Staphylococcus aureus FhuD2 is involved in the early phase of staphylococcal dissemination and generates protective immunity in mice

Siderophore-dependent ferrichelatases

Iron is a crucial secondary metabolite for bacterial proliferation, but its bioavailability under infection conditions is limited by the low solubility of ferric ion and the host's ability to sequester iron by protein chelation. In these iron limiting conditions, bacteria produce and secrete low molecular weight ferric ion chelators, siderophores, to scavenge host iron. Iron bound siderophores are recognized by surface displayed receptors and internalized by active transport preceding the liberation of the iron payload by reduction or cleavage of the siderophore. The traditional paradigms surrounding the interactions between siderophores and their corresponding receptors have relied on canonical protein-ligand binding models that do not accurately reflect the conditions experienced by siderophore binding proteins (SBPs). Research by the Raymond group suggested that a ligand displacement model does not fully describe the role of SBPs in siderophore transport where the ferric ion can be shuttled between siderophore molecules during the transport process. This work inspired further research by the Wencewicz group, which demonstrated that the Staphylococcus aureus SBP FhuD2 can catalyze the transfer of iron from the biological iron source holo-transferrin to a SBP bound iron-free siderophore. The discovery of this ferrichelatase activity represents a novel mechanism of receptor mediated active transport which raises the question: is ferrichelatase activity a unique feature of FhuD2 or a previously unappreciated hallmark of SBPs? This chapter highlights a series of protocols for the general functional characterization of SBPs and methodologies to assay ferrichelatase activity with the hopes of providing the tools to answer this question.

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.

Non-protective immune imprint underlies failure of Staphylococcus aureus IsdB vaccine

Humans frequently encounter Staphylococcus aureus (SA) throughout life. Animal studies have yielded SA candidate vaccines, yet all human SA vaccine trials have failed. One notable vaccine "failure" targeted IsdB, critical for host iron acquisition. We explored a fundamental difference between humans and laboratory animals-natural SA exposure. Recapitulating the failed phase III IsdB vaccine trial, mice previously infected with SA do not mount protective antibody responses to vaccination, unlike naive animals. Non-protective antibodies exhibit increased α2,3 sialylation that blunts opsonophagocytosis and preferentially targets a non-protective IsdB domain. IsdB vaccination of SA-infected mice recalls non-neutralizing humoral responses, further reducing vaccine efficacy through direct antibody competition. IsdB vaccine interference was overcome by immunization against the IsdB heme-binding domain. Purified human IsdB-specific antibodies also blunt IsdB passive immunization, and additional SA vaccines are susceptible to SA pre-exposure. Thus, failed anti-SA immunization trials could be explained by non-protective imprint from prior host-SA interaction.

Sensitizing Staphylococcus aureus to antibacterial agents by decoding and blocking the lipid flippase MprF

The pandemic of antibiotic resistance represents a major human health threat demanding new antimicrobial strategies. MprF is the synthase and flippase of the phospholipid lysyl-phosphatidylglycerol that increases virulence and resistance of methicillin-resistant Staphylococcus aureus (MRSA) and other pathogens to cationic host defense peptides and antibiotics. With the aim to design MprF inhibitors that could sensitize MRSA to antimicrobial agents and support the clearance of staphylococcal infections with minimal selection pressure, we developed MprF-targeting monoclonal antibodies, which bound and blocked the MprF flippase subunit. Antibody M-C7.1 targeted a specific loop in the flippase domain that proved to be exposed at both sides of the bacterial membrane, thereby enhancing the mechanistic understanding of bacterial lipid translocation. M-C7.1 rendered MRSA susceptible to host antimicrobial peptides and antibiotics such as daptomycin, and it impaired MRSA survival in human phagocytes. Thus, MprF inhibitors are recommended for new anti-virulence approaches against MRSA and other bacterial pathogens.

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.

