Insights into mechanisms used by Staphylococcus aureus to avoid destruction by human neutrophils

Adaptive Immunity Against Staphylococcus aureus

A complex interplay between host and bacterial factors allows Staphylococcus aureus to occupy its niche as a human commensal and a major human pathogen. The role of neutrophils as a critical component of the innate immune response against S. aureus, particularly for control of systemic infection, has been established in both animal models and in humans with acquired and congenital neutrophil dysfunction. The role of the adaptive immune system is less clear. Although deficiencies in adaptive immunity do not result in the marked susceptibility to S. aureus infection that neutrophil dysfunction imparts, emerging evidence suggests both T cell- and B cell-mediated adaptive immunity can influence host susceptibility and control of S. aureus. The contribution of adaptive immunity depends on the context and site of infection and can be either beneficial or detrimental to the host. Furthermore, S. aureus has evolved mechanisms to manipulate adaptive immune responses to its advantage. In this chapter, we will review the evidence for the role of adaptive immunity during S. aureus infections. Further elucidation of this role will be important to understand how it influences susceptibility to infection and to appropriately design vaccines that elicit adaptive immune responses to protect against subsequent infections.

Staphylococcus aureus Strain USA300 Perturbs Acquisition of Lysosomal Enzymes and Requires Phagosomal Acidification for Survival inside Macrophages

Methicillin-resistant Staphylococcus aureus (MRSA) causes invasive, drug-resistant skin and soft tissue infections. Reports that S. aureus bacteria survive inside macrophages suggest that the intramacrophage environment may be a niche for persistent infection; however, mechanisms by which the bacteria might evade macrophage phagosomal defenses are unclear. We examined the fate of the S. aureus-containing phagosome in THP-1 macrophages by evaluating bacterial intracellular survival and phagosomal acidification and maturation and by testing the impact of phagosomal conditions on bacterial viability. Multiple strains of S. aureus survived inside macrophages, and in studies using the MRSA USA300 clone, the USA300-containing phagosome acidified rapidly and acquired the late endosome and lysosome protein LAMP1. However, fewer phagosomes containing live USA300 bacteria than those containing dead bacteria associated with the lysosomal hydrolases cathepsin D and β-glucuronidase. Inhibiting lysosomal hydrolase activity had no impact on intracellular survival of USA300 or other S. aureus strains, suggesting that S. aureus perturbs acquisition of lysosomal enzymes. We examined the impact of acidification on S. aureus intramacrophage viability and found that inhibitors of phagosomal acidification significantly impaired USA300 intracellular survival. Inhibition of macrophage phagosomal acidification resulted in a 30-fold reduction in USA300 expression of the staphylococcal virulence regulator agr but had little effect on expression of sarA, saeR, or sigB. Bacterial exposure to acidic pH in vitro increased agr expression. Together, these results suggest that S. aureus survives inside macrophages by perturbing normal phagolysosome formation and that USA300 may sense phagosomal conditions and upregulate expression of a key virulence regulator that enables its intracellular survival.

Antibody-Based Biologics and Their Promise to Combat Staphylococcus aureus Infections

The growing incidence of serious infections mediated by methicillin-resistant Staphylococcus aureus (MRSA) strains poses a significant risk to public health. This risk is exacerbated by a prolonged void in the discovery and development of truly novel antibiotics and the absence of a vaccine. These gaps have created renewed interest in the use of biologics in the prevention and treatment of serious staphylococcal infections. In this review, we focus on efforts towards the discovery and development of antibody-based biologic agents and their potential as clinical agents in the management of serious S. aureus infections. Recent promising data for monoclonal antibodies (mAbs) targeting anthrax and Ebola highlight the potential of antibody-based biologics as therapeutic agents for serious infections.

Staphylococcal adaptation to diverse physiologic niches: an overview of transcriptomic and phenotypic changes in different biological environments

Host niches can differ strongly regarding, for example, oxygen tension, pH or nutrient availability. Staphylococcus aureus and other staphylococci are common colonizers of human epithelia as well as important human pathogens. The phenotypes that they show in different host environments, and the corresponding bacterial transcriptomes and proteomes, are currently under intense investigation. In this review, we examine the available literature describing staphylococcal phenotypes, such as expression of virulence factors, gross morphologic characteristics and growth patterns, in various physiological environments. Going forward, these studies will help researchers and clinicians to form an enhanced and more detailed picture of the interactions existing between the host and staphylococci as some of its most frequent colonizers and invaders.

IsaB Inhibits Autophagic Flux to Promote Host Transmission of Methicillin-Resistant Staphylococcus aureus

Methicillin-resistant Staphylococcus aureus (MRSA) has emerged as a major nosocomial pathogen that is widespread in both health-care facilities and in the community at large, as a result of direct host-to-host transmission. Several virulence factors are associated with pathogen transmission to naive hosts. Immunodominant surface antigen B (IsaB) is a virulence factor that helps Staphylococcus aureus to evade the host defense system. However, the mechanism of IsaB on host transmissibility remains unclear. We found that IsaB expression was elevated in transmissible MRSA. Wild-type isaB strains inhibited autophagic flux to promote bacterial survival and elicit inflammation in THP-1 cells and mouse skin. MRSA isolates with increased IsaB expression showed decreased autophagic flux, and the MRSA isolate with the lowest IsaB expression showed increased autophagic flux. In addition, recombinant IsaB rescued the virulence of the isaB deletion strain and increased the group A streptococcus (GAS) virulence in vivo. Together, these results reveal that IsaB diminishes autophagic flux, thereby allowing MRSA to evade host degradation. These findings suggest that IsaB is a suitable target for preventing or treating MRSA infection.

A comparison of Percutaneous femoral access in Endovascular Repair versus Open femoral access (PiERO): study protocol for a randomized controlled trial

BACKGROUND

Access for endovascular repair of abdominal aortic aneurysms (EVAR) is obtained through surgical cutdown or percutaneously. The only devices suitable for percutaneous closure of the 20 French arteriotomies of the common femoral artery (CFA) are the Prostar(™) and Proglide(™) devices (Abbott Vascular). Positive effects of these devices seem to consist of a lower infection rate, and shorter operation time and hospital stay. This conclusion was published in previous reports comparing techniques in patients in two different groups (cohort or randomized). Access techniques were never compared in one and the same patient; this research simplifies comparison because patient characteristics will be similar in both groups.

METHODS/DESIGN

Percutaneous access of the CFA is compared to surgical cutdown in a single patient; in EVAR surgery, access is necessary in both groins in each patient. Randomization is performed on the introduction site of the larger main device of the endoprosthesis. The contralateral device of the endoprosthesis is smaller. When we use this type of randomization, both groups will contain a similar number of main and contralateral devices. Preoperative nose cultures and perineal cultures are obtained, to compare colonization with postoperative wound cultures (in case of a surgical site infection). Furthermore, patient comfort will be considered, using VAS-scores (Visual analog scale). Punch biopsies of the groin will be harvested to retrospectively compare skin of patients who suffered a surgical site infection (SSI) to patients who did not have an SSI.

DISCUSSION

The PiERO trial is a multicenter randomized controlled clinical trial designed to show the consequences of using percutaneous access in EVAR surgery and focuses on the occurrence of surgical site infections.

TRIAL REGISTRATION

NTR4257 10 November 2013, NL44578.042.13.

