A Staphylococcus aureus pore-forming toxin subverts the activity of ADAM10 to cause lethal infection in mice

Staphylococcus aureus in vitro secretion of alpha toxin (hla) correlates with the affiliation to clonal complexes

The alpha toxin of Staphylococcus aureus is a pore forming toxin that penetrates host cell membranes causing osmotic swelling, rupture, lysis and subsequently cell death. Haemolysin alpha is toxic to a wide range of different mammalian cells; i.e., neurotoxic, dermonecrotic, haemolytic, and it can cause lethality in a wide variety of animals. In this study, the in vitro alpha toxin production of 648 previously genotyped isolates of S. aureus was measured quantitatively using antibody microarrays. Isolates originated from medical and veterinary settings and were selected in order to represent diverse clonal complexes and defined clinical conditions. Generally, the production of alpha toxin in vitro is related to the clonal complex affiliation. For clonal complexes CC22, CC30, CC45, CC479, CC705 and others, invariably no alpha toxin production was noted under the given in vitro conditions, while others, such as CC1, CC5, CC8, CC15 or CC96 secreted variable or high levels of alpha toxin. There was no correlation between alpha toxin yield and clinical course of the disease, or between alpha toxin yield and host species.

Subterfuge and sabotage: evasion of host innate defenses by invasive gram-positive bacterial pathogens

The development of a severe invasive bacterial infection in an otherwise healthy individual is one of the most striking and fascinating aspects of human medicine. A small cadre of gram-positive pathogens of the genera Streptococcus and Staphylococcus stand out for their unique invasive disease potential and sophisticated ability to counteract the multifaceted components of human innate defense. This review illustrates how these leading human disease agents evade host complement deposition and activation, impede phagocyte recruitment and activation, resist the microbicidal activities of host antimicrobial peptides and reactive oxygen species, escape neutrophil extracellular traps, and promote and accelerate phagocyte cell death through the action of pore-forming cytolysins. Understanding the molecular basis of bacterial innate immune resistance can open new avenues for therapeutic intervention geared to disabling specific virulence factors and resensitizing the pathogen to host innate immune clearance.

The psmα locus regulates production of Staphylococcus aureus alpha-toxin during infection

Staphylococcus aureus is a leading cause of human bacterial infection, causing a wide spectrum of disease ranging from skin and soft tissue infections to life-threatening pneumonia and sepsis. S. aureus toxins play an essential role in disease pathogenesis, contributing to both immunomodulation and host tissue injury. Prominent among these toxins are the membrane-active pore-forming cytolysin alpha-toxin (Hla) and the amphipathic α-helical phenol-soluble modulin (PSM) peptides. As deletion of either the hla or psm locus leads to a phenotypically similar virulence defect in skin and soft tissue infection, we sought to determine the relative contribution of each locus to disease pathogenesis. Here we show that production of Hla can be modulated by PSM expression. An S. aureus mutant lacking PSM expression exhibits a transcriptional delay in hla mRNA production and therefore fails to secrete normal levels of Hla at early phases of growth. This leads to attenuation of virulence in vitro and in murine skin and lung models of infection, correlating with reduced recovery of Hla from host tissues. Production of Hla and restoration of staphylococcal virulence can be achieved in the psm mutant by plasmid-driven overexpression of hla. Our study suggests the coordinated action of Hla and PSMs in host tissue during early pathogenesis, confirming a major role for Hla in epithelial injury during S. aureus infection. These findings highlight the possibility that therapeutics targeting PSM production may simultaneously prevent Hla-mediated tissue injury.

The accessory gene regulator (agr) controls Staphylococcus aureus virulence in a murine intracranial abscesses model

Background

Intracranial abscesses are associated with high mortality. Staphylococcus aureus is one of the main pathogens that cause intracranial infection. Until now, there is no report to identify the key effectors of S. aureus during the intracranial infection.

Methods

The murine intracranial abscesses model induced by S. aureus was constructed. The vital sign and survival rate of mice were observed to evaluate the infection. Histological examination was used to diagnose the pathological alterations of mouse tissues. The sensitivity of S. aureus to whole blood was evaluated by whole-blood killing assay.

Results

In murine intracranial abscesses model, it was shown that the mortality caused by the accessory gene regulator (agr) locus deficient strain was significant decreased compared with its parent strain. Moreover, we found that RNAIII, the effector of agr system, was essential for the intracranial infection caused by S. aureus. In the further investigation, it was shown that restoration the expression of α-toxin in agr deficient strain could partially recover the mortality in the murine intracranial abscesses model.

Conclusion

Our data suggested that the agr system of S. aureus is an important virulence determinant in the induction and mortality of intracranial abscesses in mice.

Tissue-specific patterning of host innate immune responses by Staphylococcus aureus α-toxin

Immunomodulatory cytotoxins are prominent virulence factors produced by Staphylococcus aureus, a leading cause of bacterial sepsis, skin infection, and pneumonia. S. aureus α-toxin is a pore-forming toxin that utilizes a widely expressed receptor, ADAM10, to injure the host epithelium, endothelium, and immune cells. As each host tissue is characterized by a unique composition of resident cells and recruited immune cells, the outcome of α-toxin-mediated injury may depend on the infected tissue environment. Utilizing myeloid lineage-specific Adam10 knockout mice, we show that α-toxin exerts tissue-specific effects on innate immunity to staphylococcal infection. Loss of ADAM10 expression exacerbates skin infection, yet affords protection against lethal pneumonia. These diverse outcomes are not related to altered immune cell recruitment, but rather correlate with a defect in toxin-induced IL-1β production. Extension of these studies through analysis of ADAM10 double-knockout mice affecting both the myeloid lineage and either the skin or lung epithelium highlight the prominence of toxin-induced injury to the epithelium in governing the outcome of infection. Together, these studies provide evidence of tissue specificity of pore-forming cytotoxin action in the modulation of host immunity, and illustrate that the outcome of infection is a collective manifestation of all effects of the toxin within the tissue microenvironment.