Role of Endosomal TLRs in Staphylococcus aureus Infection

Identification of the receptors involved in innate immune recognition of Staphylococcus aureus, a major cause of morbidity and mortality in humans, is essential to develop alternative strategies to treat infections caused by antibiotic-resistant strains. In the current study, we examine the role of endosomal TLRs, which sense the presence of prokaryotic-type nucleic acids, in anti-staphylococcal host defenses using infection models involving genetically defective mice. Single deficiencies in TLR7, 9, or 13 resulted in mild or no decrease in host defenses. However, the simultaneous absence of TLR7, 9, and 13 resulted in markedly increased susceptibility to cutaneous and systemic S. aureus infection concomitantly with decreased production of proinflammatory chemokines and cytokines, neutrophil recruitment to infection sites, and reduced production of reactive oxygen species. This phenotype was significantly more severe than that of mice lacking TLR2, which senses the presence of staphylococcal lipoproteins. Notably, the combined absence of TLR7, 9, and 13 resulted in complete abrogation of IL-12 p70 and IFN-β responses to staphylococcal stimulation in macrophages. Taken together, our data highlight the presence of a highly integrated endosomal detection system, whereby TLR7, 9, and 13 cooperate in sensing the presence of staphylococcal nucleic acids. We demonstrate that the combined absence of these receptors cannot be compensated for by cell surface-associated TLRs, such as TLR2, or cytosolic receptors. These data may be useful to devise strategies aimed at stimulating innate immune receptors to treat S. aureus infections.

Virulence Gene Expression of Staphylococcus aureus in Human Skin

Staphylococcus aureus is the main cause of human skin and soft tissue infections. However, S. aureus pathogenicity within the skin is not fully characterized. Here, we implemented an S. aureus cutaneous infection model using human skin explants and performed a time-course infection to study the gene expression profile of a large panel of virulence-related factors of S. aureus USA300 LAC strain, by high-throughput RT-PCR. We pinpointed the genes that were differentially regulated by the bacteria in the skin tissues and identified 12 virulence factors that were upregulated at all time points assessed. Finally, using confocal microscopy, we show that the expression of alpha-hemolysin by S. aureus varies dependent on the skin niche and that the bacteria preferentially accumulates inside sweat glands and ducts. Taken together, our study gives insights about the pathogenic lifestyle of S. aureus within human skin tissues, which may contribute for the development of anti-S. aureus therapeutic strategies.

Bacterial-fungal interactions revealed by genome-wide analysis of bacterial mutant fitness

Microbial interactions are expected to be major determinants of microbiome structure and function. Although fungi are found in diverse microbiomes, their interactions with bacteria remain largely uncharacterized. In this work, we characterize interactions in 16 different bacterial-fungal pairs, examining the impacts of 8 different fungi isolated from cheese rind microbiomes on 2 bacteria (Escherichia coli and a cheese-isolated Pseudomonas psychrophila). Using random barcode transposon-site sequencing with an analysis pipeline that allows statistical comparisons between different conditions, we observed that fungal partners caused widespread changes in the fitness of bacterial mutants compared to growth alone. We found that all fungal species modulated the availability of iron and biotin to bacterial species, which suggests that these may be conserved drivers of bacterial-fungal interactions. Species-specific interactions were also uncovered, a subset of which suggested fungal antibiotic production. Changes in both conserved and species-specific interactions resulted from the deletion of a global regulator of fungal specialized metabolite production. This work highlights the potential for broad impacts of fungi on bacterial species within microbiomes.

Iron Metabolism at the Interface between Host and Pathogen: From Nutritional Immunity to Antibacterial Development

Nutritional immunity is a form of innate immunity widespread in both vertebrates and invertebrates. The term refers to a rich repertoire of mechanisms set up by the host to inhibit bacterial proliferation by sequestering trace minerals (mainly iron, but also zinc and manganese). This strategy, selected by evolution, represents an effective front-line defense against pathogens and has thus inspired the exploitation of iron restriction in the development of innovative antimicrobials or enhancers of antimicrobial therapy. This review focuses on the mechanisms of nutritional immunity, the strategies adopted by opportunistic human pathogen Staphylococcus aureus to circumvent it, and the impact of deletion mutants on the fitness, infectivity, and persistence inside the host. This information finally converges in an overview of the current development of inhibitors targeting the different stages of iron uptake, an as-yet unexploited target in the field of antistaphylococcal drug discovery.

