Protein A suppresses immune responses during Staphylococcus aureus bloodstream infection in guinea pigs

Determinants of bacterial survival and proliferation in blood

Bloodstream infection is a major public health concern associated with high mortality and high healthcare costs worldwide. Bacteremia can trigger fatal sepsis whose prevention, diagnosis, and management have been recognized as a global health priority by the World Health Organization. Additionally, infection control is increasingly threatened by antimicrobial resistance, which is the focus of global action plans in the framework of a One Health response. In-depth knowledge of the infection process is needed to develop efficient preventive and therapeutic measures. The pathogenesis of bloodstream infection is a dynamic process resulting from the invasion of the vascular system by bacteria, which finely regulate their metabolic pathways and virulence factors to overcome the blood immune defenses and proliferate. In this review, we highlight our current understanding of determinants of bacterial survival and proliferation in the bloodstream and discuss their interactions with the molecular and cellular components of blood.

Vaccination with staphylococcal protein A protects mice against systemic complications of skin infection recurrences

Skin and soft tissue infections (SSTIs) are the most common diseases caused by Staphylococcus aureus (S. aureus), which can progress to threatening conditions due to recurrences and systemic complications. Staphylococcal protein A (SpA) is an immunomodulator antigen of S. aureus, which allows bacterial evasion from the immune system by interfering with different types of immune responses to pathogen antigens. Immunization with SpA could potentially unmask the pathogen to the immune system, leading to the production of antibodies that can protect from a second encounter with S. aureus, as it occurs in skin infection recurrences. Here, we describe a study in which mice are immunized with a mutated form of SpA mixed with the Adjuvant System 01 (SpAmut/AS01) before a primary S. aureus skin infection. Although mice are not protected from the infection under these conditions, they are able to mount a broader pathogen-specific functional immune response that results in protection against systemic dissemination of bacteria following an S. aureus second infection (recurrence). We show that this "hidden effect" of SpA can be partially explained by higher functionality of induced anti-SpA antibodies, which promotes better phagocytic activity. Moreover, a broader and stronger humoral response is elicited against several S. aureus antigens that during an infection are masked by SpA activity, which could prevent S. aureus spreading from the skin through the blood.

The characteristics of pre-existing humoral imprint determine efficacy of S. aureus vaccines and support alternative vaccine approaches

The failure of the Staphylococcus aureus (SA) IsdB vaccine trial can be explained by the recall of non-protective immune imprints from prior SA exposure. Here, we investigate natural human SA humoral imprints to understand their broader impact on SA immunizations. We show that antibody responses against SA cell-wall-associated antigens (CWAs) are non-opsonic, while antibodies against SA toxins are neutralizing. Importantly, the protective characteristics of the antibody imprints accurately predict the failure of corresponding vaccines against CWAs and support vaccination against toxins. In passive immunization platforms, natural anti-SA human antibodies reduce the efficacy of the human monoclonal antibodies suvratoxumab and tefibazumab, consistent with the results of their respective clinical trials. Strikingly, in the absence of specific humoral memory responses, active immunizations are efficacious in both naive and SA-experienced mice. Overall, our study points to a practical and predictive approach to evaluate and develop SA vaccines based on pre-existing humoral imprint characteristics.

Staphylococcal protein A-modified hydrogel facilitates in situ immunomodulation by capturing anti-HMGB1 for islet grafts

Islet transplantation is regarded as the most promising therapy for type 1 diabetes. However, both hypoxia and immune attack impair the grafted islets after transplantation, eventually failing the islet graft. Although many studies showed that biomaterials with nanoscale pores, like hydrogels, could protect islets from immune cells, the pores on biomaterials inhibited vascular endothelial cells (VECs) to creep in, which resulted in poor revascularization. Thus, a hydrogel device that can facilitate in situ immune modulations without the cost of poor revascularization should be put forward. Accordingly, we designed a spA-modified hydrogel capturing anti-HMGB1 mAB (mAB-spA Gel): the Staphylococcus aureus protein A (spA) was conjugated on the network of hydrogel to capture anti-HMGB1mAB which can inactivate immune cells, while the pore sizes of the hydrogel were more than 100μm which allows vascular endothelial cells (VECs) to creep in. In this study, we screened the optimal spA concentration in mAB-spA Gel according to the physical properties and antibody binding capability, then demonstrated that it could facilitate in situ immunomodulation without decreasing the vessel reconstruction in vitro. Further, we transplanted islet graft in vivo and showed that the survival of islets was elongated. In conclusion, mAB-spA Gel provided an alternative islet encapsulation strategy for type 1 diabetes. STATEMENT OF SIGNIFICANCE: Although various studies have shown that the backbone of the hydrogels can isolate islets grafts from immune cells and the survival of the islets can be prolonged by this way, it is also reported that when the pore size of the backbone is too small the revascularization will be adversely affected. According to this point, it is hard to adjust hydrogel's pore size to protect the islets from the immune attack while allowing endothelial vascular cells to creep in. To solve this dilemma, we designed an immunomodulatory hydrogel inhibiting the activation of T cells by immunosuppressive IgGs instead of the backbone network, so the hydrogel can prolong the survival of islets without the sacrifice of revascularization.