Modelling staphylococcal pneumonia in a human 3D lung tissue model system delineates toxin-mediated pathology

Staphylococcus aureus necrotizing pneumonia is recognized as a toxin-mediated disease, yet the tissue-destructive events remain elusive, partly as a result of lack of mechanistic studies in human lung tissue. In this study, a three-dimensional (3D) tissue model composed of human lung epithelial cells and fibroblasts was used to delineate the role of specific staphylococcal exotoxins in tissue pathology associated with severe pneumonia. To this end, the models were exposed to the mixture of exotoxins produced by S. aureus strains isolated from patients with varying severity of lung infection, namely necrotizing pneumonia or lung empyema, or to purified toxins. The necrotizing pneumonia strains secreted high levels of α-toxin and Panton-Valentine leukocidin (PVL), and triggered high cytotoxicity, inflammation, necrosis and loss of E-cadherin from the lung epithelium. In contrast, the lung empyema strain produced moderate levels of PVL, but negligible amounts of α-toxin, and triggered limited tissue damage. α-toxin had a direct damaging effect on the epithelium, as verified using toxin-deficient mutants and pure α-toxin. Moreover, PVL contributed to pathology through the lysis of neutrophils. A combination of α-toxin and PVL resulted in the most severe epithelial injury. In addition, toxin-induced release of pro-inflammatory mediators from lung tissue models resulted in enhanced neutrophil migration. Using a collection of 31 strains from patients with staphylococcal pneumonia revealed that strains producing high levels of α-toxin and PVL were cytotoxic and associated with fatal outcome. Also, the strains that produced the highest toxin levels induced significantly greater epithelial disruption. Of importance, toxin-mediated lung epithelium destruction could be inhibited by polyspecific intravenous immunoglobulin containing antibodies against α-toxin and PVL. This study introduces a novel model system for study of staphylococcal pneumonia in a human setting. The results reveal that the combination and levels of α-toxin and PVL correlate with tissue pathology and clinical outcome associated with pneumonia.

Staphylococcus aureus Biofilms Induce Macrophage Dysfunction Through Leukocidin AB and Alpha-Toxin

The macrophage response to planktonic Staphylococcus aureus involves the induction of proinflammatory microbicidal activity. However, S. aureus biofilms can interfere with these responses in part by polarizing macrophages toward an anti-inflammatory profibrotic phenotype. Here we demonstrate that conditioned medium from mature S. aureus biofilms inhibited macrophage phagocytosis and induced cytotoxicity, suggesting the involvement of a secreted factor(s). Iterative testing found the active factor(s) to be proteinaceous and partially agr-dependent. Quantitative mass spectrometry identified alpha-toxin (Hla) and leukocidin AB (LukAB) as critical molecules secreted by S. aureus biofilms that inhibit murine macrophage phagocytosis and promote cytotoxicity. A role for Hla and LukAB was confirmed by using hla and lukAB mutants, and synergy between the two toxins was demonstrated with a lukAB hla double mutant and verified by complementation. Independent confirmation of the effects of Hla and LukAB on macrophage dysfunction was demonstrated by using an isogenic strain in which Hla was constitutively expressed, an Hla antibody to block toxin activity, and purified LukAB peptide. The importance of Hla and LukAB during S. aureus biofilm formation in vivo was assessed by using a murine orthopedic implant biofilm infection model in which the lukAB hla double mutant displayed significantly lower bacterial burdens and more macrophage infiltrates than each single mutant. Collectively, these findings reveal a critical synergistic role for Hla and LukAB in promoting macrophage dysfunction and facilitating S. aureus biofilm development in vivo.

Staphylococcus aureus has a propensity to form multicellular communities known as biofilms. While growing in a biofilm, S. aureus displays increased tolerance to nutrient deprivation, antibiotic insult, and even host immune challenge. Previous studies have shown that S. aureus biofilms thwart host immunity in part by preventing macrophage phagocytosis. It remained unclear whether this was influenced solely by the considerable size of biofilms or whether molecules were also actively secreted to circumvent macrophage-mediated phagocytosis. This is the first report to demonstrate that S. aureus biofilms inhibit macrophage phagocytosis and induce macrophage death through the combined action of leukocidin AB and alpha-toxin. Loss of leukocidin AB and alpha-toxin expression resulted in enhanced S. aureus biofilm clearance in a mouse model of orthopedic implant infection, suggesting that these toxins could be targeted therapeutically to facilitate biofilm clearance in humans.

A bacterial regulatory RNA attenuates virulence, spread and human host cell phagocytosis

Staphylococcus aureus pathogenesis is directed by regulatory proteins and RNAs. We report the case of an RNA attenuating virulence and host uptake, possibly to sustain commensalism. A S. aureus sRNA, SprC (srn_3610), reduced virulence and bacterial loads in a mouse infection model. S. aureus deleted for sprC became more virulent and increased bacterial dissemination in colonized animals. Conversely, inducing SprC expression lowered virulence and the bacterial load. Without sprC, S. aureus phagocytosis by monocytes and macrophages was higher, whereas bacteria were internalized at lower yields when SprC expression was stimulated. Without sprC, higher internalization led to a greater number of extracellular bacteria, facilitating colonization. SprC expression decreased after phagocytosis, concurring with the facilitated growth of bacteria lacking the sRNA in the presence of an oxidant. The major staphylococcal autolysin facilitates S. aureus uptake by human phagocytes. ATL proved to be negatively regulated by SprC. The SprC domains involved in pairing with atl mRNA were analyzed. The addition of ATL reduced phagocytosis of bacteria lacking sprC with no effects on wild-type bacterial uptake, implying that SprC influences phagocytosis, at least in part, by controlling ATL. Since the control of SprC on ATL was modest, other factors must contribute to atl regulation.

Bacillithiol has a role in Fe-S cluster biogenesis in Staphylococcus aureus

Staphylococcus aureus does not produce the low-molecular-weight (LMW) thiol glutathione, but it does produce the LMW thiol bacillithiol (BSH). To better understand the roles that BSH plays in staphylococcal metabolism, we constructed and examined strains lacking BSH. Phenotypic analysis found that the BSH-deficient strains cultured either aerobically or anaerobically had growth defects that were alleviated by the addition of exogenous iron (Fe) or the amino acids leucine and isoleucine. The activities of the iron-sulfur (Fe-S) cluster-dependent enzymes LeuCD and IlvD, which are required for the biosynthesis of leucine and isoleucine, were decreased in strains lacking BSH. The BSH-deficient cells also had decreased aconitase and glutamate synthase activities, suggesting a general defect in Fe-S cluster biogenesis. The phenotypes of the BSH-deficient strains were exacerbated in strains lacking the Fe-S cluster carrier Nfu and partially suppressed by multicopy expression of either sufA or nfu, suggesting functional overlap between BSH and Fe-S carrier proteins. Biochemical analysis found that SufA bound and transferred Fe-S clusters to apo-aconitase, verifying that it serves as an Fe-S cluster carrier. The results presented are consistent with the hypothesis that BSH has roles in Fe homeostasis and the carriage of Fe-S clusters to apo-proteins in S. aureus.

How methicillin-resistant Staphylococcus aureus evade neutrophil killing

PURPOSE OF REVIEW

Methicillin-resistant strains of the important human pathogen Staphylococcus aureus pose a significant public health threat in the community, as they are easily transmitted, especially prone to cause invasive disease, and infect otherwise healthy individuals. The mechanistic basis for the ability of these organisms to evade the innate immune responses remains incompletely defined.

RECENT FINDINGS

The success of pathogens such as S. aureus rests, in part, on their capacity to overcome neutrophil-mediated host defense to establish infection and cause human disease. S. aureus has the potential to thwart effective neutrophil chemotaxis, and phagocytosis, and succeeds in evading killing by neutrophils. Furthermore, S. aureus surviving within neutrophils promotes neutrophil cytolysis, with release of host-derived molecules that promote local inflammation. Here, we provide a brief overview of our understanding of the mechanisms by which S. aureus - including methicillin-resistant S. aureus - avoids neutrophil-mediated host defense and causes disease.

SUMMARY

Understanding the molecular mechanisms by which S. aureus avoids neutrophil-mediated responses and initiates signaling cascades that culminate in neutrophil lysis will provide insights prerequisite to the development of novel targets for treating staphylococcal infections.