Bacterial outer membrane vesicles engineered with lipidated antigens as a platform for Staphylococcus aureus vaccine

Bacterial outer membrane vesicles (OMVs) represent an interesting vaccine platform for their built-in adjuvanticity and simplicity of production process. Moreover, OMVs can be decorated with foreign antigens using different synthetic biology approaches. However, the optimal OMV engineering strategy, which should guarantee the OMV compartmentalization of most heterologous antigens in quantities high enough to elicit protective immune responses, remains to be validated. In this work we exploited the lipoprotein transport pathway to engineer OMVs with foreign proteins. Using 5 Staphylococcus aureus protective antigens expressed in Escherichia coli as fusions to a lipoprotein leader sequence, we demonstrated that all 5 antigens accumulated in the vesicular compartment at a concentration ranging from 5 to 20% of total OMV proteins, suggesting that antigen lipidation could be a universal approach for OMV manipulation. Engineered OMVs elicited high, saturating antigen-specific antibody titers when administered to mice in quantities as low as 0.2 μg/dose. Moreover, the expression of lipidated antigens in E. coli BL21(DE3)ΔompAΔmsbBΔpagP was shown to affect the lipopolysaccharide structure, with the result that the TLR4 agonist activity of OMVs was markedly reduced. These results, together with the potent protective activity of engineered OMVs observed in mice challenged with S. aureus Newman strain, makes the 5-combo-OMVs a promising vaccine candidate to be tested in clinics.

Broad and Effective Protection against Staphylococcus aureus Is Elicited by a Multivalent Vaccine Formulated with Novel Antigens

The demand for a prophylactic vaccine against methicillin-resistant Staphylococcus aureus (MRSA) has motivated numerous dedicated research groups to design and develop such a vaccine. In this study, we have developed a multivalent vaccine, Sta-V5, composed of five conserved antigens involved in three important virulence mechanisms. This prototype vaccine conferred up to 100% protection against multiple epidemiologically relevant S. aureus isolates in five different murine disease models. The vaccine not only elicits functional antibodies that mediate opsonophagocytic killing of S. aureus but also mounts robust antigen-specific T-cell responses. In addition, our data implied that γδ T cells contribute to the protection induced by Sta-V5 in a murine skin infection model.IMPORTANCE Staphylococcus aureus infections, especially MRSA infections, are becoming a major global health issue and are resulting in mortality rates that are increasing every year. However, an effective vaccine is lacking due to the complexity of the infection process of S. aureus In this study, we found that the addition of two novel protein components to three well-studied vaccine candidates significantly improved the efficacy of the combined vaccine. Furthermore, the five-component vaccine not only elicits a robust antibody response but also induces cytokine secretion by T cells, making it a promising vaccine candidate to fill the void.

Comparative Extracellular Proteomics of Aeromonas hydrophila Reveals Iron-Regulated Secreted Proteins as Potential Vaccine Candidates

In our previous study, several iron-related outer membrane proteins in Aeromonas hydrophila, a serious pathogen of farmed fish, conferred high immunoprotectivity to fish, and were proposed as potential vaccine candidates. However, the protective efficacy of these extracellular proteins against A. hydrophila remains largely unknown. Here, we identified secreted proteins that were differentially expressed in A. hydrophila LP-2 in response to iron starvation using an iTRAQ-based quantitative proteomics method. We identified 341 proteins, of which 9 were upregulated in response to iron starvation and 24 were downregulated. Many of the differently expressed proteins were associated with protease activity. We confirmed our proteomics results with Western blotting and qPCR. We constructed three mutants by knocking out three genes encoding differentially expressed proteins (Δorf01830, Δorf01609, and Δorf03641). The physiological characteristics of these mutants were investigated. In all these mutant strains, protease activity decreased, and Δorf01609, and Δorf01830 were less virulent in zebrafish. This indicated that the proteins encoded by these genes may play important roles in bacterial infection. We next evaluated the immune response provoked by the six iron-related recombinant proteins (ORF01609, ORF01830, ORF01839, ORF02943, ORF03355, and ORF03641) in zebrafish as well as the immunization efficacy of these proteins. Immunization with these proteins significantly increased the zebrafish immune response. In addition, the relative percent survival (RPS) of the immunized zebrafish was 50-80% when challenged with three virulent A. hydrophila strains, respectively. Thus, these extracellular secreted proteins might be effective vaccine candidates against A. hydrophila infection in fish.