Functional diversity of staphylococcal surface proteins at the host-microbe interface

Surface proteins of Gram-positive pathogens are key determinants of virulence that substantially shape host-microbe interactions. Specifically, these proteins mediate host invasion and pathogen transmission, drive the acquisition of heme-iron from hemoproteins, and subvert innate and adaptive immune cell responses to push bacterial survival and pathogenesis in a hostile environment. Herein, we briefly review and highlight the multi-facetted roles of cell wall-anchored proteins of multidrug-resistant Staphylococcus aureus, a common etiological agent of purulent skin and soft tissue infections as well as severe systemic diseases in humans. In particular, we focus on the functional diversity of staphylococcal surface proteins and discuss their impact on the variety of clinical manifestations of S. aureus infections. We also describe mechanistic and underlying principles of staphylococcal surface protein-mediated immune evasion and coupled strategies S. aureus utilizes to paralyze patrolling neutrophils, macrophages, and other immune cells. Ultimately, we provide a systematic overview of novel therapeutic concepts and anti-infective strategies that aim at neutralizing S. aureus surface proteins or sortases, the molecular catalysts of protein anchoring in Gram-positive bacteria.

Engineered human antibodies for the opsonization and killing of Staphylococcus aureus

Staphylococcus aureus invariably acquires resistance mechanisms against new antibiotics. The persistent colonization with S. aureus is the key risk factor for invasive disease and a driver for the evolution of antibiotic resistant isolates. Anti-S. aureus antibodies that could promote decolonization, prevent infection, or treat disease would alleviate the selection for drug resistance. The successful development of such antibodies is complicated by Staphylococcal protein A (SpA) in the envelope of S. aureus. SpA captures immunoglobulins via their constant region, preventing antibodies from initiating anti-staphylococcal activities. Here, we demonstrate that therapeutic anti-S. aureus antibodies can be engineered to avoid sequestration by SpA. Such antibodies display extended half-lives and improve bacterial uptake and killing by immune cells.

Gram-positive organisms with their thick envelope cannot be lysed by complement alone. Nonetheless, antibody-binding on the surface can recruit complement and mark these invaders for uptake and killing by phagocytes, a process known as opsonophagocytosis. The crystallizable fragment of immunoglobulins (Fcγ) is key for complement recruitment. The cell surface of S. aureus is coated with Staphylococcal protein A (SpA). SpA captures the Fcγ domain of IgG and interferes with opsonization by anti-S. aureus antibodies. In principle, the Fcγ domain of therapeutic antibodies could be engineered to avoid the inhibitory activity of SpA. However, the SpA-binding site on Fcγ overlaps with that of the neonatal Fc receptor (FcRn), an interaction that is critical for prolonging the half-life of serum IgG. This evolutionary adaptation poses a challenge for the exploration of Fcγ mutants that can both weaken SpA–IgG interactions and retain stability. Here, we use both wild-type and transgenic human FcRn mice to identify antibodies with enhanced half-life and increased opsonophagocytic killing in models of S. aureus infection and demonstrate that antibody-based immunotherapy can be improved by modifying Fcγ. Our experiments also show that by competing for FcRn-binding, staphylococci effectively reduce the half-life of antibodies during infection. These observations may have profound impact in treating cancer, autoimmune, and asthma patients colonized or infected with S. aureus and undergoing monoclonal antibody treatment.

Multi-Antigen Outer Membrane Vesicle Engineering to Develop Polyvalent Vaccines: The Staphylococcus aureus Case

Modification of surface antigens and differential expression of virulence factors are frequent strategies pathogens adopt to escape the host immune system. These escape mechanisms make pathogens a "moving target" for our immune system and represent a challenge for the development of vaccines, which require more than one antigen to be efficacious. Therefore, the availability of strategies, which simplify vaccine design, is highly desirable. Bacterial Outer Membrane Vesicles (OMVs) are a promising vaccine platform for their built-in adjuvanticity, ease of purification and flexibility to be engineered with foreign proteins. However, data on if and how OMVs can be engineered with multiple antigens is limited. In this work, we report a multi-antigen expression strategy based on the co-expression of two chimeras, each constituted by head-to-tail fusions of immunogenic proteins, in the same OMV-producing strain. We tested the strategy to develop a vaccine against Staphylococcus aureus, a Gram-positive human pathogen responsible for a large number of community and hospital-acquired diseases. Here we describe an OMV-based vaccine in which four S. aureus virulent factors, ClfA_Y338A, LukE, SpA_KKAA and Hla_H35L have been co-expressed in the same OMVs (CLSH-OMVs_Δ60). The vaccine elicited antigen-specific antibodies with functional activity, as judged by their capacity to promote opsonophagocytosis and to inhibit Hla-mediated hemolysis, LukED-mediated leukocyte killing, and ClfA-mediated S. aureus binding to fibrinogen. Mice vaccinated with CLSH-OMVs_Δ60 were robustly protected from S. aureus challenge in the skin, sepsis and kidney abscess models. This study not only describes a generalized approach to develop easy-to-produce and inexpensive multi-component vaccines, but also proposes a new tetravalent vaccine candidate ready to move to development.

