classical SAgs. SElW is the only SAg predicted to be expressed by isolates of the CC398 lineage, an important human and livestock epidemic clone. Deletion of selw in a representative CC398 clinical isolate, S. aureus NM001, resulted in complete loss of T cell mitogenicity in vitro, and in vivo expression of SElW by S. aureus increased the bacterial load in the liver during bloodstream infection of SAg-sensitive HLA-DR4 transgenic mice. Overall, we report the characterization of a novel, highly prevalent, and potent SAg that contributes to the pathogenesis of S. aureus infection.

Staphylococcus aureus Lipase 1 Enhances Influenza A Virus Replication

Influenza A virus (IAV) causes annual epidemics and sporadic pandemics of respiratory disease. Secondary bacterial coinfection by organisms such as Staphylococcus aureus is the most common complication of primary IAV infection and is associated with high levels of morbidity and mortality. Here, we report the first identified S. aureus factor (lipase 1) that enhances IAV replication during infection via positive modulation of virus budding. The effect is observed in vivo in embryonated hen’s eggs and greatly enhances the yield of a vaccine strain, a finding that could be applied to address global shortages of influenza vaccines.

Influenza A virus (IAV) causes annual epidemics of respiratory disease in humans, often complicated by secondary coinfection with bacterial pathogens such as Staphylococcus aureus. Here, we report that the S. aureus secreted protein lipase 1 enhances IAV replication in vitro in primary cells, including human lung fibroblasts. The proviral activity of lipase 1 is dependent on its enzymatic function, acts late in the viral life cycle, and results in increased infectivity through positive modulation of virus budding. Furthermore, the proviral effect of lipase 1 on IAV is exhibited during in vivo infection of embryonated hen’s eggs and, importantly, increases the yield of a vaccine strain of IAV by approximately 5-fold. Thus, we have identified the first S. aureus protein to enhance IAV replication, suggesting a potential role in coinfection. Importantly, this activity may be harnessed to address global shortages of influenza vaccines.

Epidemiological and Clinical Evidence for the Role of Toxins in S. aureus Human Disease

Staphylococcus aureus asymptomatically colonizes approximately 30–50% of the population and is a leading cause of bacteremia, bone/joint infections, and skin infections in the US. S. aureus has become a major public health threat due to antibiotic resistance and an increasing number of failed vaccine attempts. To develop new anti-staphylococcal preventive therapies, it will take a more thorough understanding of the current role S. aureus virulence factors play in contributing to human disease. This review focuses on the clinical association of individual toxins with S. aureus infection as well as attempted treatment options. Further understanding of these associations will increase understanding of toxins and their importance to S. aureus pathogenesis.

Prevalence and Genetic Diversity of Staphylococcal Enterotoxin (-Like) Genes sey, selw, selx, selz, sel26 and sel27 in Community-Acquired Methicillin-Resistant Staphylococcus aureus

Staphylococcal enterotoxins (SEs) are virulence factors of Staphylococcus aureus associated with various toxic diseases due to their emetic and superantigenic activities. Although at least 27 SE(-like) genes have been identified in S. aureus to date, the newly identified SE(-like) genes have not yet been well characterized by their epidemiological features. In this study, the prevalence and genetic diversity of SE gene sey and SE-like genes selw, selx, selz, sel26, and sel27 were investigated for 624 clinical isolates of community-acquired methicillin-resistant S. aureus (CA-MRSA). The most prevalent SE(-like) gene was selw (92.9%), followed by selx (85.6%), sey (35.4%) and selz (5.6%), while sel26 and sel27 were not detected. Phylogenetically, sey, selw, selx, and selz were discriminated into 7, 10, 16, and 9 subtypes (groups), respectively. Among these subtypes, sey was the most conserved and showed the highest sequence identity (>98.8%), followed by selz and selx. The SE-like gene selw was the most divergent, and four out of ten genetic groups contained pseudogenes that may encode truncated product. Individual subtypes of SE(-like) genes were generally found in isolates with specific genotypes/lineages of S. aureus. This study revealed the putative ubiquity of selw and selx and the prevalence of sey and selz in some specific lineages (e.g., ST121) in CA-MRSA, suggesting a potential role of these newly described SEs(-like) in pathogenicity.