Manipulation of Autophagy in Phagocytes Facilitates Staphylococcus aureus Bloodstream Infection

The capacity for intracellular survival within phagocytes is likely a critical factor facilitating the dissemination of Staphylococcus aureus in the host. To date, the majority of work on S. aureus-phagocyte interactions has focused on neutrophils and, to a lesser extent, macrophages, yet we understand little about the role played by dendritic cells (DCs) in the direct killing of this bacterium. Using bone marrow-derived DCs (BMDCs), we demonstrate for the first time that DCs can effectively kill S. aureus but that certain strains of S. aureus have the capacity to evade DC (and macrophage) killing by manipulation of autophagic pathways. Strains with high levels of Agr activity were capable of causing autophagosome accumulation, were not killed by BMDCs, and subsequently escaped from the phagocyte, exerting significant cytotoxic effects. Conversely, strains that exhibited low levels of Agr activity failed to accumulate autophagosomes and were killed by BMDCs. Inhibition of the autophagic pathway by treatment with 3-methyladenine restored the bactericidal effects of BMDCs. Using an in vivo model of systemic infection, we demonstrated that the ability of S. aureus strains to evade phagocytic cell killing and to survive temporarily within phagocytes correlated with persistence in the periphery and that this effect is critically Agr dependent. Taken together, our data suggest that strains of S. aureus exhibiting high levels of Agr activity are capable of blocking autophagic flux, leading to the accumulation of autophagosomes. Within these autophagosomes, the bacteria are protected from phagocytic killing, thus providing an intracellular survival niche within professional phagocytes, which ultimately facilitates dissemination.

Differential Interaction of the Staphylococcal Toxins Panton-Valentine Leukocidin and γ-Hemolysin CB with Human C5a Receptors

Staphylococcus aureus is well adapted to the human host. Evasion of the host phagocyte response is critical for successful infection. The staphylococcal bicomponent pore-forming toxins Panton-Valentine leukocidin LukSF-PV (PVL) and γ-hemolysin CB (HlgCB) target human phagocytes through interaction with the complement receptors C5aR1 and C5aR2. Currently, the apparent redundancy of both toxins cannot be adequately addressed in experimental models of infection because mice are resistant to PVL and HlgCB. The molecular basis for species specificity of the two toxins in animal models is not completely understood. We show that PVL and HlgCB feature distinct activity toward neutrophils of different mammalian species, where activity of PVL is found to be restricted to fewer species than that of HlgCB. Overexpression of various mammalian C5a receptors in HEK cells confirms that cytotoxicity toward neutrophils is driven by species-specific interactions of the toxins with C5aR1. By taking advantage of the species-specific engagement of the toxins with their receptors, we demonstrate that PVL and HlgCB differentially interact with human C5aR1 and C5aR2. In addition, binding studies illustrate that different parts of the receptor are involved in the initial binding of the toxin and the subsequent formation of lytic pores. These findings allow a better understanding of the molecular mechanism of pore formation. Finally, we show that the toxicity of PVL, but not of HlgCB, is neutralized by various C5aR1 antagonists. This study offers directions for the development of improved preclinical models for infection, as well as for the design of drugs antagonizing leukocidin toxicity.

Phagocyte subsets and lymphocyte clonal deletion behind ineffective immune response to Staphylococcus aureus

Lack of known mechanisms of protection against Staphylococcus aureus in humans is hindering development of efficacious vaccines. Preclinical as well as clinical data suggest that antibodies play an important role against S. aureus. For instance, certain hypogammaglobulinaemic patients are at increased risk of staphylococcal infections. However, development of effective humoral response may be dampened by converging immune-evasion mechanisms of S. aureus. We hypothesize that B-cell proliferation induced by staphylococcal protein A (SpA) and continuous antigen exposure, without the proper T-cell help and cytokine stimuli, leads to antigen-activated B-cell deletion and anergy. Recent findings suggest an important role of type I neutrophils (PMN-I) and conventionally activated macrophages (M1) against S. aureus, while alternatively activated macrophages (M2) favour biofilm persistence and sepsis. In addition, neutrophil-macrophage cooperation promotes extravasation and activation of neutrophils as well as clearance of bacteria ensnared in neutrophil extracellular traps. Activation of these processes is modulated by cytokines and T cells. Indeed, low CD4(+) T-cell counts represent an important risk factor for skin infections and bacteraemia in patients. Altogether, these observations could lead to the identification of predictive correlates of protection and ways for shifting the balance of the response to the benefit of the host through vaccination.

Signaling mechanism by the Staphylococcus aureus two-component system LytSR: role of acetyl phosphate in bypassing the cell membrane electrical potential sensor LytS

The two-component system LytSR has been linked to the signal transduction of cell membrane electrical potential perturbation and is involved in the adaptation of Staphylococcus aureus to cationic antimicrobial peptides. It consists of a membrane-bound histidine kinase, LytS, which belongs to the family of multiple transmembrane-spanning domains receptors, and a response regulator, LytR, which belongs to the novel family of non-helix-turn-helix DNA-binding domain proteins. LytR regulates the expression of cidABC and lrgAB operons, the gene products of which are involved in programmed cell death and lysis. Invivo studies have demonstrated involvement of two overlapping regulatory networks in regulating the lrgAB operon, both depending on LytR. One regulatory network responds to glucose metabolism and the other responds to changes in the cell membrane potential. Herein, we show that LytS has autokinase activity and can catalyze a fast phosphotransfer reaction, with 50% of its phosphoryl group lost within 1 minute of incubation with LytR. LytS has also phosphatase activity. Notably, LytR undergoes phosphorylation by acetyl phosphate at a rate that is 2-fold faster than the phosphorylation by LytS. This observation is significant in lieu of the in vivo observations that regulation of the lrgAB operon is LytR-dependent in the presence of excess glucose in the medium. The latter condition does not lead to perturbation of the cell membrane potential but rather to the accumulation of acetate in the cell. Our study provides insights into the molecular basis for regulation of lrgAB in a LytR-dependent manner under conditions that do not involve sensing by LytS.

Mesenchymal Stromal/Stem Cell and Minocycline-Loaded Hydrogels Inhibit the Growth of Staphylococcus aureus that Evades Immunomodulation of Blood-Derived Leukocytes

Mesenchymal stromal/stem cells (MSCs) have demonstrated favorable wound healing properties in addition to their differentiation capacity. MSCs encapsulated in biomaterials such as gelatin and polyethylene glycol (PEG) composite hydrogels have displayed an immunophenotype change that leads to the release of cytokines and growth factors to accelerate wound healing. However, therapeutic potential of implanted MSC-loaded hydrogels may be limited by non-specific protein adsorption that facilitates adhesion of bacterial pathogens such as planktonic Staphylococcus aureus (SA) to the surface with subsequent biofilm formation resistant to immune cell recognition and antibiotic activity. In this study, we demonstrate that blood-derived primary leukocytes and bone marrow-derived MSCs cannot inhibit colony-forming abilities of planktonic or biofilm-associated SA. However, we show that hydrogels loaded with MSCs and minocycline significantly inhibit colony-forming abilities of planktonic SA while maintaining MSC viability and multipotency. Our results suggest that minocycline and MSC-loaded hydrogels may decrease the bioburden of SA at implant sites in wounds, and may improve the wound healing capabilities of MSC-loaded hydrogels.

Insights into the Staphylococcus aureus-host interface: global changes in host and pathogen gene expression in a rabbit skin infection model

Staphylococcus aureus is an important cause of human skin and soft tissue infections (SSTIs) globally. Notably, 80% of all SSTIs are caused by S. aureus, of which ∼63% are abscesses and/or cellulitis. Although progress has been made, our knowledge of the host and pathogen factors that contribute to the pathogenesis of SSTIs is incomplete. To provide a more comprehensive view of this process, we monitored changes in the S. aureus transcriptome and selected host proinflammatory molecules during abscess formation and resolution in a rabbit skin infection model. Within the first 24 h, S. aureus transcripts involved in DNA repair, metabolite transport, and metabolism were up-regulated, suggesting an increase in the machinery encoding molecules involved in replication and cell division. There was also increased expression of genes encoding virulence factors, namely secreted toxins and fibronectin and/or fibrinogen-binding proteins. Of the host genes tested, we found that transcripts encoding IL-8, IL1β, oncostatin M-like, CCR1, CXCR1 (IL8RA), CCL4 (MIP-1β) and CCL3 (MIP1α)-like proteins were among the most highly up-regulated transcripts during S. aureus abscess formation. Our findings provide additional insight into the pathogenesis of S. aureus SSTIs, including a temporal component of the host response. These results serve as a springboard for future studies directed to better understand how/why mild or moderate SSTIs progress to invasive disease.