Discovery of protective B-cell epitopes for development of antimicrobial vaccines and antibody therapeutics

Protective antibodies play an essential role in immunity to infection by neutralizing microbes or their toxins and recruiting microbicidal effector functions. Identification of the protective B-cell epitopes, those parts of microbial antigens that contact the variable regions of the protective antibodies, can lead to development of antibody therapeutics, guide vaccine design, enable assessment of protective antibody responses in infected or vaccinated individuals, and uncover or localize pathogenic microbial functions that could be targeted by novel antimicrobials. Monoclonal antibodies are required to link in vivo or in vitro protective effects to specific epitopes and may be obtained from experimental animals or from humans, and their binding can be localized to specific regions of antigens by immunochemical assays. The epitopes are then identified with mapping methods such as X-ray crystallography of antigen-antibody complexes, antibody inhibition of hydrogen-deuterium exchange in the antigen, antibody-induced alteration of the nuclear magnetic resonance spectrum of the antigen, and experimentally validated computational docking of antigen-antibody complexes. The diversity in shape, size and structure of protective B-cell epitopes, and the increasing importance of protective B-cell epitope discovery to development of vaccines and antibody therapeutics are illustrated through examples from different microbe categories, with emphasis on epitopes targeted by broadly neutralizing antibodies to pathogens of high antigenic variation. Examples include the V-shaped Ab52 glycan epitope in the O-antigen of Francisella tularensis, the concave CR6261 peptidic epitope in the haemagglutinin stem of influenza virus H1N1, and the convex/concave PG16 glycopeptidic epitope in the gp120 V1/V2 loop of HIV type 1.

Cardiolipin synthetase is involved in antagonistic interaction (reverse CAMP phenomenon) of Mycoplasma species with Staphylococcus aureus beta-hemolysis

Mycoplasma hyorhinis has been implicated in a variety of swine diseases. However, little is known about the hemolytic capabilities of Mycoplasma species in general or M. hyorhinis in particular. In this study, we show that M. hyorhinis possesses beta-hemolytic activity which may be involved in the invasion process. M. hyorhinis also possesses antagonistic cooperativity (reverse CAMP phenomenon) with Staphylococcus aureus beta-hemolysis, resulting in the protection of erythrocytes from the beta-hemolytic activity of S. aureus (reverse CAMP). The reversed CAMP phenomenon has been attributed to phospholipase D (PLD) activity. In silico analysis of the M. hyorhinis genome revealed the absence of the pld gene but the presence of the cls gene encoding cardiolipin synthetase, which contains two PLD active domains. The transformation of Mycoplasma gallisepticum that has neither the cls gene nor the reverse CAMP phenomenon with the cls gene from M. hyorhinis resulted in the reverse CAMP phenomenon, suggesting for the first time that reverse CAMP can be induced by cardiolipin synthetase.

Staphylococcus aureus alpha-toxin mediates general and cell type-specific changes in metabolite concentrations of immortalized human airway epithelial cells

Staphylococcus aureus alpha-toxin (Hla) is a potent pore-forming cytotoxin that plays an important role in the pathogenesis of S. aureus infections, including pneumonia. The impact of Hla on the dynamics of the metabolome in eukaryotic host cells has not been investigated comprehensively. Using ¹H-NMR, GC-MS and HPLC-MS, we quantified the concentrations of 51 intracellular metabolites and assessed alterations in the amount of 25 extracellular metabolites in the two human bronchial epithelial cell lines S9 and 16HBE14o⁻ under standard culture conditions and after treatment with sub-lethal amounts (2 µg/ml) of recombinant Hla (rHla) in a time-dependent manner. Treatment of cells with rHla caused substantial decreases in the concentrations of intracellular metabolites from different metabolic pathways in both cell lines, including ATP and amino acids. Concomitant increases in the extracellular concentrations were detected for various intracellular compounds, including nucleotides, glutathione disulfide and NAD⁺. Our results indicate that rHla has a major impact on the metabolome of eukaryotic cells as a consequence of direct rHla-mediated alterations in plasma membrane permeability or indirect effects mediated by cellular signalling. However, cell-specific changes also were observed. Glucose consumption and lactate production rates suggest that the glycolytic activity of S9 cells, but not of 16HBE14o⁻ cells, is increased in response to rHla. This could contribute to the observed higher level of resistance of S9 cells against rHla-induced membrane damage.

Adhesion, invasion and evasion: the many functions of the surface proteins of Staphylococcus aureus

Staphylococcus aureus is an important opportunistic pathogen and persistently colonizes about 20% of the human population. Its surface is 'decorated' with proteins that are covalently anchored to the cell wall peptidoglycan. Structural and functional analysis has identified four distinct classes of surface proteins, of which microbial surface component recognizing adhesive matrix molecules (MSCRAMMs) are the largest class. These surface proteins have numerous functions, including adhesion to and invasion of host cells and tissues, evasion of immune responses and biofilm formation. Thus, cell wall-anchored proteins are essential virulence factors for the survival of S. aureus in the commensal state and during invasive infections, and targeting them with vaccines could combat S. aureus infections.

Combinatorial synthesis and in vitro evaluation of a biaryl hydroxyketone library as antivirulence agents against MRSA

Antibiotic resistance coupled with decreased development of new antibiotics necessitates the search for novel antibacterial agents. Antivirulence agents offer an alternative to conventional antibiotics. In this work, we report on a family of small-molecule antivirulence agents against methicillin-resistant Staphylococcus aureus (MRSA), the most widespread bacterial pathogen. Structure-activity relationship studies led to the development of a concise synthesis of a 148-member biarylhydroxyketone library. An acylation bond-forming process afforded resorcinols (1) and aryloxy acetonitriles (2) as synthons. A Lewis-acid-activated Friedel-Crafts' acylation step involving a nitrile functionality of 2 by ZnCl2, followed by nucleophilic attack by 1 was executed to obtain biaryl hydroxyketones in excellent yields. A large number of products crystallized. This strategy affords a range of biarylhydroxyketones in a single step. This is the first collective synthetic study documenting access to this class of compounds through a single synthetic operation. In vitro efficacy of compounds in this library was evaluated by a rabbit erythrocyte hemolysis assay. The most efficacious compound, 4f-12, inhibits hemolysis by 98.1 ± 0.1% compared to control in the absence of the compound.

Three-Dimensional Human Skin Models to Understand Staphylococcus aureus Skin Colonization and Infection

Staphylococcus aureus is both a major bacterial pathogen as well as a common member of the human skin microbiota. Due to its widespread prevalence as an asymptomatic skin colonizer and its importance as a source of skin and soft tissue infections, an improved understanding of how S. aureus attaches to, grows within, and breaches the stratified layers of the epidermis is of critical importance. Three-dimensional organotypic human skin culture models are informative and tractable experimental systems for future investigations of the interactions between S. aureus and the multi-faceted skin tissue. We propose that S. aureus virulence factors, primarily appreciated for their role in pathogenesis of invasive infections, play alternative roles in promoting asymptomatic bacterial growth within the skin. Experimental manipulations of these cultures will provide insight into the many poorly understood molecular interactions occurring at the interface between S. aureus and stratified human skin tissue.