Staphylococcus aureus infections in children

PURPOSE OF REVIEW

Staphylococcus aureus is the most common invasive bacterial pathogen infecting children in the U.S. and many parts of the world. This major human pathogen continues to evolve, and recognition of recent trends in epidemiology, therapeutics and future horizons is of high importance.

RECENT FINDINGS

Over the past decade, a relative rise of methicillin-susceptible S. aureus (MSSA) has occurred, such that methicillin-resistant S. aureus (MRSA) no longer dominates the landscape of invasive disease. Antimicrobial resistance continues to develop, however, and novel therapeutics or preventive modalities are urgently needed. Unfortunately, several recent vaccine attempts proved unsuccessful in humans.

SUMMARY

Recent scientific breakthroughs highlight the opportunity for novel interventions against S. aureus by interfering with virulence rather than by traditional antimicrobial mechanisms. A S. aureus vaccine remains elusive; the reasons for this are multifactorial, and lessons learned from prior unsuccessful attempts may create a path toward an effective preventive. Finally, new diagnostic modalities have the potential to greatly enhance clinical care for invasive S. aureus disease in children.

Orientin mediates protection against MRSA-induced pneumonia by inhibiting Sortase A

Drug-resistant pathogenic Staphylococcus aureus (S. aureus) has severely threatened human health and arouses widespread concern. Sortase A (SrtA) is an essential virulence factor of S. aureus, which is responsible for the covalent anchoring of a variety of virulence-related proteins to the cell wall. SrtA has always been regarded as an ideal pharmacological target against S. aureus infections. In this research, we have determined that orientin, a natural compound isolated from various medicinal plants, can effectively inhibit the activity of SrtA with an IC₅₀ of 50.44 ± 0.51 µM. We further demonstrated that orientin inhibited the binding of S. aureus to fibrinogen and diminished biofilm formation and the attaching of Staphylococcal protein A (SpA) to the cell wall in vitro. Using the fluorescence quenching assay, we demonstrated a direct interaction between orientin and SrtA. Further mechanistic studies revealed that the residues Glu-105, Thr-93, and Cys-184 were the key sites for the binding of SrtA to orientin. Importantly, we demonstrated that treatment with orientin attenuated S. aureus virulence of in vivo and protected mice against S. aureus-induced lethal pneumonia. These findings indicate that orientin is a potential drug to counter S. aureus infections and limit the development of drug resistance.

Staphylococcal Protein A Induces Leukocyte Necrosis by Complexing with Human Immunoglobulins

One of the defining features of Staphylococcus aureus is its ability to evade and impair the human immune response through expression of staphylococcal protein A (SpA). Herein, we describe a previously unknown mechanism by which SpA can form toxic immune complexes when in the presence of human serum, which leads to the loss of human leukocytes. Further, we demonstrate that these toxic complexes are formed specifically through SpA's interaction with intact human IgG and that, in the presence of purified IgG Fab and Fc fragments, SpA shows no such toxicity. The mechanism of action of this toxicity appears to be one mediated by necrosis and not by apoptosis, as previously hypothesized, with up to 90% of human B cells rapidly becoming necrotic following stimulation with SpA-IgG complexes. This phenomenon depends on the immunoglobulin binding capacity of SpA, as a nonbinding mutant of SpA did not induce necrosis. Importantly, immune sera raised against SpA had the capacity to significantly reduce the observed toxicity. An unprecedented toxic effect of SpA-IgG complexes on monocytes was also observed, suggesting the existence of a novel mechanism independent from the interaction of SpA with the B cell receptor. Together, these data implicate SpA in inducing indiscriminate leukocyte toxicity upon formation of complexes with IgG and highlight the requirement for vaccination strategies to inhibit this mechanism. IMPORTANCE Staphylococcus aureus is one of the largest health care threats faced by humankind, with a reported mortality rate within the United States greater than that of HIV/AIDS, tuberculosis, and viral hepatitis combined. One of the defining features of S. aureus as a human pathogen is its ability to evade and impair the human immune response through expression of staphylococcal protein A. Herein, we show that SpA induces necrosis in various immune cells by complexing with human immunoglobulins. Vaccination of mice with a nontoxigenic SpA mutant induced sera capable of inhibiting this mechanism. These observations shed new light on the toxic mechanisms of this key staphylococcal virulence factor and on protective modalities of SpA-based vaccination.