Release of Staphylococcus aureus extracellular vesicles and their application as a vaccine platform

Secretion of extracellular vesicles (EVs), a process common to eukaryotes, archae, and bacteria, represents a secretory pathway that allows cell-free intercellular communication. Microbial EVs package diverse proteins and influence the host-pathogen interaction, but the mechanisms underlying EV production in Gram-positive bacteria are poorly understood. Here we show that EVs purified from community-associated methicillin-resistant Staphylococcus aureus package cytosolic, surface, and secreted proteins, including cytolysins. Staphylococcal alpha-type phenol-soluble modulins promote EV biogenesis by disrupting the cytoplasmic membrane; whereas, peptidoglycan cross-linking and autolysin activity modulate EV production by altering the permeability of the cell wall. We demonstrate that EVs purified from a S. aureus mutant that is genetically engineered to express detoxified cytolysins are immunogenic in mice, elicit cytolysin-neutralizing antibodies, and protect the animals in a lethal sepsis model. Our study reveals mechanisms underlying S. aureus EV production and highlights the usefulness of EVs as a S. aureus vaccine platform.

Extracellular vesicles (EVs) influence host-pathogen interactions, but EV biogenesis in gram-positive bacteria remains elusive. Here authors characterize EVs from Staphylococcus aureus and show that phenol-soluble modulins and autolysins promote EV biogenesis by disrupting the membrane and cell wall.

In Vivo Analysis of Staphylococcus aureus-Infected Mice Reveals Differential Temporal and Spatial Expression Patterns of fhuD2

Staphylococcus aureus is an opportunistic human pathogen and a major cause of invasive infections such as bacteremia, endocarditis, pneumonia, and wound infections. FhuD2 is a staphylococcal lipoprotein involved in the uptake of iron-hydroxymate and is under the control of the iron uptake regulator Fur. This protein is part of an investigational multicomponent vaccine formulation that has shown protective efficacy in several murine models of infection. Even though fhuD2 expression has been shown to be upregulated in murine kidneys infected with S. aureus, it is not known whether the bacterium undergoes increased iron deprivation during prolonged infection. Furthermore, different S. aureus infection niches might provide different environments and levels of iron availability, resulting in different fhuD2 expression patterns among organs of the same host. To address these questions, we characterized the in vitro expression of the fhuD2 gene and confirmed Fur-dependent regulation of its expression. We further investigated its expression in mice infected with a bioluminescent reporter strain of S. aureus expressing the luciferase operon under the control of the fhuD2 promoter. The emission of bioluminescence in different organs was followed over a 7-day time course, and quantitative real-time PCR analysis of the RNA transcribed from the endogenous fhuD2 gene was performed. Using this approach, we were able to show that fhuD2 expression was induced during infection in all organs analyzed and that differences in expression were observed at different time points and in different infected organs. Our data suggest that S. aureus undergoes increased iron deprivation during the progression of infection in diverse host organs and accordingly induces dedicated iron acquisition mechanisms. Since FhuD2 plays a central role in providing the pathogen with the required iron, further knowledge of the patterns of fhuD2 expression in vivo during infection will be instrumental in better defining the role of this antigen in S. aureus pathogenesis and as a vaccine antigen.