Staphylococcus aureus toxins

Staphylococcus aureus is a dangerous pathogen that causes a variety of severe diseases. The virulence of S. aureus is defined by a large repertoire of virulence factors, among which secreted toxins play a preeminent role. Many S. aureus toxins damage biological membranes, leading to cell death. In particular, S. aureus produces potent hemolysins and leukotoxins. Among the latter, some were recently identified to lyse neutrophils after ingestion, representing an especially powerful weapon against bacterial elimination by innate host defense. Furthermore, S. aureus secretes many factors that inhibit the complement cascade or prevent recognition by host defenses. Several further toxins add to this multi-faceted program of S. aureus to evade elimination in the host. This review will give an overview over S. aureus toxins focusing on recent advances in our understanding of how leukotoxins work in receptor-mediated or receptor-independent fashions.

Phenol-soluble modulins--critical determinants of staphylococcal virulence

Phenol-soluble modulins (PSMs) are a recently discovered family of amphipathic, alpha-helical peptides that have multiple roles in staphylococcal pathogenesis and contribute to a large extent to the pathogenic success of virulent staphylococci, such as Staphylococcus aureus. PSMs may cause lysis of many human cell types including leukocytes and erythrocytes, stimulate inflammatory responses, and contribute to biofilm development. PSMs appear to have an original role in the commensal lifestyle of staphylococci, where they facilitate growth and spreading on epithelial surfaces. Aggressive, cytolytic PSMs seem to have evolved from that original role and are mainly expressed in highly virulent S. aureus. Here, we will review the biochemistry, genetics, and role of PSMs in the commensal and pathogenic lifestyles of staphylococci, discuss how diversification of PSMs defines the aggressiveness of staphylococcal species, and evaluate potential avenues to target PSMs for drug development against staphylococcal infections.

Staphylococcus aureus α-toxin: nearly a century of intrigue

Staphylococcus aureus secretes a number of host-injurious toxins, among the most prominent of which is the small β-barrel pore-forming toxin α-hemolysin. Initially named based on its properties as a red blood cell lytic toxin, early studies suggested a far greater complexity of α-hemolysin action as nucleated cells also exhibited distinct responses to intoxication. The hemolysin, most aptly referred to as α-toxin based on its broad range of cellular specificity, has long been recognized as an important cause of injury in the context of both skin necrosis and lethal infection. The recent identification of ADAM10 as a cellular receptor for α-toxin has provided keen insight on the biology of toxin action during disease pathogenesis, demonstrating the molecular mechanisms by which the toxin causes tissue barrier disruption at host interfaces lined by epithelial or endothelial cells. This review highlights both the historical studies that laid the groundwork for nearly a century of research on α-toxin and key findings on the structural and functional biology of the toxin, in addition to discussing emerging observations that have significantly expanded our understanding of this toxin in S. aureus disease. The identification of ADAM10 as a proteinaceous receptor for the toxin not only provides a greater appreciation of truths uncovered by many historic studies, but now affords the opportunity to more extensively probe and understand the role of α-toxin in modulation of the complex interaction of S. aureus with its human host.

Metalloproteinases and their natural inhibitors in inflammation and immunity

Over the past 50 years, steady growth in the field of metalloproteinase biology has shown that the degradation of extracellular matrix components represents only a fraction of the functions performed by these enzymes and has highlighted their fundamental roles in immunity. Metalloproteinases regulate aspects of immune cell development, effector function, migration and ligand-receptor interactions. They carry out ectodomain shedding of cytokines and their cognate receptors. Together with their endogenous inhibitors TIMPs (tissue inhibitor of metalloproteinases), these enzymes regulate signalling downstream of the tumour necrosis factor receptor and the interleukin-6 receptor, as well as that downstream of the epidermal growth factor receptor and Notch, which are all pertinent for inflammatory responses. This Review discusses the metalloproteinase family as a crucial component in immune cell development and function.

Oxacillin alters the toxin expression profile of community-associated methicillin-resistant Staphylococcus aureus

The emergence of community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA) is a growing cause for concern. These strains are more virulent than health care-associated MRSA (HA-MRSA) due to higher levels of toxin expression. In a previous study, we showed that the high-level expression of PBP2a, the alternative penicillin binding protein encoded by the mecA gene on type II staphylococcal cassette chromosome mec (SCCmec) elements, reduced toxicity by interfering with the Agr quorum sensing system. This was not seen in strains carrying the CA-MRSA-associated type IV SCCmec element. These strains express significantly lower levels of PBP2a than the other MRSA type, which may explain their relatively high toxicity. We hypothesized that as oxacillin is known to increase mecA expression levels, it may be possible to attenuate the toxicity of CA-MRSA by using this antibiotic. Subinhibitory oxacillin concentrations induced PBP2a expression, repressed Agr activity, and, as a consequence, decreased phenol-soluble modulin (PSM) secretion by CA-MRSA strains. However, consistent with other studies, oxacillin also increased the expression levels of alpha-toxin and Panton-Valentine leucocidin (PVL). The net effect of these changes on the ability to lyse diverse cell types was tested, and we found that where the PSMs and alpha-toxin are important, oxacillin reduced overall lytic activity, but where PVL is important, it increased lytic activity, demonstrating the pleiotropic effect of oxacillin on toxin expression by CA-MRSA.

Assessment of an anti-alpha-toxin monoclonal antibody for prevention and treatment of Staphylococcus aureus-induced pneumonia

Alpha-toxin (AT) is a major virulence factor in the disease pathogenesis of Staphylococcus aureus. We previously identified a monoclonal antibody (MAb) against AT that reduced disease severity in a mouse dermonecrosis model. Here, we evaluate the activity of an affinity-optimized variant, LC10, in a mouse model of S. aureus pneumonia. Passive immunization with LC10 increased survival and reduced bacterial numbers in the lungs and kidneys of infected mice and showed protection against diverse S. aureus clinical isolates. The lungs of S. aureus-infected mice exhibited bacterial pneumonia, including widespread inflammation, whereas the lungs of mice that received LC10 exhibited minimal inflammation and retained healthy architecture. Consistent with reduced immune cell infiltration, LC10-treated animals had significantly lower (P < 0.05) proinflammatory cytokine and chemokine levels in the bronchoalveolar lavage fluid than did those of the control animals. This reduction in inflammation and damage to the LC10-treated animals resulted in reduced vascular protein leakage and CO2 levels in the blood. LC10 was also assessed for its therapeutic activity in combination with vancomycin or linezolid. Treatment with a combination of LC10 and vancomycin or linezolid resulted in a significant increase (P < 0.05) in survival relative to the monotherapies and was deemed additive to synergistic by isobologram analysis. Consistent with improved survival, the lungs of animals treated with antibiotic plus LC10 exhibited less inflammatory tissue damage than those that received monotherapy. These data provide insight into the mechanisms of protection provided by AT inhibition and support AT as a promising target for immunoprophylaxis or adjunctive therapy against S. aureus pneumonia.