Staphylococcus aureus Pneumonia: Preceding Influenza Infection Paves the Way for Low-Virulent Strains

Staphylococcus aureus is a facultative pathogenic bacterium that colonizes the nasopharyngeal area of healthy individuals, but can also induce severe infection, such as pneumonia. Pneumonia caused by mono- or superinfected S.aureus leads to high mortality rates. To establish an infection, S. aureus disposes of a wide variety of virulence factors, which can vary between clinical isolates. Our study aimed to characterize pneumonia isolates for their virulent capacity. For this, we analyzed isolates from colonization, pneumonia due to S. aureus, and pneumonia due to S. aureus/influenza virus co-infection. A total of 70 strains were analyzed for their virulence genes and the host–pathogen interaction was analyzed through functional assays in cell culture systems. Strains from pneumonia due to S. aureus mono-infection showed enhanced invasion and cytotoxicity against professional phagocytes than colonizing and co-infecting strains. This corresponded to the high presence of cytotoxic components in pneumonia strains. By contrast, strains obtained from co-infection did not exhibit these virulence characteristics and resembled strains from colonization, although they caused the highest mortality rate in patients. Taken together, our results underline the requirement of invasion and toxins to cause pneumonia due to S. aureus mono-infection, whereas in co-infection even low-virulent strains can severely aggravate pneumonia.

Identification, cloning and characterization of SpEX exotoxin produced by Staphylococcus pseudintermedius

Staphylococci have evolved numerous strategies to evade their hosts’ immune systems. Some staphylococcal toxins target essential components of host innate immunity, one of the two main branches of the immune system. Analysis of the Staphylococcus pseudintermedius secretome using liquid chromatography mass spectrometry guided by genomic data, was used to identify an S. pseudintermedius exotoxin provisionally named SpEX. This exoprotein has low overall amino acid identity with the Staphylococcus aureus group of proteins named staphylococcal superantigen like proteins (SSLs) and staphylococcal enterotoxin- like toxin X (SEIX), but predictive modeling showed that it shares similar folds and domain architecture to these important virulence factors. In this study, we found SpEX binds to complement component C5, prevents complement mediated lysis of sensitized bovine red blood cells, kills polymorphonuclear leukocytes and monocytes and inhibits neutrophil migration at sub-lethal concentrations. A mutant version of SpEX, produced through amino acid substitution at selected positions, had diminished cytotoxicity. Anti-SpEX produced in dogs reduced the inhibitory effect of native SpEX on canine neutrophil migration and protected immune cells from the toxic effects of the native recombinant protein. These results suggest that SpEX likely plays an important role in S. pseudintermedius virulence and that attenuated SpEX may be an important candidate for inclusion in a vaccine against S. pseudintermedius infections.

Cell intrinsic functions of neutrophils and their manipulation by pathogens

Neutrophils are a crucial first line of defense against infection, migrating rapidly into tissues where they deploy granule components and toxic oxidants for efficient phagocytosis and microbe killing. Subsequent apoptosis and clearance of dying neutrophils are essential for control of infection and resolution of the inflammatory response. A subset of microbial pathogens survive exposure to neutrophils by manipulating phagocytosis, phagosome-granule fusion, oxidant production, and lifespan. Elucidating how they accomplish this unusual feat provides new insights into normal neutrophil function. In this review, we highlight recent discoveries about the ways in which neutrophils use cell-intrinsic mechanisms to control infection, and how these defenses are subverted by pathogens.

Immunity to Staphylococcus aureus: Implications for Vaccine Development

Cell-mediated immunity seems to be critical for prevention and resolution of invasive S. aureus infections, but an imbalance in this immunity may also produce SIRS and death or an inadequate protective response with prolonged bacteremia and death. This dysregulation is likely at the heart of mortality and severe disease in humans. Anti-toxin antibodies may also come into play in reducing the severity of S. aureus infections, but these antibodies might also address superantigen-induced immune dysregulation. Thus, while changing intrinsic T cell responses may be therapeutically difficult, monoclonal antibodies against superantigens may have utility in addressing dysfunctional immune responses to S. aureus. The models above are hypotheses for examining, and potentially dramatically improving immune response to and safety of S. aureus vaccines.

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.

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.

Staphylococcal Superantigens: Pyrogenic Toxins Induce Toxic Shock

Staphylococcal enterotoxin B (SEB) and related superantigenic toxins produced by Staphylococcus aureus are potent activators of the immune system. These protein toxins bind to major histocompatibility complex (MHC) class II molecules and specific Vβ regions of T-cell receptors (TCRs), resulting in the activation of both monocytes/macrophages and T lymphocytes. The bridging of TCRs with MHC class II molecules by superantigens triggers an early “cytokine storm” and massive polyclonal T-cell proliferation. Proinflammatory cytokines, tumor necrosis factor α, interleukin 1 (IL-1), IL-2, interferon γ (IFNγ), and macrophage chemoattractant protein 1 elicit fever, inflammation, multiple organ injury, hypotension, and lethal shock. Upon MHC/TCR ligation, superantigens induce signaling pathways, including mitogen-activated protein kinase cascades and cytokine receptor signaling, which results in NFκB activation and the phosphoinositide 3-kinase/mammalian target of rapamycin pathways. In addition, gene profiling studies have revealed the essential roles of innate antimicrobial defense genes in the pathogenesis of SEB. The genes expressed in a murine model of SEB-induced shock include intracellular DNA/RNA sensors, apoptosis/DNA damage-related molecules, endoplasmic reticulum/mitochondrial stress responses, immunoproteasome components, and IFN-stimulated genes. This review focuses on the signaling pathways induced by superantigens that lead to the activation of inflammation and damage response genes. The induction of these damage response genes provides evidence that SEB induces danger signals in host cells, resulting in multiorgan injury and toxic shock. Therapeutics targeting both host inflammatory and cell death pathways can potentially mitigate the toxic effects of staphylococcal superantigens.