Aspartate tightens the anchoring of staphylococcal lipoproteins to the cytoplasmic membrane

In gram-negative bacteria, the ABC transporter LolCDE complex translocates outer membrane-specific lipoproteins (Lpp) from the inner membrane to the outer membrane. Lpp possessing aspartate (Asp) at position +2 are not translocated because it functions as a LolCDE avoidance signal. In gram-positive bacteria, lacking an outer membrane and the Lol system, Lpp are only anchored at the outer leaflet of the cytoplasmic membrane. However, the release of Lpp particularly in pathogenic or commensal species is crucial for immune modulation. Here, we provide evidence that in Staphylococcus aureus Asp at position +2 plays a role in withholding Lpp to the cytoplasmic membrane. Screening of published exoproteomic data of S. aureus revealed that Lpp mainly with Gly or Ser at position +2 were found in exoproteome, but there was no Lpp with Asp+2. The occurrence of Lpp with Asp+2 is infrequent in gram-positive bacteria. In S. aureus USA300 only seven of the 67 Lpp possess Asp+2; among them five Lpp represented Lpl lipoproteins involved in host cell invasion. Our study demonstrated that replacing the Asp+2 present in Lpl8 with a Ser enhances its release into the supernatant. However, there is no different release of Asp+2 and Ser+2 in mprF mutant that lacks the positive charge of lysyl-phosphatidylglycerol (Lys-PG). Moreover, substitution of Ser+2 by Asp in SitC (MntC) did not lead to a decreased release indicating that in staphylococci positions +3 and +4 might also be important for a tighter anchoring of Lpp. Here, we show that Asp in position +2 and adjacent amino acids contribute in tightening the anchoring of Lpp by interaction of the negative charged Asp with the positive charged Lys-PG.

Quantitative Proteomic Analysis Reveals Changes in the Benchmark Corynebacterium pseudotuberculosis Biovar Equi Exoproteome after Passage in a Murine Host

Corynebacterium pseudotuberculosis biovar equi is the etiologic agent of ulcerative lymphangitis. To investigate proteins that could be related to the virulence of this pathogen, we combined an experimental passage process using a murine model and high-throughput proteomics with a mass spectrometry, data-independent acquisition (LC-MS^E) approach to identify and quantify the proteins released into the supernatants of strain 258_equi. To our knowledge, this approach allowed characterization of the exoproteome of a C. pseudotuberculosis equi strain for the first time. Interestingly, the recovery of this strain from infected mouse spleens induced a change in its virulence potential, and it became more virulent in a second infection challenge. Proteomic screening performed from culture supernatant of the control and recovered conditions revealed 104 proteins that were differentially expressed between the two conditions. In this context, proteomic analysis of the recovered condition detected the induction of proteins involved in bacterial pathogenesis, mainly related to iron uptake. In addition, KEGG enrichment analysis showed that ABC transporters, bacterial secretion systems and protein export pathways were significantly altered in the recovered condition. These findings show that secretion and secreted proteins are key elements in the virulence and adaptation of C. pseudotuberculosis. Collectively, bacterial pathogenesis-related proteins were identified that contribute to the processes of adherence, intracellular growth and evasion of the immune system. Moreover, this study enhances our understanding of the factors that may influence the pathogenesis of C. pseudotuberculosis.

Structural Basis for Xenosiderophore Utilization by the Human Pathogen Staphylococcus aureus

Staphylococcus aureus produces a cocktail of metallophores (staphylopine, staphyloferrin A, and staphyloferrin B) to scavenge transition metals during infection of a host. In addition, S. aureus displays the extracellular surface lipoproteins FhuD1 and FhuD2 along with the ABC transporter complex FhuCBG to facilitate the use of hydroxamate xenosiderophores such as desferrioxamine B (DFOB) for iron acquisition. DFOB is used as a chelation therapy to treat human iron overload diseases and has been linked to an increased risk of S. aureus infections. We used a panel of synthetic DFOB analogs and a FhuD2-selective trihydroxamate sideromycin to probe xenosiderophore utilization in S. aureus and establish structure-activity relationships for Fe(III) binding, FhuD2 binding, S. aureus growth promotion, and competition for S. aureus cell entry. Fe(III) binding assays and FhuD2 intrinsic fluorescence quenching experiments revealed that diverse chemical modifications of the terminal ends of linear ferrioxamine siderophores influences Fe(III) affinity but not FhuD2 binding. Siderophore-sideromycin competition assays and xenosiderophore growth promotion assays revealed that S. aureus SG511 and ATCC 11632 can distinguish between competing siderophores based exclusively on net charge of the siderophore-Fe(III) complex. Our work provides a roadmap for tuning hydroxamate xenosiderophore scaffolds to suppress (net negative charge) or enhance (net positive or neutral charge) uptake by S. aureus for applications in metal chelation therapy and siderophore-mediated antibiotic delivery, respectively.