Staphylococcus aureus persistence in non-professional phagocytes

S. aureus is a frequent cause of chronic and therapy-refractory infections. The ability of S. aureus to invade different types of non-professional phagocytes, to escape from the host lysosomal degradation machinery and to persist within the intracellular location for long time periods are most likely essential steps in pathogenesis. During the course from acute to chronic infection the bacteria need to dynamically react to the environmental changes and to adapt to the intracellular environment. In this context the bacteria change to SCV-like phenotypes that exhibit some characteristics of stable SCV-mutants, like upregulation of adhesins and downregulation of toxins. The exact formation mechanism and further typical features of these dynamically forming SCVs are largely unknown. In this review, recent data on the essential steps to establish chronic infections will be summarized and the clinical consequences of the dynamic bacterial adaptation mechanisms will be discussed.

Nanoparticle-detained toxins for safe and effective vaccination

Toxoid vaccines--vaccines based on inactivated bacterial toxins--are routinely used to promote antitoxin immunity for the treatment and prevention of bacterial infections. Following chemical or heat denaturation, inactivated toxins can be administered to mount toxin-specific immune responses. However, retaining faithful antigenic presentation while removing toxin virulence remains a major challenge and presents a trade-off between efficacy and safety in toxoid development. Here, we show a nanoparticle-based toxin-detainment strategy that safely delivers non-disrupted pore-forming toxins for immune processing. Using erythrocyte membrane-coated nanoparticles and staphylococcal α-haemolysin, we demonstrate effective virulence neutralization via spontaneous particle entrapment. Compared with vaccination with heat-denatured toxin, mice vaccinated with the nanoparticle-detained toxin showed superior protective immunity against toxin-mediated adverse effects. We find that the non-disruptive detoxification approach benefited the immunogenicity and efficacy of toxoid vaccines. We anticipate that this study will open new possibilities in the preparation of antitoxin vaccines against the many virulence factors that threaten public health.

Phenol-soluble modulins

PSMs are a recently discovered family of short, amphipathic, α-helical peptides in staphylococci. Several PSMs are key virulence determinants, particularly in highly virulent Staphylococcus aureus strains. PSMα peptides of S. aureus facilitate neutrophil lysis after phagocytosis, and are key contributors to several infection types, including skin infection and bacteremia. Furthermore, all PSMs contribute to biofilm structuring and the dissemination of biofilm-associated infection. Cytolytic PSMs as produced by S. aureus appear to have evolved from original functions in the non-infectious lifestyle of staphylococci. The surfactant properties of PSMs, which they all share, are believed to facilitate growth on epithelial surfaces. The basic role of PSMs in staphylococcal physiology is underscored, for example, by their exceptionally strict and direct control by quorum-sensing and the presence of a dedicated secretion system. Targeting PSMs for anti-staphylococcal drug development may be a promising approach to overcome the problems associated with widespread antibiotic resistance in staphylococci.

Perivascular macrophages mediate neutrophil recruitment during bacterial skin infection

Transendothelial migration of neutrophils in postcapillary venules is a key event in the inflammatory response against pathogens and tissue damage. The precise regulation of this process is incompletely understood. We report that perivascular macrophages are critical for neutrophil migration into skin infected with the pathogen Staphylococcus aureus. Using multiphoton intravital microscopy we showed that neutrophils extravasate from inflamed dermal venules in close proximity to perivascular macrophages, which are a major source of neutrophil chemoattractants. The virulence factor α-hemolysin produced by S. aureus lyses perivascular macrophages, which leads to decreased neutrophil transmigration. Our data illustrate a previously unrecognized role for perivascular macrophages in neutrophil recruitment to inflamed skin and indicate that S. aureus uses hemolysin-dependent killing of these cells as an immune evasion strategy.

Cell targeting by the Staphylococcus aureus pore-forming toxins: it's not just about lipids

Staphylococcus aureus employs numerous pore-forming cytotoxins to injure host immune cells and promote infection. Until recently, it was unclear how these cytotoxins targeted specific cell types for lysis. Membrane lipids were initially postulated to be cytotoxin receptor candidates. However, the cell-type specificity and species-dependent targeting of these toxins did not support lipids as sole receptors. The recent identification of proteinaceous receptors for several S. aureus cytotoxins now provides an explanation for the observed tropism. These findings also have important implications for the implementation of animal models to study S. aureus pathogenesis, and for the development of novel therapeutics.

Staphylococcus aureus alpha toxin suppresses effective innate and adaptive immune responses in a murine dermonecrosis model

An optimal host response against Staphylococcus aureus skin and soft tissue infections (SSTI) is dependent on IL-1β and IL-17 mediated abscess formation. Alpha toxin (AT), an essential virulence factor for SSTI, has been reported to damage tissue integrity; however its effect on the immune response has not been investigated. Here, we demonstrate that infection with USA300 AT isogenic mutant (Δhla), or passive immunization with an AT neutralizing mAb, 2A3, 24 h prior to infection with wild type USA300 (WT), resulted in dermonecrotic lesion size reduction, and robust neutrophil infiltration. Infiltration correlates with increase in proinflammatory cytokines and chemokines, as well as enhanced bacterial clearance relative to immunization with a negative control mAb. In addition, infection with Δhla, or with WT +2A3, resulted in an early influx of innate IL-17⁺γδT cells and a more rapid induction of an adaptive immune response as measured by Th1 and Th17 cell recruitment at the site of infection. These results are the first direct evidence of a role for AT in subverting the innate and adaptive immune responses during a S. aureus SSTI. Further, these effects of AT can be overcome with a high affinity anti-AT mAb resulting in a reduction in disease severity.

Mechanisms of cytolysin-induced cell damage -- a role for auto- and paracrine signalling

Cytolysins inflict cell damage by forming pores in the plasma membrane. The Na(+) conductivity of these pores results in an ion influx that exceeds the capacity of the Na(+) /K(+) -pump to extrude Na(+) . This net load of intracellular osmolytes results in swelling and eventual lysis of the attacked cell. Many nucleated cells have the capacity to reduce the potential damage of pore-forming proteins, whereas erythrocytes have been regarded as essentially defenceless against cytolysin-induced cell damage. This review addresses how autocrine/paracrine signalling and the cells intrinsic volume regulation markedly influence the fate of the cell after membrane insertion of cytolysins. Moreover, it regards the various steps that may explain the relative large degree of diversity between cell types and species as well as highlights some of the current gaps in the mechanistic understanding of cytolysin-induced cell injury.