Staphylococcus aureus Inhibits Neutrophil-derived IL-8 to Promote Cell Death

While Staphylococcus aureus accelerates human neutrophil cell death, the underlying host- and pathogen-derived mechanisms remain incompletely defined. Previous studies demonstrated that the S. aureus SaeR/S sensory system is essential for pathogen survival following neutrophil phagocytosis. Herein, we demonstrate that the SaeR/S system promoted accelerated cell death, suppressed phosphorylation of nuclear factor-κB, and reduced interleukin-8 (IL-8) production in human neutrophils. Treatment of neutrophils with recombinant IL-8 significantly reduced bacterial burden and apoptosis. Our findings demonstrate a mechanism by which S. aureus suppresses the early neutrophil-derived IL-8 response to disrupt cell fate and promote disease.

Significance of four methionine sulfoxide reductases in Staphylococcus aureus

Staphylococcus aureus is a major human pathogen and emergence of antibiotic resistance in clinical staphylococcal isolates raises concerns about our ability to control these infections. Cell wall-active antibiotics cause elevated synthesis of methionine sulfoxide reductases (Msrs: MsrA1 and MsrB) in S. aureus. MsrA and MsrB enzymes reduce S-epimers and R-epimers of methionine sulfoxide, respectively, that are generated under oxidative stress. In the S. aureus chromosome, there are three msrA genes (msrA1, msrA2 and msrA3) and one msrB gene. To understand the precise physiological roles of Msr proteins in S. aureus, mutations in msrA1, msrA2 and msrA3 and msrB genes were created by site-directed mutagenesis. These mutants were combined to create a triple msrA (msrA1, msrA2 and msrA3) and a quadruple msrAB (msrA1, msrA2, msrA3, msrB) mutant. These mutants were used to determine the roles of Msr proteins in staphylococcal growth, antibiotic resistance, adherence to human lung epithelial cells, pigment production, and survival in mice relative to the wild-type strains. MsrA1-deficient strains were sensitive to oxidative stress conditions, less pigmented and less adherent to human lung epithelial cells, and showed reduced survival in mouse tissues. In contrast, MsrB-deficient strains were resistant to oxidants and were highly pigmented. Lack of MsrA2 and MsrA3 caused no apparent growth defect in S. aureus. In complementation experiments with the triple and quadruple mutants, it was MsrA1 and not MsrB that was determined to be critical for adherence and phagocytic resistance of S. aureus. Overall, the data suggests that MsrA1 may be an important virulence factor and MsrB probably plays a balancing act to counter the effect of MsrA1 in S. aureus.

ω-Hydroxyemodin limits staphylococcus aureus quorum sensing-mediated pathogenesis and inflammation

Antibiotic-resistant pathogens are a global health threat. Small molecules that inhibit bacterial virulence have been suggested as alternatives or adjuncts to conventional antibiotics, as they may limit pathogenesis and increase bacterial susceptibility to host killing. Staphylococcus aureus is a major cause of invasive skin and soft tissue infections (SSTIs) in both the hospital and community settings, and it is also becoming increasingly antibiotic resistant. Quorum sensing (QS) mediated by the accessory gene regulator (agr) controls virulence factor production essential for causing SSTIs. We recently identified ω-hydroxyemodin (OHM), a polyhydroxyanthraquinone isolated from solid-phase cultures of Penicillium restrictum, as a suppressor of QS and a compound sought for the further characterization of the mechanism of action. At concentrations that are nontoxic to eukaryotic cells and subinhibitory to bacterial growth, OHM prevented agr signaling by all four S. aureus agr alleles. OHM inhibited QS by direct binding to AgrA, the response regulator encoded by the agr operon, preventing the interaction of AgrA with the agr P2 promoter. Importantly, OHM was efficacious in a mouse model of S. aureus SSTI. Decreased dermonecrosis with OHM treatment was associated with enhanced bacterial clearance and reductions in inflammatory cytokine transcription and expression at the site of infection. Furthermore, OHM treatment enhanced the immune cell killing of S. aureus in vitro in an agr-dependent manner. These data suggest that bacterial disarmament through the suppression of S. aureus QS may bolster the host innate immune response and limit inflammation.

Nfu facilitates the maturation of iron-sulfur proteins and participates in virulence in Staphylococcus aureus

The acquisition and metabolism of iron (Fe) by the human pathogen Staphylococcus aureus is critical for disease progression. S. aureus requires Fe to synthesize inorganic cofactors called iron-sulfur (Fe-S) clusters, which are required for functional Fe-S proteins. In this study we investigated the mechanisms utilized by S. aureus to metabolize Fe-S clusters. We identified that S. aureus utilizes the Suf biosynthetic system to synthesize Fe-S clusters and we provide genetic evidence suggesting that the sufU and sufB gene products are essential. Additional biochemical and genetic analyses identified Nfu as an Fe-S cluster carrier, which aids in the maturation of Fe-S proteins. We find that deletion of the nfu gene negatively impacts staphylococcal physiology and pathogenicity. A nfu mutant accumulates both increased intracellular non-incorporated Fe and endogenous reactive oxygen species (ROS) resulting in DNA damage. In addition, a strain lacking Nfu is sensitive to exogenously supplied ROS and reactive nitrogen species. Congruous with ex vivo findings, a nfu mutant strain is more susceptible to oxidative killing by human polymorphonuclear leukocytes and displays decreased tissue colonization in a murine model of infection. We conclude that Nfu is necessary for staphylococcal pathogenesis and establish Fe-S cluster metabolism as an attractive antimicrobial target.

The Role of Two-Component Signal Transduction Systems in Staphylococcus aureus Virulence Regulation

Staphylococcus aureus is a versatile, opportunistic human pathogen that can asymptomatically colonize a human host but can also cause a variety of cutaneous and systemic infections. The ability of S. aureus to adapt to such diverse environments is reflected in the presence of complex regulatory networks fine-tuning metabolic and virulence gene expression. One of the most widely distributed mechanisms is the two-component signal transduction system (TCS) which allows a pathogen to alter its gene expression profile in response to environmental stimuli. The simpler TCSs consist of only a transmembrane histidine kinase (HK) and a cytosolic response regulator. S. aureus encodes a total of 16 conserved pairs of TCSs that are involved in diverse signalling cascades ranging from global virulence gene regulation (e.g. quorum sensing by the Agr system), the bacterial response to antimicrobial agents, cell wall metabolism, respiration and nutrient sensing. These regulatory circuits are often interconnected and affect each other's expression, thus fine-tuning staphylococcal gene regulation. This manuscript gives an overview of the current knowledge of staphylococcal environmental sensing by TCS and its influence on virulence gene expression and virulence itself. Understanding bacterial gene regulation by TCS can give major insights into staphylococcal pathogenicity and has important implications for knowledge-based drug design and vaccine formulation.

Binding mode characterization of novel RNA polymerase inhibitors using a combined biochemical and NMR approach

Bacterial RNA polymerase (RNAP) represents a validated target for the development of broad-spectrum antibiotics. However, the medical value of RNAP inhibitors in clinical use is limited by the prevalence of resistant strains. To overcome this problem, we focused on the exploration of alternative target sites within the RNAP. Previously, we described the discovery of a novel RNAP inhibitor class containing an ureidothiophene-2-carboxylic acid core structure. Herein, we demonstrate that these compounds are potent against a set of methicillin-resistant Staphylococcus aureus (MRSA) strains (MIC 2-16 μg mL(-1)) and rifampicin-resistant Escherichia coli TolC strains (MIC 12.5-50 μg mL(-1)). Additionally, an abortive transcription assay revealed that these compounds inhibit the bacterial transcription process during the initiation phase. Furthermore, the binding mode of the ureidothiophene-2-carboxylic acids was characterized by mutagenesis studies and ligand-based NMR spectroscopy. Competition saturation transfer difference (STD) NMR experiments with the described RNAP inhibitor myxopyronin A (Myx) suggest that the ureidothiophene-2-carboxylic acids compete with Myx for the same binding site in the RNAP switch region. INPHARMA (interligand NOE for pharmacophore mapping) experiments and molecular docking simulations provided a binding model in which the ureidothiophene-2-carboxylic acids occupy the region of the Myx western chain binding site and slightly occlude that of the eastern chain. These results demonstrate that the ureidothiophene-2-carboxylic acids are a highly attractive new class of RNAP inhibitors that can avoid the problem of resistance.