Association between the accessory gene regulator (agr) locus and the presence of superantigen genes in clinical isolates of methicillin-resistant Staphylococcus aureus

OBJECTIVE

Methicillin-resistant Staphylococcus aureus cause to a variety of hard to cure infections. MRSA isolates also, produce an arsenal of virulence factors contribute to severe infections. The aim of this study was to find out the relationship between agr locus and presence of S. aureus superantigens (SAgs).

RESULTS

Clinical isolates in two groups from two different states of Iran were collected. Antibiotic resistance patterns, agr typing, and virulence factor genes prevalence were identified and relationship between them was analyzed using SPSS software version16. Most of the samples were collected from wound 39 isolates in Group 1 and 61 isolates in Group 2. Frequency of MRSA strains was 38.1% in Group 1 and 52.1% in Group 2. Also, the most common resistance among both groups was to penicillin. agr positive isolates were detected in 132 isolates of Group 1 and 104 isolates of Group 2. In Conclusion, a significant relationship between the SAgs frequency and agr locus in both groups has been indicated. The production of superantigens in S. aureus plays an important role in the classification of agr locus, and this locus can affect differently in methicillin-resistant strains.

Manipulation of Innate and Adaptive Immunity by Staphylococcal Superantigens

Staphylococcal superantigens (SAgs) constitute a family of potent exotoxins secreted by Staphylococcus aureus and other select staphylococcal species. SAgs function to cross-link major histocompatibility complex (MHC) class II molecules with T cell receptors (TCRs) to stimulate the uncontrolled activation of T lymphocytes, potentially leading to severe human illnesses such as toxic shock syndrome. The ubiquity of SAgs in clinical S. aureus isolates suggests that they likely make an important contribution to the evolutionary fitness of S. aureus. Although the apparent redundancy of SAgs in S. aureus has not been explained, the high level of sequence diversity within this toxin family may allow for SAgs to recognize an assorted range of TCR and MHC class II molecules, as well as aid in the avoidance of humoral immunity. Herein, we outline the major diseases associated with the staphylococcal SAgs and how a dysregulated immune system may contribute to pathology. We then highlight recent research that considers the importance of SAgs in the pathogenesis of S. aureus infections, demonstrating that SAgs are more than simply an immunological diversion. We suggest that SAgs can act as targeted modulators that drive the immune response away from an effective response, and thus aid in S. aureus persistence.

Basis of Virulence in Enterotoxin-Mediated Staphylococcal Food Poisoning

The Staphylococcus aureus enterotoxins are a superfamily of secreted virulence factors that share structural and functional similarities and possess potent superantigenic activity causing disruptions in adaptive immunity. The enterotoxins can be separated into two groups; the classical (SEA-SEE) and the newer (SEG-SElY and counting) enterotoxin groups. Many members from both these groups contribute to the pathogenesis of several serious human diseases, including toxic shock syndrome, pneumonia, and sepsis-related infections. Additionally, many members demonstrate emetic activity and are frequently responsible for food poisoning outbreaks. Due to their robust tolerance to denaturing, the enterotoxins retain activity in food contaminated previously with S. aureus. The genes encoding the enterotoxins are found mostly on a variety of different mobile genetic elements. Therefore, the presence of enterotoxins can vary widely among different S. aureus isolates. Additionally, the enterotoxins are regulated by multiple, and often overlapping, regulatory pathways, which are influenced by environmental factors. In this review, we also will focus on the newer enterotoxins (SEG-SElY), which matter for the role of S. aureus as an enteropathogen, and summarize our current knowledge on their prevalence in recent food poisoning outbreaks. Finally, we will review the current literature regarding the key elements that govern the complex regulation of enterotoxins, the molecular mechanisms underlying their enterotoxigenic, superantigenic, and immunomodulatory functions, and discuss how these activities may collectively contribute to the overall manifestation of staphylococcal food poisoning.