The impact of α-toxin on host cell plasma membrane permeability and cytokine expression during human blood infection by CA-MRSA USA300

This investigation examines the influence of α-toxin (Hla) expression by CA-MRSA on host immune cell integrity and cytokine expression during infection of human blood. Flow cytometry analysis of human blood infected by Staphylococcus aureus PFGE type USA300 or a USA300Δhla demonstrated that Hla expression significantly increased plasma membrane permeability of human CD14(+) monocytes. The increased susceptibility of human CD14(+) monocytes to Hla toxicity paralleled the high cell-surface expression on these cell types of ADAM10. USA300 rapidly associated with PMNs and monocytes but not T cells following inoculation of human blood. Transcription analysis indicated a strong up-regulation of proinflammatory cytokine transcription following infection of human blood by USA300 and USA300Δhla. CBAs and ELISAs determined that IL-6, IL-10, TNF-α, IFN-γ, IL-1β, IL-8, and IL-4 are significantly up-regulated during the initial phases of human blood infection by USA300 relative to mock-infected blood but failed to distinguish any significant differences in secreted cytokine protein concentrations during infection by USA300Δhla relative to USA300. Collectively, these findings demonstrate that expression of Hla by USA300 has a significant impact on human CD14(+) monocyte plasma membrane integrity but is not exclusively responsible for the proinflammatory cytokine profile induced by USA300 during the initial stages of human blood infection.

Bacteria activate sensory neurons that modulate pain and inflammation

Nociceptor sensory neurons are specialized to detect potentially damaging stimuli, protecting the organism by initiating the sensation of pain and eliciting defensive behaviours. Bacterial infections produce pain by unknown molecular mechanisms, although they are presumed to be secondary to immune activation. Here we demonstrate that bacteria directly activate nociceptors, and that the immune response mediated through TLR2, MyD88, T cells, B cells, and neutrophils and monocytes is not necessary for Staphylococcus aureus-induced pain in mice. Mechanical and thermal hyperalgesia in mice is correlated with live bacterial load rather than tissue swelling or immune activation. Bacteria induce calcium flux and action potentials in nociceptor neurons, in part via bacterial N-formylated peptides and the pore-forming toxin α-haemolysin, through distinct mechanisms. Specific ablation of Nav1.8-lineage neurons, which include nociceptors, abrogated pain during bacterial infection, but concurrently increased local immune infiltration and lymphadenopathy of the draining lymph node. Thus, bacterial pathogens produce pain by directly activating sensory neurons that modulate inflammation, an unsuspected role for the nervous system in host-pathogen interactions.

Characterization of a mouse-adapted Staphylococcus aureus strain

More effective antibiotics and a protective vaccine are desperately needed to combat the ‘superbug’ Staphylococcus aureus. While in vivo pathogenicity studies routinely involve infection of mice with human S. aureus isolates, recent genetic studies have demonstrated that S. aureus lineages are largely host-specific. The use of such animal-adapted S. aureus strains may therefore be a promising approach for developing more clinically relevant animal infection models. We have isolated a mouse-adapted S. aureus strain (JSNZ) which caused a severe outbreak of preputial gland abscesses among male C57BL/6J mice. We aimed to extensively characterize this strain on a genomic level and determine its virulence potential in murine colonization and infection models. JSNZ belongs to the MLST type ST88, rare among human isolates, and lacks an hlb-converting phage encoding human-specific immune evasion factors. Naive mice were found to be more susceptible to nasal and gastrointestinal colonization with JSNZ than with the human-derived Newman strain. Furthermore, naïve mice required antibiotic pre-treatment to become colonized with Newman. In contrast, JSNZ was able to colonize mice in the absence of antibiotic treatment suggesting that this strain can compete with the natural flora for space and nutrients. In a renal abscess model, JSNZ caused more severe disease than Newman with greater weight loss and bacterial burden. In contrast to most other clinical isolates, JSNZ can also be readily genetically modified by phage transduction and electroporation. In conclusion, the mouse-adapted strain JSNZ may represent a valuable tool for studying aspects of mucosal colonization and for screening novel vaccines and therapies directed at preventing colonization.

Kinase AKT1 negatively controls neutrophil recruitment and function in mice

Neutrophils are critically involved in host defense and inflammatory injury. However, intrinsic signaling mechanisms controlling neutrophil recruitment and activities are poorly defined. In this article, we showed that protein kinase AKT1 (also known as PKBα) is the dominant isoform expressed in neutrophils and is downregulated upon bacterial infection and neutrophil activation. AKT1 deficiency resulted in severe disease progression accompanied by recruitment of neutrophils and enhanced bactericidal activity in the acute inflammatory lung injury (ALI) and the Staphylococcus aureus infection mouse models. Moreover, the depletion of neutrophils efficiently reversed the aggravated inflammatory response, but adoptive transfer of AKT1(-/-) neutrophils could potentiate the inflammatory immunity, indicating an intrinsic effect of the neutrophil in modulating inflammation in AKT1(-/-) mice. In the ALI model, the infiltration of neutrophils into the inflammatory site was associated with enhanced migration capacity, whereas inflammatory stimuli could promote neutrophil apoptosis. In accordance with these findings, neutralization of CXCR2 attenuated neutrophil infiltration and delayed the occurrence of inflammation. Finally, the enhanced bactericidal activity and inflammatory immunity of AKT-deficient neutrophils were mediated by a STAT1-dependent, but not a mammalian target of rapamycin-dependent, pathway. Thus, our findings indicated that the AKT1-STAT1 signaling axis negatively regulates neutrophil recruitment and activation in ALI and S. aureus infection in mice.

Neutrophils versus Staphylococcus aureus: a biological tug of war

The pathogen Staphylococcus aureus is well adapted to its human host. Neutrophil-mediated killing is a crucial defense system against S. aureus; however, the pathogen has evolved many strategies to resist killing. We first describe the discrete steps of neutrophil activation and migration to the site of infection and the killing of microbes by neutrophils in general. We then highlight the different approaches utilized by S. aureus to resist the different steps of neutrophil attack. Various molecules are discussed in their evolutionary context. Most of the molecules secreted by S. aureus to combat neutrophil attacks at the site of infection show clear human specificity. Many elements of human neutrophil defenses appear redundant, and so the evasion strategies of staphylococci display redundant functions as well. All efforts by S. aureus to resist neutrophil-mediated killing stress the importance of these mechanisms in the pathophysiology of staphylococcal diseases. However, the highly human-specific nature of most host-pathogen interactions hinders the in vivo establishment of their contribution to staphylococcal pathophysiology.