The staphylococcal toxins γ-haemolysin AB and CB differentially target phagocytes by employing specific chemokine receptors

Evasion of the host phagocyte response by Staphylococcus aureus is crucial to successful infection with the pathogen. γ-haemolysin AB and CB (HlgAB, HlgCB) are bicomponent pore-forming toxins present in almost all human S. aureus isolates. Cellular tropism and contribution of the toxins to S. aureus pathophysiology are poorly understood. Here we identify the chemokine receptors CXCR1, CXCR2 and CCR2 as targets for HlgAB, and the complement receptors C5aR and C5L2 as targets for HlgCB. The receptor expression patterns allow the toxins to efficiently and differentially target phagocytic cells. Murine neutrophils are resistant to HlgAB and HlgCB. CCR2 is the sole murine receptor orthologue compatible with γ-haemolysin. In a murine peritonitis model, HlgAB contributes to S. aureus bacteremia in a CCR2-dependent manner. HlgAB-mediated targeting of CCR2(+) cells highlights the involvement of inflammatory macrophages during S. aureus infection. Functional quantification identifies HlgAB and HlgCB as major secreted staphylococcal leukocidins.

Staphylococcus aureus gene expression in a rat model of infective endocarditis

Background

Diabetes mellitus is a frequent underlying comorbidity in patients with Staphylococcus aureus endocarditis, and it represents a risk factor for complications and a negative outcome. The pathogenesis of staphylococcal endocardial infections in diabetic hosts has been poorly characterized, and little is known about S. aureus gene expression in endocardial vegetations.

Methods

We utilized a rat model of experimental S. aureus endocarditis to compare the pathogenesis of staphylococcal infection in diabetic and nondiabetic hosts and to study the global S. aureus transcriptome in endocardial vegetations in vivo.

Results

Diabetic rats had higher levels of bacteremia and larger endocardial vegetations than nondiabetic control animals. Microarray analyses revealed that 61 S. aureus genes were upregulated in diabetic rats, and the majority of these bacterial genes were involved in amino acid and carbohydrate metabolism. When bacterial gene expression in vivo (diabetic or nondiabetic endocardial vegetations) was compared to in vitro growth conditions, higher in vivo expression of genes encoding toxins and proteases was observed. Additionally, genes involved in the production of adhesins, capsular polysaccharide, and siderophores, as well as in amino acid and carbohydrate transport and metabolism, were upregulated in endocardial vegetations. To test the contribution of selected upregulated genes to the pathogenesis of staphylococcal endocarditis, isogenic deletion mutants were utilized. A mutant defective in production of the siderophore staphyloferrin B was attenuated in the endocarditis model, whereas the virulence of a surface adhesin (ΔsdrCDE) mutant was similar to that of the parental S. aureus strain.

Conclusions

Our results emphasize the relevance of diabetes mellitus as a risk factor for infectious endocarditis and provide a basis for understanding gene expression during staphylococcal infections in vivo.

Electronic supplementary material

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Cytotoxic Virulence Predicts Mortality in Nosocomial Pneumonia Due to Methicillin-Resistant Staphylococcus aureus

The current study identified bacterial factors that may improve management of methicillin-resistant Staphylococcus aureus (MRSA) nosocomial pneumonia. Isolates were obtained from 386 patients enrolled in a randomized, controlled study of antibiotic efficacy. Isolates were screened for production of virulence factors and for vancomycin susceptibility. After adjustment for host factors such as severity of illness and treatment modality, cytotoxic activity was strongly and inversely associated with mortality; however, it had no effect on clinical cure. Isolates having low cytotoxicity, which were derived largely from healthcare-associated clones, exhibited a greater prevalence of vancomycin heteroresistance, and they were recovered more often from patients who were older and frailer. Additionally, a clone with low cytotoxic activity was associated with death and poor clinical improvement. Clone specificity and attenuated virulence appear to be associated with outcome. To our knowledge, these are the first correlations between MRSA virulence and mortality in nosocomial pneumonia.

What role does the quorum-sensing accessory gene regulator system play during Staphylococcus aureus bacteremia?

Staphylococcus aureus is a major cause of bacteremia, which frequently results in serious secondary infections such as infective endocarditis, osteomyelitis, and septic arthritis. The ability of S. aureus to cause such a wide range of infections has been ascribed to its huge armoury of different virulence factors, many of which are under the control of the quorum-sensing accessory gene regulator (Agr) system. However, a significant fraction of S. aureus bacteremia cases are caused by agr-defective isolates, calling into question the role of Agr in invasive staphylococcal infections. This review draws on recent work to define the role of Agr during bacteremia and explain why the loss of this major virulence regulator is sometimes a price worth paying for S. aureus.

Site-specific mutation of the sensor kinase GraS in Staphylococcus aureus alters the adaptive response to distinct cationic antimicrobial peptides

The Staphylococcus aureus two-component regulatory system, GraRS, is involved in resistance to killing by distinct host defense cationic antimicrobial peptides (HD-CAPs). It is believed to regulate downstream target genes such as mprF and dltABCD to modify the S. aureus surface charge. However, the detailed mechanism(s) by which the histidine kinase, GraS, senses specific HD-CAPs is not well defined. Here, we studied a well-characterized clinical methicillin-resistant S. aureus (MRSA) strain (MW2), its isogenic graS deletion mutant (ΔgraS strain), a nonameric extracellular loop mutant (ΔEL strain), and four residue-specific ΔEL mutants (D37A, P39A, P39S, and D35G D37G D41G strains). The ΔgraS and ΔEL strains were unable to induce mprF and dltA expression and, in turn, demonstrated significantly increased susceptibilities to daptomycin, polymyxin B, and two prototypical HD-CAPs (hNP-1 and RP-1). Further, P39A, P39S, and D35G-D37G-D41G ΔEL mutations correlated with moderate increases in HD-CAP susceptibility. Reductions of mprF and dltA induction by PMB were also found in the ΔEL mutants, suggesting these residues are pivotal to appropriate activation of the GraS sensor kinase. Importantly, a synthetic exogenous soluble EL mimic of GraS protected the parental MW2 strain against hNP-1- and RP-1-mediated killing, suggesting a direct interaction of the EL with HD-CAPs in GraS activation. In vivo, the ΔgraS and ΔEL strains displayed dramatic reductions in achieved target tissue MRSA counts in an endocarditis model. Taken together, our results provide new insights into potential roles of GraS in S. aureus sensing of HD-CAPs to induce adaptive survival responses to these molecules.

SdrC induces staphylococcal biofilm formation through a homophilic interaction

The molecular pathogenesis of many Staphylococcus aureus infections involves growth of bacteria as biofilm. In addition to polysaccharide intercellular adhesin (PIA) and extracellular DNA, surface proteins appear to mediate the transition of bacteria from planktonic growth to sessile lifestyle as well as biofilm growth, and can enable these processes even in the absence of PIA expression. However, the molecular mechanisms by which surface proteins contribute to biofilm formation are incompletely understood. Here we demonstrate that self-association of the serine-aspartate repeat protein SdrC promotes both bacterial adherence to surfaces and biofilm formation. However, this homophilic interaction is not required for the attachment of bacteria to abiotic surfaces. We identified the subdomain that mediates SdrC dimerization and subsequent cell-cell interactions. In addition, we determined that two adjacently located amino acid sequences within this subdomain are required for the SdrC homophilic interaction. Comparative amino acid sequence analysis indicated that these binding sites are conserved. In summary, our study identifies SdrC as a novel molecular determinant in staphylococcal biofilm formation and describes the mechanism responsible for intercellular interactions. Furthermore, these findings contribute to a growing body of evidence suggesting that homophilic interactions between surface proteins present on neighbouring bacteria induce biofilm growth.