A protein A based Staphylococcus aureus vaccine with improved safety

Exposure to Staphylococcus aureus does not lead to immunity as evidenced by the persistent colonization of one third of the human population. S. aureus immune escape is mediated by factors that preempt complement activation, destroy phagocytes, and modify B and T cell responses. One such factor, Staphylococcal protein A (SpA) encompasses five Immunoglobulin binding domains (IgBDs) that associate with the Fcγ domain to block phagocytosis. IgBDs also associate with Fab encoded by V_H3 clan related genes. SpA binding to V_H3-IgM that serves as a B cell receptor results in B cell expansion and secretion of antibodies with no specificity for S. aureus. SpA crosslinking of V_H3-IgG and V_H3-IgE bound to cognate receptors of mast cells and basophils promotes histamine release and anaphylaxis. Earlier work developed a prototype variant SpA_KKAA with four amino acid substitutions in each IgBD. When tested in animal models, SpA_KKAA elicited neutralizing antibodies and protection against infection. We show here that SpA_KKAA retains crosslinking activity for V_H3-IgG and V_H3-IgE. We use a rational approach to design and test 67 new SpA variants for loss of V_H3 binding and anaphylactic activities. We identify two detoxified candidates that elicit SpA-neutralizing antibodies and protect animals from S. aureus colonization and bloodstream infection. The new detoxified SpA candidates bear three instead of four amino acid substitutions thus increasing the development of SpA-specific antibodies. We propose that detoxified SpA variants unable to crosslink V_H3-idiotypic immunoglobulin may be suitably developed as clinical-grade vaccines for safety and efficacy testing in humans.

Adoptive Transfer of Serum Samples From Children With Invasive Staphylococcal Infection and Protection Against Staphylococcus aureus Sepsis

A successful Staphylococcus aureus vaccine remains elusive, and one controversy in the field is whether humans generate a protective adaptive immune response to infection. We developed a bacterial challenge murine assay that directly assesses the protective capacity of adoptively transferred human serum samples. We first validated the model by showing that postpneumococcal vaccine serum samples from humans induced effective clearance of Streptococcus pneumoniae in mice. We then found that human serum samples adoptively transferred from children with invasive S. aureus infections exhibited protection from disease in a murine model, with some samples conferring near complete protection. These findings demonstrate that human serum samples are capable of conferring a protective adaptive response generated by humans during invasive staphylococcal disease, allowing for the study of protective factors in a murine model. Identification of the protective factors present in the most efficacious serum samples would be of high interest as potential staphylococcal vaccine candidates or passive therapeutics.

Protein A of Staphylococcus aureus strain NCTC8325 interacted with heparin

Heparin, known for its anticoagulant activity, is commonly used as the coatings of medical devices. The attaching of Staphylococcus aureus, a prominent human and animal pathogen, to the heparin coatings usually leads to catheter-related bloodstream infections. Hence, the study of the interaction between heparin and S. aureus surface proteins is desired. Here, we found that protein A (SpA) of S. aureus was a heparin-binding protein, contributing to the interaction between S. aureus and heparin. The cell-wall-anchored SpA was one of the most critical S. aureus virulence factors with a lysin-like motif (LysM). When SpA was mutated to remove the LysM motif, the heparin-binding capability of SpA dropped 50%. The in-frame deletion of spa also reduced the heparin-binding capability of S. aureus. There was 1.3-fold more of heparin bound to wild type S. aureus than the Δspa::Em strain. These results would help understand the host-microbe interaction and the infection by S. aureus.

The Complex Diseases of Staphylococcus pseudintermedius in Canines: Where to Next?

Staphylococcus pseudintermedius is a pathogenic bacterium of concern within the veterinary sector and is involved in numerous infections in canines, including topical infections such as canine pyoderma and otitis externa, as well as systemic infections within the urinary, respiratory and reproductive tract. The high prevalence of methicillin-resistant Staphylococcus pseudintermedius (MRSP) within such infections is a growing concern. Therefore, it is crucial to understand the involvement of S. pseudintermedius in canine disease pathology to gain better insight into novel treatment avenues. Here, we review the literature focused on S. pseudintermedius infection in multiple anatomic locations in dogs and the role of MRSP in treatment outcomes at these niches. Multiple novel treatment avenues for MRSP have been pioneered in recent years and these are discussed with a specific focus on vaccines and phage therapy as potential therapeutic options. Whilst both undertakings are in their infancy, phage therapy is versatile and has shown high success in both animal and human medical use. It is clear that further research is required to combat the growing problems associated with MRSP in canines.

Glycosylation-dependent opsonophagocytic activity of staphylococcal protein A antibodies

All currently licensed antibodies against bacteria target exotoxins. For most pathogens, neutralization of toxin(s) is not sufficient to prevent bacterial replication. Antibodies against surface determinants represent better candidates to enhance opsonophagocytic killing, but the mechanisms of action of such antibodies have not been systematically studied. Staphylococcal protein A is a conserved surface protein of Staphylococcus aureus and a crucial virulence determinant that manipulates B-cell responses and blocks deposition of opsonin. Monoclonal antibodies directed against SpA represent potential therapeutic agents as well as a formidable tool to identify and optimize effector functions of antibodies that can promote bacterial clearance.