The mutated staphylococcal H35A α-toxin inhibits adhesion and invasion of Staphylococcus aureus and group A streptococci

In previous studies we demonstrated that the staphylococcal α-toxin inhibits adhesion and invasion of S. aureus by epithelial cells through binding to α5β1 integrin, a receptor of fibronectin. Moreover, we revealed that a H35A mutation abolishes the cytotoxicity of α-toxin completely. These findings led us to hypothesize that the H35A mutated α-toxin may be explored as a potential inhibitor for bacterial adhesion and invasion of epithelial cells. In this study, we examined the impact of the H35A α-toxin on staphylococcal capacity of adhering to and invading into epithelial cells and found that the addition of H35A α-toxin in the culture medium dramatically inhibited S. aureus' ability to adhere to and internalize into epithelial cells. Importantly, we demonstrated that both the staphylococcal α-toxin and H35A mutated α-toxin are capable of retarding the adhesion and invasion of epithelial cells by Streptococcus pyogenes. These findings suggest that the H35A toxoid has the potential to be utilized as an inhibitor of S. aureus and S. pyogenes ability to adhere to and invade epithelial cells.

Role of pore-forming toxins in bacterial infectious diseases

Pore-forming toxins (PFTs) are the most common bacterial cytotoxic proteins and are required for virulence in a large number of important pathogens, including Streptococcus pneumoniae, group A and B streptococci, Staphylococcus aureus, Escherichia coli, and Mycobacterium tuberculosis. PFTs generally disrupt host cell membranes, but they can have additional effects independent of pore formation. Substantial effort has been devoted to understanding the molecular mechanisms underlying the functions of certain model PFTs. Likewise, specific host pathways mediating survival and immune responses in the face of toxin-mediated cellular damage have been delineated. However, less is known about the overall functions of PFTs during infection in vivo. This review focuses on common themes in the area of PFT biology, with an emphasis on studies addressing the roles of PFTs in in vivo and ex vivo models of colonization or infection. Common functions of PFTs include disruption of epithelial barrier function and evasion of host immune responses, which contribute to bacterial growth and spreading. The widespread nature of PFTs make this group of toxins an attractive target for the development of new virulence-targeted therapies that may have broad activity against human pathogens.

Discovery of antivirulence agents against methicillin-resistant Staphylococcus aureus

Antivirulence agents inhibit the production of disease-causing virulence factors but are neither bacteriostatic nor bactericidal. Antivirulence agents against methicillin-resistant Staphylococcus aureus (MRSA) strain USA300, the most widespread community-associated MRSA strain in the United States, were discovered by virtual screening against the response regulator AgrA, which acts as a transcription factor for the expression of several of the most prominent S. aureus toxins and virulence factors involved in pathogenesis. Virtual screening was followed by similarity searches in the databases of commercial vendors. The small-molecule compounds discovered inhibit the production of the toxins alpha-hemolysin and phenol-soluble modulin α in a dose-dependent manner without inhibiting bacterial growth. These antivirulence agents are small-molecule biaryl compounds in which the aromatic rings either are fused or are separated by a short linker. One of these compounds is the FDA-approved nonsteroidal anti-inflammatory drug diflunisal. This represents a new use for an old drug. Antivirulence agents might be useful in prophylaxis and as adjuvants in antibiotic therapy for MRSA infections.

Role of pore-forming toxins in neonatal sepsis

Protein toxins are important virulence factors contributing to neonatal sepsis. The major pathogens of neonatal sepsis, group B Streptococci, Escherichia coli, Listeria monocytogenes, and Staphylococcus aureus, secrete toxins of different molecular nature, which are key for defining the disease. Amongst these toxins are pore-forming exotoxins that are expressed as soluble monomers prior to engagement of the target cell membrane with subsequent formation of an aqueous membrane pore. Membrane pore formation is not only a means for immediate lysis of the targeted cell but also a general mechanism that contributes to penetration of epithelial barriers and evasion of the immune system, thus creating survival niches for the pathogens. Pore-forming toxins, however, can also contribute to the induction of inflammation and hence to the manifestation of sepsis. Clearly, pore-forming toxins are not the sole factors that drive sepsis progression, but they often act in concert with other bacterial effectors, especially in the initial stages of neonatal sepsis manifestation.

Staphylococcus aureus toxins--their functions and genetics

The outcome of encounters between Staphylococcus (S.) aureus and its human host ranges from life-threatening infection through allergic reactions to symptom-free colonization. The pan-genome of this bacterial species encodes numerous toxins, known or strongly suspected to cause specific diseases or symptoms. Three toxin families are in the focus of this review, namely (i) pore-forming toxins, (ii) exfoliative toxins and (iii) superantigens. The majority of toxin-encoding genes are located on mobile genetic elements (MGEs), resulting in a pronounced heterogeneity in the endowment with toxin genes of individual S. aureus strains. Recent population genomic analysis have provided a framework for an improved understanding of the temporal and spatial scales of the motility of MGEs and their associated toxin genes. The distribution of toxin genes among clonal lineages within the species S. aureus is not random, and phylogenetic (sub-)lineages within clonal complexes feature characteristic toxin signatures. When studying pathogenesis, this lineage association, which is caused by the clonal nature of S. aureus makes it difficult to discriminate effects of specific toxins from contributions of the genetic background and/or other associated genetic factors.

Community-associated MRSA: what makes them special?

While infections with methicillin-resistant Staphylococcus aureus (MRSA) were traditionally restricted to the hospital setting, novel MRSA strains emerged over the last two decades that have the capacity to infect otherwise healthy people outside of the hospital setting. These community-associated (CA-)MRSA strains combine methicillin resistance with enhanced virulence and fitness. Interestingly, CA-MRSA strains emerged globally and from different backgrounds, indicating that the "trade-off" between maintaining sufficient levels of methicillin resistance and obtaining enhanced virulence at a low fitness cost was achieved on several occasions in convergent evolution. However, frequently this process comprised similar changes. First and foremost, all CA-MRSA strains typically carry a novel type of methicillin resistance locus that appears to cause less of a fitness burden. Additionally, acquisition of specific toxin genes, most notably that encoding Panton-Valentine leukocidin (PVL), and adaptation of gene expression of genome-encoded toxins, such as alpha-toxin and phenol-soluble modulins (PSMs), further contributed to the evolution of CA-MRSA. Finally, the exceptional epidemiological success of the USA300 CA-MRSA clone in particular may have been due to yet another gene acquisition, namely that of the speG gene, which is located on the arginine catabolic mobile element (ACME) and involved in detoxifying harmful host-derived polyamines.