Foreign Body Infection Models to Study Host-Pathogen Response and Antimicrobial Tolerance of Bacterial Biofilm

The number of implanted medical devices is steadily increasing and has become an effective intervention improving life quality, but still carries the risk of infection. These infections are mainly caused by biofilm-forming staphylococci that are difficult to treat due to the decreased susceptibility to both antibiotics and host defense mechanisms. To understand the particular pathogenesis and treatment tolerance of implant-associated infection (IAI) animal models that closely resemble human disease are needed. Applications of the tissue cage and catheter abscess foreign body infection models in the mouse will be discussed herein. Both models allow the investigation of biofilm and virulence of various bacterial species and a comprehensive insight into the host response at the same time. They have also been proven to serve as very suitable tools to study the anti-adhesive and anti-infective efficacy of different biomaterial coatings. The tissue cage model can additionally be used to determine pharmacokinetics, efficacy and cytotoxicity of antimicrobial compounds as the tissue cage fluid can be aspirated repeatedly without the need to sacrifice the animal. Moreover, with the advance in innovative imaging systems in rodents, these models may offer new diagnostic measures of infection. In summary, animal foreign body infection models are important tools in the development of new antimicrobials against IAI and can help to elucidate the complex interactions between bacteria, the host immune system, and prosthetic materials.

Basis of virulence in a Panton-Valentine leukocidin-negative community-associated methicillin-resistant Staphylococcus aureus strain

Community-associated (CA) infections with methicillin-resistant Staphylococcus aureus (MRSA) are on a global rise. However, analysis of virulence characteristics has been limited almost exclusively to the US endemic strain USA300. CA-MRSA strains that do not produce Panton-Valentine leukocidin (PVL) have not been investigated on a molecular level. Therefore, we analyzed virulence determinants in a PVL-negative CA-MRSA strain, ST72, from Korea. Genome-wide analysis identified 3 loci that are unique to that strain, but did not affect virulence. In contrast, phenol-soluble modulins (PSMs) and the global virulence regulator Agr strongly affected lysis of neutrophils and erythrocytes, while α-toxin and Agr had a major impact on in vivo virulence. Our findings substantiate the general key roles these factors play in CA-MRSA virulence. However, our analyses also showed noticeable differences to strain USA300, inasmuch as α-toxin emerged as a much more important factor than PSMs in experimental skin infection caused by ST72.

Living bacteria rheology: population growth, aggregation patterns, and collective behavior under different shear flows

The activity of growing living bacteria was investigated using real-time and in situ rheology-in stationary and oscillatory shear. Two different strains of the human pathogen Staphylococcus aureus-strain COL and its isogenic cell wall autolysis mutant, RUSAL9-were considered in this work. For low bacteria density, strain COL forms small clusters, while the mutant, presenting deficient cell separation, forms irregular larger aggregates. In the early stages of growth, when subjected to a stationary shear, the viscosity of the cultures of both strains increases with the population of cells. As the bacteria reach the exponential phase of growth, the viscosity of the cultures of the two strains follows different and rich behaviors, with no counterpart in the optical density or in the population's colony-forming units measurements. While the viscosity of strain COL culture keeps increasing during the exponential phase and returns close to its initial value for the late phase of growth, where the population stabilizes, the viscosity of the mutant strain culture decreases steeply, still in the exponential phase, remains constant for some time, and increases again, reaching a constant plateau at a maximum value for the late phase of growth. These complex viscoelastic behaviors, which were observed to be shear-stress-dependent, are a consequence of two coupled effects: the cell density continuous increase and its changing interacting properties. The viscous and elastic moduli of strain COL culture, obtained with oscillatory shear, exhibit power-law behaviors whose exponents are dependent on the bacteria growth stage. The viscous and elastic moduli of the mutant culture have complex behaviors, emerging from the different relaxation times that are associated with the large molecules of the medium and the self-organized structures of bacteria. Nevertheless, these behaviors reflect the bacteria growth stage.

Evolution of hypervirulence by a MRSA clone through acquisition of a transposable element

Staphylococcus aureus has evolved as a pathogen that causes a range of diseases in humans. There are two dominant modes of evolution thought to explain most of the virulence differences between strains. First, virulence genes may be acquired from other organisms. Second, mutations may cause changes in the regulation and expression of genes. Here we describe an evolutionary event in which transposition of an IS element has a direct impact on virulence gene regulation resulting in hypervirulence. Whole-genome analysis of a methicillin-resistant S. aureus (MRSA) strain USA500 revealed acquisition of a transposable element (IS256) that is absent from close relatives of this strain. Of the multiple copies of IS256 found in the USA500 genome, one was inserted in the promoter sequence of repressor of toxins (Rot), a master transcriptional regulator responsible for the expression of virulence factors in S. aureus. We show that insertion into the rot promoter by IS256 results in the derepression of cytotoxin expression and increased virulence. Taken together, this work provides new insight into evolutionary strategies by which S. aureus is able to modify its virulence properties and demonstrates a novel mechanism by which horizontal gene transfer directly impacts virulence through altering toxin regulation.

Differential responses of osteoblasts and macrophages upon Staphylococcus aureus infection

BACKGROUND

Staphylococcus aureus (S. aureus) is one of the primary causes of bone infections which are often chronic and difficult to eradicate. Bacteria like S. aureus may survive upon internalization in cells and may be responsible for chronic and recurrent infections. In this study, we compared the responses of a phagocytic cell (i.e. macrophage) to a non-phagocytic cell (i.e. osteoblast) upon S. aureus internalization.

RESULTS

We found that upon internalization, S. aureus could survive for up to 5 and 7 days within macrophages and osteoblasts, respectively. Significantly more S. aureus was internalized in macrophages compared to osteoblasts and a significantly higher (100 fold) level of live intracellular S. aureus was detected in macrophages compared to osteoblasts. However, the percentage of S. aureus survival after infection was significantly lower in macrophages compared to osteoblasts at post-infection days 1-6. Interestingly, macrophages had relatively lower viability in shorter infection time periods (i.e. 0.5-4 h; significant at 2 h) but higher viability in longer infection time periods (i.e. 6-8 h; significant at 8 h) compared to osteoblasts. In addition, S. aureus infection led to significant changes in reactive oxygen species production in both macrophages and osteoblasts. Moreover, infected osteoblasts had significantly lower alkaline phosphatase activity at post-infection day 7 and infected macrophages had higher phagocytosis activity compared to non-infected cells.

CONCLUSIONS

S. aureus was found to internalize and survive within osteoblasts and macrophages and led to differential responses between osteoblasts and macrophages. These findings may assist in evaluation of the pathogenesis of chronic and recurrent infections which may be related to the intracellular persistence of bacteria within host cells.

Cationic methacrylate polymers as topical antimicrobial agents against Staphylococcus aureus nasal colonization

The in vitro and in vivo antimicrobial activity of primary ammonium ethyl methacrylate homopolymers (AEMPs) was investigated. AEMPs with different degrees of polymerization (DP = 7.7–12) were prepared by reversible addition–fragmentation chain-transfer (RAFT) polymerization. The AEMPs showed higher inhibitory effects against Gram-positive bacteria, including methicillin-resistant Staphylococcus aureus (MRSA), than Gram-negative bacteria. The AEMPs also showed potent anti-S. aureus activity in the presence of fetal bovine serum, whereas the activity of the antibiotic mupirocin was reduced under the same conditions. The AEMPs showed very little or no hemolytic activity. The cytotoxicity of AEMPs against mammalian cells HEp-2 and COS-7 was concentration-dependent, and the cell viability significantly decreased at higher polymer concentrations. The AEMPs significantly reduced the number of viable S. aureus cells in the nasal environment of cotton rats when compared to that of the control. This study demonstrates that AEMPs have potential for use in treating topical S. aureus infections.