Antibodies may bind to bacterial pathogens or their toxins to control infections, and their effector activity is mediated through the recruitment of complement component C1q or the engagement with Fcγ receptors (FcγRs). For bacterial pathogens that rely on a single toxin to cause disease, immunity correlates with toxin neutralization. Most other bacterial pathogens, including Staphylococcus aureus, secrete numerous toxins and evolved multiple mechanisms to escape opsonization and complement killing. Several vaccine candidates targeting defined surface antigens of S. aureus have failed to meet clinical endpoints. It is unclear that such failures can be solely attributed to the poor selection of antibody targets. Thus far, studies to delineate antibody-mediated uptake and killing of Gram-positive pathogens remain extremely limited. Here, we exploit 3F6-hIgG1, a human monoclonal antibody that binds and neutralizes the abundant surface-exposed Staphylococcal protein A (SpA). We find that galactosylation of 3F6-hIgG1 that favors C1q recruitment is indispensable for opsonophagocytic killing of staphylococci and for protection against bloodstream infection in animals. However, the simple removal of fucosyl residues, which results in reduced C1q binding and increased engagement with FcγR, maintains the opsonophagocytic killing and protective attributes of the antibody. We confirm these results by engineering 3F6-hIgG1 variants with biased binding toward C1q or FcγRs. While the therapeutic benefit of monoclonal antibodies against infectious disease agents may be debatable, the functional characterization of such antibodies represents a powerful tool for the development of correlates of protection that may guide future vaccine trials.

Inhibition of Host Arginase Activity Against Staphylococcal Bloodstream Infection by Different Metabolites

Staphylococcus aureus is a notorious bacterial pathogen that often causes soft tissue and bloodstream infections and invariably garners resistance mechanisms against new antibiotics. Modulation of the host immune response by metabolites is a powerful tool against bacterial infections, but has not yet been used against S. aureus infections. In this study, we identified four metabolite biomarkers: L-proline, L-isoleucine, L-leucine, and L-valine (PILV), through a metabolomics study using animal models of S. aureus bloodstream infection. The exogenous administration of each metabolite or of PILV showed anti-infective effects, and a higher protection was achieved with PILV in comparison to individual metabolites. During the staphylococcal infection, the expression of most host arginase and nitric oxide synthase (NOS) isozymes was simultaneously induced in mouse liver, kidney, and blood samples. However, the induction of arginase isozymes was dramatically stronger than that of NOS isozymes. This elevated arginase activity was inhibited by the metabolite biomarkers thus killing S. aureus, and PILV exhibited the strongest inhibition of arginase activity and bacterial inhibition. The suppression of arginase activity also contributed to the metabolite-mediated phagocytic killing of S. aureus in mouse and human blood. Our findings demonstrate the metabolite-mediated arginase inhibition as a therapeutic intervention for S. aureus infection.

Characterisation of antibiotic resistance, virulence, clonality and mortality in MRSA and MSSA bloodstream infections at a tertiary-level hospital in Hungary: a 6-year retrospective study

BACKGROUND

Staphylococcus aureus bloodstream infections (BSI) cause significant morbidity and mortality due to the frequent antibiotic resistance, toxin and adhesin production of the bacterium. These characteristics differ significantly in methicillin resistant (MRSA) and methicillin sensitive S. aureus (MSSA) and also among isolates of different MRSA clones, contributing to the outcome of S. aureus bacteraemia.

METHODS

In this study, all MRSA BSI isolates from Semmelweis University, Budapest, Hungary, isolated between 2011-2016 and the same number of matched MSSA (overall 306 isolates) were characterised in terms of antibiotic susceptibility, virulence genes, clonality and their association with all-cause 30-day mortality. Effect of patient related variables, such as age, gender and comorbidities were also investigated.

RESULTS

ST22-MRSA-IV and ST5-MRSA-II were the most prevalent clones in our study. SCCmec I isolates showed the highest resistance rates and SCCmec II carried most virulence genes. Infections caused by SCCmec IV isolates were associated with the highest mortality rate (42.2%), despite the similar comorbidity rates of the different patient groups. All-cause 30-day mortality was 39.9% in the MRSA and 30.7% in the MSSA group. Increased teicoplanin MIC was associated with high mortality rate. Resistance to ciprofloxacin, erythromycin and clindamycin was common in MRSA, whereas MSSA isolates were more sensitive to all antibiotics with the exception of doxycycline. All MRSA isolates were sensitive to glycopeptides and linezolid; resistance to rifampicin and sulfamethoxazole-trimethoprim was low. MRSA isolates carried more adhesion genes, superantigens were more frequent in MSSA. Panton-Valentine leukocidin was found in 2.3% of the isolates.

CONCLUSION

This study provides insight into the clonal composition and associated mortality of BSI S. aureus isolates in Hungary. The results suggest that the outcome of the infection is determined by the antibiotic resistance, genotype of the bacterium, and patient-related factors; rather than the virulence factors carried by the bacteria.

Mechanisms of Immune Evasion and Bone Tissue Colonization That Make Staphylococcus aureus the Primary Pathogen in Osteomyelitis

PURPOSE OF REVIEW

Staphylococcus aureus is the primary pathogen responsible for osteomyelitis, which remains a major healthcare burden. To understand its dominance, here we review the unique pathogenic mechanisms utilized by S. aureus that enable it to cause incurable osteomyelitis.