Crystallization and preliminary X-ray diffraction analysis of the complex between a human anti-alpha toxin antibody fragment and alpha toxin

Staphylococcus aureus alpha toxin (AT) has been crystallized in complex with the Fab fragment of a human antibody (MEDI4893). This constitutes the first reported crystals of AT bound to an antibody. The monoclinic crystals belonged to space group P2₁, with unit-cell parameters a=85.52, b=148.50, c=93.82 Å, β=99.82°. The diffraction of the crystals extended to 2.56 Å resolution. The asymmetric unit contained two MEDI4893 Fab-AT complexes. This corresponds to a crystal volume per protein weight (VM) of 2.3 Å3 Da(-1) and a solvent content of 47%. The three-dimensional structure of this complex will contribute to an understanding of the molecular basis of the interaction of MEDI4893 with AT. It will also shed light on the mechanism of action of this antibody, the current evaluation of which in the field of S. aureus-mediated diseases makes it a particularly interesting case study. Finally, this study will provide the three-dimensional structure of AT in a monomeric state for the first time.

A serologic correlate of protective immunity against community-onset Staphylococcus aureus infection

BACKGROUND

Staphylococcus aureus is among the leading causes of human infection. Widespread drug resistance, emergence of highly virulent strains, and the ability of S. aureus to colonize >30% of the human population contribute to this organism's pathogenic success. Human serologic responses to S. aureus and their relationship to protective immunity remain incompletely defined, challenging the strategic development of efficacious vaccines.

METHODS

We measured humoral responses to 2 staphylococcal exotoxins, α-hemolysin (Hla) and Panton-Valentine leukocidin (PVL; LukF-PV/LukS-PV subunits), both premier targets of current vaccine and immunotherapy development. We correlated acute and convalescent serum antibody levels with incidence of recurrent infection over 12 months follow-up in 235 children with S. aureus colonization, primary or recurrent skin and soft tissue infection, or invasive disease.

RESULTS

Cutaneous infection elicited transient increases in anti-Hla and anti-PVL antibodies; however, subsequent infection risk was similar between primary and recurrent cutaneous infection cohorts. Patients with invasive infections had the lowest preexisting titers against Hla and LukF but displayed the highest convalescent titers. Across cohorts, convalescent anti-Hla titers correlated with protection against subsequent S. aureus infection.

CONCLUSIONS

Cutaneous S. aureus infection does not reliably provoke durable, protective immune responses. This study provides the first link between protection from disease recurrence and the humoral response to Hla, a virulence factor already implicated in disease pathogenesis. These observations can be utilized to refine ongoing vaccine and immunotherapy efforts and inform the design of clinical trials.

Pathogenic pore-forming proteins: function and host response

Organisms from all kingdoms produce pore-forming proteins, with the best-characterized being of bacterial origin. The last decade of research has revealed that the channels formed by these proteins can be very diverse, thus differentially affecting target cell-membrane permeability and consequent cellular outcome. The responses to these toxins are also extremely diverse due to multiple downstream effects of pore-induced changes in ion balance. Determining the secondary effects of pore-forming toxins is essential to understand their contribution to infection.

Adherens junctions as targets of microorganisms: a focus on Helicobacter pylori

Mucosal epithelia are targeted by several microorganisms as a way of adhesion, internalization, and/or exploitation of the host properties to induce disease. Helicobacter pylori are worldwide prevalent bacteria that colonize the human stomach. Persistent infection of the gastric mucosa with H. pylori and concurrent chronic gastritis are risk factors for ulcer disease and gastric carcinoma. Therefore, interactions at the H. pylori-epithelial interface are important to understand the pathogenesis of these bacteria and the host responses that contribute to disease development. Here, we provide an overview of the interactions between microorganisms and the adherens junctions with an emphasis on H. pylori.

Filaggrin-dependent secretion of sphingomyelinase protects against staphylococcal α-toxin-induced keratinocyte death

BACKGROUND

The skin of patients with atopic dermatitis (AD) has defects in keratinocyte differentiation, particularly in expression of the epidermal barrier protein filaggrin. AD skin lesions are often exacerbated by Staphylococcus aureus-mediated secretion of the virulence factor α-toxin. It is unknown whether lack of keratinocyte differentiation predisposes to enhanced lethality from staphylococcal toxins.

OBJECTIVE

We investigated whether keratinocyte differentiation and filaggrin expression protect against cell death induced by staphylococcal α-toxin.

METHODS

Filaggrin-deficient primary keratinocytes were generated through small interfering RNA gene knockdown. RNA expression was determined by using real-time PCR. Cell death was determined by using the lactate dehydrogenase assay. Keratinocyte cell survival in filaggrin-deficient (ft/ft) mouse skin biopsies was determined based on Keratin 5 staining. α-Toxin heptamer formation and acid sphingomyelinase expression were determined by means of immunoblotting.

RESULTS

We found that filaggrin expression, occurring as the result of keratinocyte differentiation, significantly inhibits staphylococcal α-toxin-mediated pathogenicity. Furthermore, filaggrin plays a crucial role in protecting cells by mediating the secretion of sphingomyelinase, an enzyme that reduces the number of α-toxin binding sites on the keratinocyte surface. Finally, we determined that sphingomyelinase enzymatic activity directly prevents α-toxin binding and protects keratinocytes against α-toxin-induced cytotoxicity.

CONCLUSIONS

The current study introduces the novel concept that S aureus α-toxin preferentially targets and destroys filaggrin-deficient keratinocytes. It also provides a mechanism to explain the increased propensity for S aureus-mediated exacerbation of AD skin disease.

Ectodomain shedding and ADAMs in development

Proteolytic enzymes belonging to the A Disintegin And Metalloproteinase (ADAM) family are able to cleave transmembrane proteins close to the cell surface, in a process referred to as ectodomain shedding. Substrates for ADAMs include growth factors, cytokines, chemokines and adhesion molecules, and, as such, many ADAM proteins play crucial roles in cell-cell adhesion, extracellular and intracellular signaling, cell differentiation and cell proliferation. In this Review, we summarize the fascinating roles of ADAMs in embryonic and adult tissue development in both vertebrates and invertebrates.

Pore-forming bacterial toxins and antimicrobial peptides as modulators of ADAM function

Membrane-perturbating proteins and peptides are widespread agents in biology. Pore-forming bacterial toxins represent major virulence factors of pathogenic microorganisms. Membrane-damaging peptides constitute important antimicrobial effectors of innate immunity. Membrane perturbation can incur multiple responses in mammalian cells. The present discussion will focus on the interplay between membrane-damaging agents and the function of cell-bound metalloproteinases of the ADAM family. These transmembrane enzymes have emerged as the major proteinase family that mediate the proteolytic release of membrane-associated proteins, a process designated as "shedding". They liberate a large spectrum of functionally active molecules including inflammatory cytokines, growth factor receptors and cell adhesion molecules, thereby regulating such vital cellular functions as cell-cell adhesion, cell proliferation and cell migration. ADAM activation may constitute part of the cellular recovery machinery on the one hand, but likely also promotes inflammatory processes on the other. The mechanisms underlying ADAM activation and the functional consequences thereof are currently the subject of intensive research. Attention here is drawn to the possible involvement of purinergic receptors and ceramide generation in the context of ADAM activation following membrane perturbation by membrane-active agents.