The AgrD N-terminal leader peptide of Staphylococcus aureus has cytolytic and amyloidogenic properties

Staphylococcus aureus virulence is coordinated through the Agr quorum-sensing system to produce an array of secreted molecules. One important class of secreted virulence factors is the phenol-soluble modulins (PSMs). PSMs are small-peptide toxins that have recently been characterized for their roles in infection, biofilm development, and subversion of the host immune system. In this work, we demonstrate that the signal peptide of the S. aureus quorum-sensing signal, AgrD, shares structural and functional similarities with the PSM family of toxins. The efficacy of this peptide (termed N-AgrD) beyond AgrD propeptide trafficking has never been described before. We observe that N-AgrD, like the PSMs, is found in the amyloid fibrils of S. aureus biofilms and is capable of forming and seeding amyloid fibrils in vitro. N-AgrD displays cytolytic and proinflammatory properties that are abrogated after fibril formation. These data suggest that the N-AgrD leader peptide affects S. aureus biology in a manner similar to that described previously for the PSM peptide toxins. Taken together, our findings suggest that peptide cleavage products can affect cellular function beyond their canonical roles and may represent a class of virulence factors warranting further exploration.

Differential regulation of staphylococcal virulence by the sensor kinase SaeS in response to neutrophil-derived stimuli

Two-component systems (TCSs) are highly conserved across bacteria and are used to rapidly sense and respond to changing environmental conditions. The human pathogen Staphylococcus aureus uses the S. aureus exoprotein expression (sae) TCS to sense host signals and activate transcription of virulence factors essential to pathogenesis. Despite its importance, the mechanism by which the histidine kinase SaeS recognizes specific host stimuli is unknown. After mutagenizing the predicted extracellular loop of SaeS, we discovered one methionine residue (M31) was essential for the ability of S. aureus to transcribe sae target genes, including hla, lukAB/lukGH, and hlgA. This single M31A mutation also significantly reduced cytotoxicity in human neutrophils to levels observed in cells following interaction with ΔsaeS. Another important discovery was that mutation of two aromatic anchor residues (W32A and F33A) disrupted the normal basal signaling of SaeS in the absence of inducing signals, yet both mutant kinases had appropriate activation of effector genes following exposure to neutrophils. Although the transcriptional profile of aromatic mutation W32A was consistent with that of WT in response to human α-defensin 1, mutant kinase F33A did not properly transcribe the γ-toxin genes in response to this stimulus. Taken together, our results provide molecular evidence for how SaeS recognizes host signals and triggers activation of select virulence factors to facilitate evasion of innate immunity. These findings have important implications for signal transduction in prokaryotes and eukaryotes due to conservation of aromatic anchor residues across both of these domains and the important role they play in sensor protein structure and function.

Phagocytosis of Staphylococcus aureus by human neutrophils prevents macrophage efferocytosis and induces programmed necrosis

Community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA) pose a significant threat to human health. Polymorphonuclear leukocytes (PMN) are the first responders during staphylococcal infection, but 15-50% of the initial ingested inoculum survives within the PMN phagosome and likely contributes directly or indirectly to disease pathogenesis. We hypothesize that surviving intracellular CA-MRSA undermine effective phagocyte-mediated defense by causing a decrease in macrophage uptake of PMN containing viable S. aureus and by promoting PMN lysis. In support of this hypothesis, PMN harboring viable CA-MRSA strain USA300 (PMN-SA) upregulated the "don't eat me" signal CD47, remained bound to the surface, and were inefficiently ingested by macrophages. In addition, coculture with PMN-SA altered the macrophage phenotype. Compared to macrophages fed USA300 alone, macrophages challenged with PMN-SA produced more IL-8 and less IL-1 receptor antagonist, TNF-α, activated caspase-1, and IL-1β. Although they exhibited some features of apoptosis within 3 h following ingestion of S. aureus, including phosphatidylserine exposure and mitochondrial membrane depolarization, PMN-SA had sustained levels of proliferating cell nuclear Ag expression, absence of caspase activation, and underwent lysis within 6 h following phagocytosis. PMN lysis was dependent on receptor-interacting protein 1, suggesting that PMN-SA underwent programmed necrosis or necroptosis. These data are the first demonstration, to our knowledge, that bacteria can promote sustained expression of proliferating cell nuclear Ag and that human PMN undergo necroptosis. Together, these findings demonstrate that S. aureus surviving within PMN undermine the innate immune response and may provide insight into the pathogenesis of S. aureus disease.

Phagocytosis and killing of Staphylococcus aureus by human neutrophils

Neutrophils are essential for host defense against Staphylococcus aureus infections. Although significant progress has been made, our understanding of neutrophil interactions with S. aureus remains incomplete. To provide a more comprehensive view of this process, we investigated phagocytosis and killing of S. aureus by human neutrophils using varied assay conditions in vitro. A greater percentage of bacteria were internalized by adherent neutrophils compared to those in suspension, and, unexpectedly, uptake of S. aureus by adherent neutrophils occurred efficiently in the absence of opsonins. An antibody specific for S. aureus promoted uptake of unopsonized bacteria in suspension, but had little or no capacity to enhance phagocytosis of S. aureus opsonized with normal human serum or by adherent neutrophils. Collectively, these results indicate that assay conditions can have a significant influence on the phagocytosis and killing of S. aureus by neutrophils. More importantly, the results suggest a vaccine approach directed to enhance opsonophagocytosis alone is not sufficient to promote increased killing of S. aureus by human neutrophils. With the emergence and reemergence of antibiotic-resistant microorganisms, establishing parameters that are optimal for studying neutrophil-S. aureus interactions will pave the way towards developing immune-directed strategies for anti-staphylococcal therapies.

Myeloid-derived suppressor cells contribute to Staphylococcus aureus orthopedic biofilm infection

Myeloid-derived suppressor cells (MDSCs) are a heterogeneous population of immature monocytes and granulocytes that are potent inhibitors of T cell activation. A role for MDSCs in bacterial infections has only recently emerged, and nothing is known about MDSC function in the context of Staphylococcus aureus infection. Because S. aureus biofilms are capable of subverting immune-mediated clearance, we examined whether MDSCs could play a role in this process. CD11b(+)Gr-1(+) MDSCs represented the main cellular infiltrate during S. aureus orthopedic biofilm infection, accounting for >75% of the CD45+ population. Biofilm-associated MDSCs inhibited T cell proliferation and cytokine production, which correlated with a paucity of T cell infiltrates at the infection site. Analysis of FACS-purified MDSCs recovered from S. aureus biofilms revealed increased arginase-1, inducible NO synthase, and IL-10 expression, key mediators of MDSC suppressive activity. Targeted depletion of MDSCs and neutrophils using the mAb 1A8 (anti-Ly6G) improved bacterial clearance by enhancing the intrinsic proinflammatory attributes of infiltrating monocytes and macrophages. Furthermore, the ability of monocytes/macrophages to promote biofilm clearance in the absence of MDSC action was revealed with RB6-C85 (anti-Gr-1 or anti-Ly6G/Ly6C) administration, which resulted in significantly increased S. aureus burdens both locally and in the periphery, because effector Ly 6C monocytes and, by extension, mature macrophages were also depleted. Collectively, these results demonstrate that MDSCs are key contributors to the chronicity of S. aureus biofilm infection, as their immunosuppressive function prevents monocyte/macrophage proinflammatory activity, which facilitates biofilm persistence.