RECENT FINDINGS

Using an arsenal of toxins and virulence proteins, S. aureus kills and usurps immune cells during infection, to produce non-neutralizing pathogenic antibodies that thwart adaptive immunity. S. aureus also has specific mechanisms for distinct biofilm formation on implants, necrotic bone tissue, bone marrow, and within the osteocyte lacuno-canicular networks (OLCN) of live bone. In vitro studies have also demonstrated potential for intracellular colonization of osteocytes, osteoblasts, and osteoclasts. S. aureus has evolved a multitude of virulence mechanisms to achieve life-long infection of the bone, most notably colonization of OLCN. Targeting S. aureus proteins involved in these pathways could provide new targets for antibiotics and immunotherapies.

Absence of Protein A Expression Is Associated With Higher Capsule Production in Staphylococcal Isolates

Staphylococcus aureus is a major human pathogen, and a leading cause of soft tissue and blood stream infections. One of the causes of its success as a pathogen is the peculiar array of immune evasion factors through which the bacterium avoids host defenses, where the staphylococcal protein A (SpA) plays a major role thanks to its IgG binding activities. Moreover, SpA has recently been proposed as a promising vaccine antigen. In this study, we evaluated the expression of SpA in a collection of staphylococcal strains, about 7% of which did not express SpA (SpA^- strains), despite the presence of the gene. By a comparative genomic analysis, we identified that a mutation in the spa 5′ UTR sequence affecting the RBS is responsible for the loss of SpA in a subset of SpA^- strains. Using a high-throughput qRT-PCR approach on a selected panel of virulence-related genes, we identified that the SpA^- phenotype is associated with lower spa transcript levels and increased expression and production of capsule as well as other changes in the transcription of several key virulence factors. Our data suggest that the SpA^- phenotype has occurred in geographically distinct strains through different molecular mechanisms including both mutation, leading likely to translation alterations, and transcriptional deregulation. Furthermore, we provide evidence that SpA^- strains are highly susceptible to phagocytic uptake mediated by anti-capsule antibodies. These data suggest that S. aureus may alter its virulence factor expression pattern as an adaptation to the host or environment. Vaccination strategies targeting both SpA and capsule could therefore result in broader coverage against staphylococcal isolates than SpA alone.

Immune Evasion by Staphylococcus aureus

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

Sortases, Surface Proteins, and Their Roles in Staphylococcus aureus Disease and Vaccine Development

Sortases cleave short peptide motif sequences at the C-terminal end of secreted surface protein precursors and either attach these polypeptides to the peptidoglycan of Gram-positive bacteria or promote their assembly into pilus structures that are also attached to peptidoglycan. Sortase A, the enzyme first identified in the human pathogen Staphylococcus aureus, binds LPXTG motif sorting signals, cleaves between threonine (T) and glycine (G) residues, and forms an acyl enzyme between its active-site cysteine thiol and the carboxyl group of threonine (T). Sortase A acyl enzyme is relieved by the nucleophilic attack of the cross bridge amino group within lipid II, thereby generating surface protein linked to peptidoglycan precursor. Such products are subsequently incorporated into the cell wall envelope by enzymes of the peptidoglycan synthesis pathway. Surface proteins linked to peptidoglycan may be released from the bacterial envelope to diffuse into host tissues and fulfill specific biological functions. S. aureus sortase A is essential for host colonization and for the pathogenesis of invasive diseases. Staphylococcal sortase-anchored surface proteins fulfill key functions during the infectious process, and vaccine-induced antibodies targeting surface proteins may provide protection against S. aureus. Alternatively, small-molecule inhibitors of sortase may be useful agents for the prevention of S. aureus colonization and invasive disease.

Vaccination with the Staphylococcus aureus secreted proteins EapH1 and EapH2 impacts both S. aureus carriage and invasive disease

INTRODUCTION

There is a need for an efficacious vaccine reducing infections due to Staphylococcus aureus, a common cause of community and hospital infection. Infecting organisms originate from S. aureus populations colonising the nares and bowel. Antimicrobials are widely used to transiently reduce S. aureus colonisation prior to surgery, a practice which is selecting for resistant S. aureus isolates. S. aureus secretes multiple proteins, including the protease inhibitors extracellular adhesion protein homologue 1 and 2 (EapH1 and EapH2).

METHODS

Mice were vaccinated intramuscularly or intranasally with Adenovirus serotype 5 and Modified Vaccinia Ankara viral vectors expressing EapH1 and EapH2 proteins, or with control viruses. Using murine S. aureus colonisation models, we monitored S. aureus colonisation by sequential stool sampling. Monitoring of S. aureus invasive disease after intravenous challenge was performed using bacterial load and abscess numbers in the kidney.

RESULTS

Intramuscular vaccination with Adenovirus serotype 5 and Modified Vaccinia Ankara viral vectors expressing EapH1 and EapH2 proteins significantly reduces bacterial recovery in the murine renal abscess model of infection, but the magnitude of the effect is small. A single intranasal vaccination with an adenoviral vaccine expressing these proteins reduced S. aureus gastrointestinal (GI) tract colonisation.