Staphylococcus aureus Panton-Valentine leukocidin induces an inflammatory response in human phagocytes via the NLRP3 inflammasome

The Staphylococcus aureus pore-forming toxin PVL is most likely causative for life-threatening necrotizing infections, which are characterized by massive tissue inflammation and necrosis. Whereas the cytotoxic action of PVL on human neutrophils is already well established, the PVL effects on other sensitive cell types, such as monocytes and macrophages, are less clear. In this study, we used different types of human leukocytes (neutrophils, monocytes, macrophages, lymphocytes) to investigate cell-specific binding of PVL subunits and subsequent proinflammatory and cytotoxic effects. In all PVL-sensitive cells, we identified the binding of the subunit LukS-PV as the critical factor for PVL-induced cytotoxicity, which was followed by binding of LukF-PV. LukS-PV binds to monocytes, macrophages, and neutrophils but not to lymphocytes. Additionally, we showed that PVL binding to monocytes and macrophages leads to release of caspase-1-dependent proinflammatory cytokines IL-1β and IL-18. PVL activates the NLRP3 inflammasome, a signaling complex of myeloid cells that is involved in caspase-1-dependent IL-1β processing in response to pathogens and endogenous danger signals. Specific inhibition of this pathway at several steps significantly reduced inflammasome activation and subsequent pyronecrosis. Furthermore, we found that PAMPs and DAMPs derived from dying neutrophils can dramatically enhance this response by up-regulating pro-IL-1β in monocytes/macrophages. This study analyzes a specific host signaling pathway that mediates PVL-induced inflammation and cytotoxicity, which has high relevance for CA-MRSA-associated and PVL-mediated pathogenic processes, such as necrotizing infections.

Factors contributing to epidemic MRSA clones replacement in a hospital setting

The mechanisms governing the epidemiology dynamics and success determinants of a specific healthcare-associated methicillin-resistant S. aureus (HA-MRSA) clone in hospital settings are still unclear. Important epidemiological changes have occurred in Europe since 2000 that have been related to the appearance of the ST22-IV clone. Between 2006 and 2010, we observed the establishment of the ST22-IV clone displacing the predominant Italian clone, ST228-I, in a large Italian university hospital. To investigate the factors associated with a successful spread of epidemic MRSA clones we studied the biofilm production, the competitive behavior in co-culture, the capacity of invasion of the A549 cells, and the susceptibility to infection in a murine model of acute pneumonia of the two major HA-MRSA clones, ST22-IV and ST228-I. We showed that persistence of ST22-IV is associated with its increased biofilm production and capacity to inhibit the growth of ST228-I in co-culture. Compared to ST228-I, ST22-IV had a significantly higher capacity to invade the A549 cells and a higher virulence in a murine model of acute lung infection causing severe inflammation and determining death in all the mice within 60 hours. On the contrary, ST228-I was associated with mice survival and clearance of the infection. ST22-IV, compared with ST228-I, caused a higher number of persistent, long lasting bacteremia. These data suggest that ST22-IV could have exploited its capacity to i) increase its biofilm production over time, ii) maintain its growth kinetics in the presence of a competitor and iii) be particularly invasive and virulent both in vitro and in vivo, to replace other well-established MRSA clones, becoming the predominant European clone.

Staphylococcus aureus α-toxin modulates skin host response to viral infection

BACKGROUND

Patients with atopic dermatitis (AD) with a history of eczema herpeticum have increased staphylococcal colonization and infections. However, whether Staphylococcus aureus alters the outcome of skin viral infection has not been determined.

OBJECTIVE

We investigated whether S aureus toxins modulated host response to herpes simplex virus (HSV) 1 and vaccinia virus (VV) infections in normal human keratinocytes (NHKs) and in murine infection models.

METHODS

NHKs were treated with S aureus toxins before incubation of viruses. BALB/c mice were inoculated with S aureus 2 days before VV scarification. Viral loads of HSV-1 and VV were evaluated by using real-time PCR, a viral plaque-forming assay, and immunofluorescence staining. Small interfering RNA duplexes were used to knockdown the gene expression of the cellular receptor of α-toxin, a disintegrin and metalloprotease 10 (ADAM10). ADAM10 protein and α-toxin heptamers were detected by using Western blot assays.

RESULTS

We demonstrate that sublytic staphylococcal α-toxin increases viral loads of HSV-1 and VV in NHKs. Furthermore, we demonstrate in vivo that the VV load is significantly greater (P < .05) in murine skin inoculated with an α-toxin-producing S aureus strain compared with murine skin inoculated with the isogenic α-toxin-deleted strain. The viral enhancing effect of α-toxin is mediated by ADAM10 and is associated with its pore-forming property. Moreover, we demonstrate that α-toxin promotes viral entry in NHKs.

CONCLUSION

The current study introduces the novel concept that staphylococcal α-toxin promotes viral skin infection and provides a mechanism by which S aureus infection might predispose the host toward disseminated viral infections.

Abscess formation and alpha-hemolysin induced toxicity in a mouse model of Staphylococcus aureus peritoneal infection

Staphylococcus aureus is a frequent cause of skin infection and sepsis in humans. Preclinical vaccine studies with S. aureus have used a mouse model with intraperitoneal challenge and survival determination as a measure for efficacy. To appreciate the selection of protective antigens in this model, we sought to characterize the pathological attributes of S. aureus infection in the peritoneal cavity. Testing C57BL/6J and BALB/c mice, >10(9) CFU of S. aureus Newman were needed to produce a lethal outcome in 90% of animals infected via intraperitoneal injection. Both necropsy and histopathology revealed the presence of intraperitoneal abscesses in the vicinity of inoculation sites. Abscesses were comprised of fibrin as well as collagen deposits and immune cells with staphylococci replicating at the center of these lesions. Animals that succumbed to challenge harbored staphylococci in abscess lesions and in blood. The establishment of lethal infections, but not the development of intraperitoneal abscesses, was dependent on S. aureus expression of alpha-hemolysin (Hla). Active immunization with nontoxigenic Hla(H35L) or passive immunization with neutralizing monoclonal antibodies protected mice against early lethal events associated with intraperitoneal S. aureus infection but did not affect the establishment of abscess lesions. These results characterize a mouse model for the study of intraperitoneal abscess formation by S. aureus, a disease that occurs frequently in humans undergoing continuous ambulatory peritoneal dialysis for end-stage renal disease.