Separately or combined, LukG/LukH is functionally unique compared to other staphylococcal bicomponent leukotoxins

Staphylococcus aureus is a major human pathogen that elaborates several exotoxins. Among these are the bicomponent leukotoxins (BCLs), which include γ-hemolysin, Panton-Valentine leukocidin (PVL), and LukDE. The toxin components are classified as either F or S proteins, which are secreted individually and assemble on cell surfaces to form hetero-oligomeric pores resulting in lysis of PMNs and/or erythrocytes. F and S proteins of γ-hemolysin, PVL and LukDE have ∼ 70% sequence homology within the same class and several heterologous combinations of F and S members from these three bicomponent toxin groups are functional. Recently, an additional BCL pair, LukGH (also called LukAB) that has only 30% homology to γ-hemolysin, PVL and LukDE, has been characterized from S. aureus. Our results showed that LukGH was more cytotoxic to human PMNs than PVL. However, LukGH-induced calcium ion influx in PMNs was markedly attenuated and slower than that induced by PVL and other staphylococcal BCLs. In contrast to other heterologous BCL combinations, LukG in combination with heterologous S components, and LukH in combination with heterologous F components did not induce calcium ion entry or cell lysis in human PMNs or rabbit erythrocytes. Like PVL, LukGH induced IL-8 production by PMNs. While individual components LukG and LukH had no cytolytic or calcium influx activity, they each induced high levels of IL-8 transcription and secretion. IL-8 production induced by LukG or LukH was dependent on NF-κB. Therefore, our results indicate LukGH differs functionally from other staphylococcal BCLs.

Bacterial serine/threonine protein kinases in host-pathogen interactions

In bacterial pathogenesis, monitoring and adapting to the dynamically changing environment in the host and an ability to disrupt host immune responses are critical. The virulence determinants of pathogenic bacteria include the sensor/signaling proteins of the serine/threonine protein kinase (STPK) family that have a dual role of sensing the environment and subverting specific host defense processes. STPKs can sense a wide range of signals and coordinate multiple cellular processes to mount an appropriate response. Here, we review some of the well studied bacterial STPKs that are essential virulence factors and that modify global host responses during infection.

Identification of the Staphylococcus aureus vfrAB operon, a novel virulence factor regulatory locus

During a screen of the Nebraska Transposon Mutant Library, we identified 71 mutations in the Staphylococcus aureus genome that altered hemolysis on blood agar medium. Although many of these mutations disrupted genes known to affect the production of alpha-hemolysin, two of them were associated with an apparent operon, designated vfrAB, that had not been characterized previously. Interestingly, a ΔvfrB mutant exhibited only minor effects on the transcription of the hla gene, encoding alpha-hemolysin, when grown in broth, as well as on RNAIII, a posttranscriptional regulatory RNA important for alpha-hemolysin translation, suggesting that VfrB may function at the posttranscriptional level. Indeed, a ΔvfrB mutant had increased aur and sspAB protease expression under these conditions. However, disruption of the known secreted proteases in the ΔvfrB mutant did not restore hemolytic activity in the ΔvfrB mutant on blood agar. Further analysis revealed that, in contrast to the minor effects of VfrB on hla transcription when strains were cultured in liquid media, the level of hla transcription was decreased 50-fold in the absence of VfrB on solid media. These results demonstrate that while VfrB represses protease expression when strains are grown in broth, hla regulation is highly responsive to factors associated with growth on solid media. Intriguingly, the ΔvfrB mutant displayed increased pathogenesis in a model of S. aureus dermonecrosis, further highlighting the complexity of VfrB-dependent virulence regulation. The results of this study describe a phenotype associated with a class of highly conserved yet uncharacterized proteins found in Gram-positive bacteria, and they shed new light on the regulation of virulence factors necessary for S. aureus pathogenesis.

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.

Protecting against post-influenza bacterial pneumonia by increasing phagocyte recruitment and ROS production

Seasonal and especially pandemic influenza predispose patients to secondary bacterial pneumonias, which are a major cause of deaths and morbidity. Staphylococcus aureus is a particularly common and deadly form of post-influenza pneumonia, and increasing staphylococcal drug resistance makes the development of new therapies urgent. We explored an innate immune-mediated model of the lung to define novel mechanisms by which the host can be protected against secondary staphylococcal pneumonia after sub-lethal influenza infection. We found that stimulating the innate immunity in the lung by overexpression of GM-CSF will result in resistance to S. aureus pneumonia after sublethal influenza infection. Resistance was mediated by alveolar macrophages and neutrophils, and was associated with increased production of reactive oxygen species (ROS) by alveolar macrophages. Resistance was abrogated by treatment with agents that scavenged ROS. We conclude that stimulating innate immunity in the lung markedly reduces susceptibility to post-influenza staphylococcal pneumonia and that this may represent a novel immunomodulatory strategy for prevention and treatment of secondary bacterial pneumonia after influenza.

Assembly of NADPH oxidase in human neutrophils is modulated by the opacity-associated protein expression State of Neisseria gonorrhoeae

Neisseria gonorrhoeae (the gonococcus, Gc) triggers a potent inflammatory response and recruitment of neutrophils to the site of infection. Gc survives exposure to neutrophils despite these cells' antimicrobial products, such as reactive oxygen species (ROS). ROS production in neutrophils is initiated by NADPH oxidase, which converts oxygen into superoxide. The subunits of NADPH oxidase are spatially separated between granules (gp91(phox)/p22(phox)) and the cytoplasm (p47(phox), p67(phox), and p40(phox)). Activation of neutrophils promotes the coassembly of NADPH oxidase subunits at phagosome and/or plasma membranes. While Gc-expressing opacity-associated (Opa) proteins can induce neutrophils to produce ROS, Opa-negative (Opa-) Gc does not stimulate neutrophil ROS production. Using constitutively Opa- and OpaD-positive (OpaD+) Gc bacteria in strain FA1090, we now show that the difference in ROS production levels in primary human neutrophils between these backgrounds can be attributed to differential assembly of NADPH oxidase. Neutrophils infected with Opa- Gc showed limited translocation of NADPH oxidase cytoplasmic subunits to cellular membranes, including the bacterial phagosome. In contrast, these subunits rapidly translocated to neutrophil membranes following infection with OpaD+ Gc. gp91(phox) and p22(phox) were recruited to Gc phagosomes regardless of bacterial Opa expression. These results suggest that Opa- Gc interferes with the recruitment of neutrophil NADPH oxidase cytoplasmic subunits to membranes, in particular, the p47(phox) "organizing" subunit, to prevent assembly of the holoenzyme, resulting in an absence of the oxidative burst.

Phenol-soluble modulins: novel virulence-associated peptides of staphylococci

 Phenol-soluble modulins (PSMs), a novel class of small peptides with an amphipathic α-helical structure and strong surfactant-like properties, are produced by most staphylococci, especially pathogenic Staphylococcus aureus and Staphylococcus epidermidis. PSMs can: induce the production of proinflammatory cytokines; recruit, activate and lyse neutrophils to help staphylococci evade immune damage; lyse erythrocytes and are associated with the hemolysis of staphylococcal disease; facilitate the structuring and detachment of staphylococcal biofilms and disseminate biofilm-associated infection; and kill competing microbes and act as weapons in interbacterial warfare. Therefore, PSMs are considered to be critical virulence-associated factors and to play important roles in the pathogenesis of staphylococci. This review summarizes the classification, structure, expression regulation and biological functions of PSMs. The possible means to prevent PSM-associated diseases are also outlined in order to emphasize the need to investigate PSMs as potential targets for drug and vaccine design against staphylococcal infections.

Expression, purification, crystallization and preliminary X-ray analysis of the receiver domain of Staphylococcus aureus LytR protein

The response-regulatory protein LytR belongs to a family of transcription factors involved in the regulation of important virulence factors in pathogenic bacteria. The protein consists of a receiver domain and an effector domain, which play an important role in controlled cell death and lysis. The LytR receiver domain (LytR(N)) has been overexpressed, purified and crystallized using the sitting-drop and hanging-drop vapour-diffusion methods. The crystals grew as needles, with unit-cell parameters a = b = 84.82, c = 157.3 Å, α = β = 90, γ = 120°. LytR(N) crystallized in space group P6122 and the crystals diffracted to a maximum resolution of 2.34 Å. Based on the Matthews coefficient (V(M) = 5.44 Å(3) Da(-1)), one molecule is estimated to be present in the asymmetric unit.