Antibody-mediated protection against Staphylococcus aureus dermonecrosis and sepsis by a whole cell vaccine

Staphylococcus aureus is a very important human pathogen that causes significant morbidity and mortality worldwide. Several vaccine clinical trials based on generating antibody against staphylococcal surface polysaccharides or proteins have been unsuccessful. A killed whole cell lysate preparation (SaWCA) was made by lysing a USA 300 strain with lysostaphin followed by sonication and harvest of the supernatant fraction. Immunization with SaWCA and cholera toxin (CT) generated robust IL-17A but relatively modest antibody responses, and provided protection in the skin abscess but not in the dermonecrosis or invasive infection model. In contrast, parenteral immunization with SaWCA and alum produced robust antibody and IL-17A responses and protected mice in all three models. Sera generated after immunization with SaWCA had measurable antibodies directed against six tested conserved surface proteins, and promoted opsonophagocytosis activity (OPA) against two S. aureus strains. Passive transfer of SaWCA-immune serum protected mice against dermonecrosis and invasive infection but provided no demonstrable effect against skin abscesses, suggesting that antibodies alone may not be sufficient for protection in this model. Thus, immunization with a SA lysate preparation generates potent antibody and T cell responses, and confers protection in systemic and cutaneous staphylococcal infection models.

Ferric hydroxamate uptake system contributes to Edwardsiella ictaluri virulence

Edwardsiella ictaluri is a Gram-negative facultative intracellular pathogen causing enteric septicemia in fish, particularly in channel catfish. Ferric iron is an essential micronutrient for bacterial survival, and some bacterial pathogens use secreted hydroxamate-type siderophores to chelate iron in host tissues. Siderophore-iron complexes are taken up by these bacteria via the ferric hydroxamate uptake (Fhu) system. In E. ictaluri, the Fhu system consists of fhuC, fhuD, fhuB, and fhuA genes. However, the importance of the Fhu system in E. ictaluri virulence has not been investigated completely. Here, we present construction of E. ictaluri fhuD and fhuB mutants (EiΔfhuD and EiΔfhuB) by in-frame gene deletion and evaluation of the mutants' virulence and immunogenicity in channel catfish fingerlings and fry. Immersion challenges showed that EiΔfhuD was not significantly attenuated (p < 0.05) in catfish fingerlings, whereas EiΔfhuB was significantly attenuated (p < 0.01). Catfish fingerlings immunized with EiΔfhuD and EiΔfhuB showed 100% and 97.62% survival, respectively. Fry immersion challenges indicated EiΔfhuB was also significantly attenuated (p < 0.05) in two-week old fry compared to the wild-type (48.96% vs. 82.14% mortalities). The survival rate in the fry vaccinated with EiΔfhuB was significantly higher (p < 0.05) than that of non-vaccinated fry (96.77% vs. 21.42% survival). Our data indicates that the fhuB gene, but not the fhuD gene, contributes to E. ictaluri virulence.

Lipoproteins of Gram-Positive Bacteria: Key Players in the Immune Response and Virulence

Since the discovery in 1973 of the first of the bacterial lipoproteins (Lpp) in Escherichia coli, Braun's lipoprotein, the ever-increasing number of publications indicates the importance of these proteins. Bacterial Lpp belong to the class of lipid-anchored proteins that in Gram-negative bacteria are anchored in both the cytoplasmic and outer membranes and in Gram-positive bacteria are anchored only in the cytoplasmic membrane. In contrast to the case for Gram-negative bacteria, in Gram-positive bacteria lipoprotein maturation and processing are not vital. Physiologically, Lpp play an important role in nutrient and ion acquisition, allowing particularly pathogenic species to better survive in the host. Bacterial Lpp are recognized by Toll-like receptor 2 (TLR2) of the innate immune system. The important role of Lpp in Gram-positive bacteria, particularly in the phylum Firmicutes, as key players in the immune response and pathogenicity has emerged only in recent years. In this review, we address the role of Lpp in signaling and modulating the immune response, in inflammation, and in pathogenicity. We also address the potential of Lpp as promising vaccine candidates.