CONCLUSION

Vaccination against EapH1 / EapH2 proteins may offer an antibiotic independent way to reduce S. aureus colonisation, as well as contributing to protection against S. aureus invasive disease.

Characterization of recombinant wild-type and nontoxigenic protein A from Staphylococcus pseudintermedius

Background:Staphylococcus pseudintermedius is an opportunistic pathogen that is the major cause of pyoderma affecting dogs. Conventional antimicrobial treatment for infections caused by this organism have failed in recent years due to widespread resistance and alternative treatment strategies are a high priority. Protein A encoded in Staphylococcus aureus by spa protects the bacterium by binding IgG and acts as a superantigen. Staphylococcus pseudintermedius possess two genes orthologous to S. aureus spa, spsP, and spsQ. Methods: SpsQ and SpsQ-M, a non-toxigenic SpsQ, were cloned and expressed as recombinant proteins and their cytotoxic effect on canine B cells was measured. The neutralizing ability of antibody raised against them in clinically healthy dogs was evaluated. Results:S. pseudintermedius SpsQ induced apoptosis of canine B cells. Specific amino acid substitutions diminished SpsQ-M binding to immunoglobulin and its super-antigenic activity, while its antigenicity was maintained. This recombinant, non-toxigenic S. pseudintermedius SpsQ stimulated the production of antibodies in dogs that specifically reacted with SpsQ and greatly diminished its cytotoxic effect on canine B cells. Conclusions: The production of neutralizing antibody suggests that attenuated, non-toxic SpsQ produced in this study is a good candidate for inclusion in a vaccine for use in the treatment and prevention of S. pseudintermedius infections.

Abbreviations: SpA: Staphylococcus aureus protein A; SpsP: Staphylococcus pseudintermedius protein A; SpsQ: Staphylococcus pseudintermedius protein A; SpsQ-M: attenuated Staphylococcus pseudintermedius protein A; MRSP: methicillin resistant Staphylococcus pseudintermedius; IgA: immunoglobulin A; IgG: immunoglobulin G; IgM: immunoglobulin M; VH: variable region of immunoglobulin heavy chain; IgBD: immunoglobulin binding domains; MFI: mean fluorescent intensity; SEM: standard error of the mean; PBMC: Peripheral blood mononuclear cells; CD21: complement receptor type 2; ST: Sequence type; OD: Optical density; ORF: open reading frame; PBS: Phosphate buffered saline; Tween 20: Polyethylene glycol sorbitan monolaurate 20; HRP: horseradish peroxidase; TMB- 3,3',5,5'-Tetramethylbenzidine

Animal and Human Tissue Models of Vertical Listeria monocytogenes Transmission and Implications for Other Pregnancy-Associated Infections

Intrauterine infections lead to serious complications for mother and fetus, including preterm birth, maternal and fetal death, and neurological sequelae in the surviving offspring. Improving maternal and child heath is a global priority. Yet, the development of strategies to prevent and treat pregnancy-related diseases has lagged behind progress made in other medical fields. One of the challenges is finding tractable model systems that replicate the human maternal-fetal interface. Animal models offer the ability to study pathogenesis and host defenses in vivo However, the anatomy of the maternal-fetal interface is highly divergent across species. While many tools are available to study host responses in the pregnant mouse model, other animals have placentas that are more similar to that of humans. Here we describe new developments in animal and human tissue models to investigate the pathogenesis of listeriosis at the maternal-fetal interface. We highlight gaps in existing knowledge and make recommendations on how they can be filled.

Staphylococcal Protein A Contributes to Persistent Colonization of Mice with Staphylococcus aureus

Staphylococcus aureus persistently colonizes the nasopharynx in humans, which increases the risk for invasive diseases, such as skin infection and bacteremia. Nasal colonization triggers IgG responses against staphylococcal surface antigens; however, these antibodies cannot prevent subsequent colonization or disease. Here, we describe S. aureus WU1, a multilocus sequence type 88 (ST88) isolate that persistently colonizes the nasopharynx in mice. We report that staphylococcal protein A (SpA) is required for persistence of S. aureus WU1 in the nasopharynx. Compared to animals colonized by wild-type S. aureus, mice colonized with the Δspa variant mount increased IgG responses against staphylococcal colonization determinants. Immunization of mice with a nontoxigenic SpA variant, which cannot cross-link B cell receptors and divert antibody responses, elicits protein A-neutralizing antibodies that promote IgG responses against colonizing S. aureus and diminish pathogen persistence.IMPORTANCE Staphylococcus aureus persistently colonizes the nasopharynx in about one-third of the human population, thereby promoting community- and hospital-acquired infections. Antibiotics are currently used for decolonization of individuals at increased risk of infection. However, the efficacy of antibiotics is limited by recolonization and selection for drug-resistant strains. Here, we propose a model of how staphylococcal protein A (SpA), a B cell superantigen, modifies host immune responses during colonization to support continued persistence of S. aureus in the nasopharynx. We show that this mechanism can be thwarted by vaccine-induced anti-SpA antibodies that promote IgG responses against staphylococcal antigens and diminish colonization.

Targeting fundamental pathways to disrupt Staphylococcus aureus survival: clinical implications of recent discoveries

The emergence of community-associated methicillin-resistant Staphylococcus aureus during the past decade along with an impending shortage of effective antistaphylococcal antibiotics have fueled impressive advances in our understanding of how S. aureus overcomes the host environment to establish infection. Backed by recent technologic advances, studies have uncovered elaborate metabolic, nutritional, and virulence strategies deployed by S. aureus to survive the restrictive and hostile environment imposed by the host, leading to a plethora of promising antimicrobial approaches that have potential to remedy the antibiotic resistance crisis. In this Review, we highlight some of the critical and recently elucidated bacterial strategies that are potentially amenable to intervention, discuss their relevance to human diseases, and address the translational challenges posed by current animal models.

The recombination dynamics of Staphylococcus aureus inferred from spA gene

Background

Given the role of spA as a pivotal virulence factor decisive for Staphylococcus aureus ability to escape from innate and adaptive immune responses, one can consider it as an object subject to adaptive evolution and that variations in spA may uncover pathogenicity variations.

Results

The population genetic structure was deduced from the extracellular domains of SpA gene sequence (domains A-E and the X-region) and compared to the MLST-analysis of 41 genetically diverse methicillin-resistant (MRSA) and methicillin-susceptible (MSSA) S. aureus strains. Incongruence between tree topologies was noticeable and in the inferred spA tree most MSSA isolates were clustered in a distinct group. Conversely, the distribution of strains according to their spA-type was not always congruent with the tree inferred from the complete spA gene foreseeing that spA is a mosaic gene composed of different segments exhibiting different evolutionary histories. Evidences of a network-like organization were identified through several conflicting phylogenetic signals and indeed several intragenic recombination events (within subdomains of the gene) were detected within and between CC’s of MRSA strains. The alignment of SpA sequences enabled the clustering of several isoforms as a result of non-randomly distributed amino acid variations, located in two clusters of polymorphic sites in domains D to B and Xr (a). Nevertheless, evidences of cluster specific structural arrangements were detected reflecting alterations on specific residues with potential impact on S. aureus pathogenicity.

Conclusions

The detection of positive selection operating on spA combined with frequent non-synonymous mutations, domain duplication and frequent intragenic recombination events represent important mechanisms acting in the evolutionary adaptive mechanism promoting spA genetic plasticity. These findings argue that crucial allelic forms correlated with pathogenicity can be identified by sequences analysis enabling the design of more robust schemes.

Electronic supplementary material

The online version of this article (doi:10.1186/s12866-016-0757-9) contains supplementary material, which is available to authorized users.

Innate Immune Signaling Activated by MDR Bacteria in the Airway

Health care-associated bacterial pneumonias due to multiple-drug resistant (MDR) pathogens are an important public health problem and are major causes of morbidity and mortality worldwide. In addition to antimicrobial resistance, these organisms have adapted to the milieu of the human airway and have acquired resistance to the innate immune clearance mechanisms that normally prevent pneumonia. Given the limited efficacy of antibiotics, bacterial clearance from the airway requires an effective immune response. Understanding how specific airway pathogens initiate and regulate innate immune signaling, and whether this response is excessive, leading to host-induced pathology may guide future immunomodulatory therapy. We will focus on three of the most important causes of health care-associated pneumonia, Staphylococcus aureus, Pseudomonas aeruginosa, and Klebsiella pneumoniae, and review the mechanisms through which an inappropriate or damaging innate immune response is stimulated, as well as describe how airway pathogens cause persistent infection by evading immune activation.

Vaccines for Staphylococcus aureus and Target Populations

Staphylococcus aureus is a leading pathogen in surgical site, intensive care unit, and skin infections, as well as healthcare-associated pneumonias. These infections are associated with an enormous burden of morbidity, mortality, and increase of hospital length of stay and patient cost. S. aureus is impressively fast in acquiring antibiotic resistance, and multidrug-resistant strains are a serious threat to human health. Due to resistance or insufficient effectiveness, antibiotics and bundle measures leave a tremendous unmet medical need worldwide. There are no licensed vaccines on the market despite the significant efforts done by public and private initiatives. Indeed, vaccines tested in clinical trials in the last two decades have failed to show efficacy. However, they targeted single antigens and contained no adjuvants and efficacy trials were performed in severely ill subjects. Herein, we provide a comprehensive evaluation of potential target populations for efficacy trials taking into account key factors such as population size, incidence of S. aureus infection, disease outcome, primary endpoints, as well as practical advantages and disadvantages. We describe the whole-blood assay as a potential surrogate of protection, and we show the link between phase III clinical trial data of failed vaccines with their preclinical observations. Finally, we give our perspective on how new vaccine formulations and clinical development approaches may lead to successful S. aureus vaccines.