Adaptive immune response to lipoproteins of Staphylococcus aureus in healthy subjects

Staphylococcus aureus is a frequent commensal but also a dangerous pathogen, causing many forms of infection ranging from mild to life‐threatening conditions. Among its virulence factors are lipoproteins, which are anchored in the bacterial cell membrane. Lipoproteins perform various functions in colonization, immune evasion, and immunomodulation. These proteins are potent activators of innate immune receptors termed Toll‐like receptors 2 and 6. This study addressed the specific B‐cell and T‐cell responses directed to lipoproteins in human S. aureus carriers and non‐carriers. 2D immune proteomics and ELISA approaches revealed that titers of antibodies (IgG) binding to S. aureus lipoproteins were very low. Proliferation assays and cytokine profiling data showed only subtle responses of T cells; some lipoproteins did not elicit proliferation. Hence, the robust activation of the innate immune system by S. aureus lipoproteins does not translate into a strong adaptive immune response. Reasons for this may include inaccessibility of lipoproteins for B cells as well as ineffective processing and presentation of the antigens to T cells.

Crystal structure of FhuD at 1.6 Å resolution: a ferrichrome-binding protein from the animal and human pathogen Staphylococcus pseudintermedius

Staphylococcus pseudintermedius is a leading cause of disease in dogs, and zoonosis causes human infections. Methicillin-resistant S. pseudintermedius strains are emerging, resembling the global health threat of S. aureus. Therefore, it is increasingly important to characterize potential targets for intervention against S. pseudintermedius. Here, FhuD, an S. pseudintermedius surface lipoprotein implicated in iron uptake, was characterized. It was found that FhuD bound ferrichrome in an iron-dependent manner, which increased the thermostability of FhuD by >15 °C. The crystal structure of ferrichrome-free FhuD was determined via molecular replacement at 1.6 Å resolution. FhuD exhibits the class III solute-binding protein (SBP) fold, with a ligand-binding cavity between the N- and C-terminal lobes, which is here occupied by a PEG molecule. The two lobes of FhuD were oriented in a closed conformation. These results provide the first detailed structural characterization of FhuD, a potential therapeutic target of S. pseudintermedius.

One Dose of Staphylococcus aureus 4C-Staph Vaccine Formulated with a Novel TLR7-Dependent Adjuvant Rapidly Protects Mice through Antibodies, Effector CD4+ T Cells, and IL-17A

A rapidly acting, single dose vaccine against Staphylococcus aureus would be highly beneficial for patients scheduled for major surgeries or in intensive care units. Here we show that one immunization with a multicomponent S. aureus candidate vaccine, 4C-Staph, formulated with a novel TLR7-dependent adjuvant, T7-alum, readily protected mice from death and from bacterial dissemination, both in kidney abscess and peritonitis models, outperforming alum-formulated vaccine. This increased efficacy was paralleled by higher vaccine-specific and α-hemolysin-neutralizing antibody titers and Th1/Th17 cell responses. Antibodies played a crucial protective role, as shown by the lack of protection of 4C-Staph/T7-alum vaccine in B-cell-deficient mice and by serum transfer experiments. Depletion of effector CD4⁺ T cells not only reduced survival but also increased S. aureus load in kidneys of mice immunized with 4C-Staph/T7-alum. The role of IL-17A in the control of bacterial dissemination in 4C-Staph/T7-alum vaccinated mice was indicated by in vivo neutralization experiments. We conclude that single dose 4C-Staph/T7-alum vaccine promptly and efficiently protected mice against S. aureus through the combined actions of antibodies, CD4⁺ effector T cells, and IL-17A. These data suggest that inclusion of an adjuvant that induces not only fast antibody responses but also IL-17-producing cell-mediated effector responses could efficaciously protect patients scheduled for major surgeries or in intensive care units.

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.

Iron acquisition in the cystic fibrosis lung and potential for novel therapeutic strategies

Iron acquisition is vital to microbial survival and is implicated in the virulence of many of the pathogens that reside in the cystic fibrosis (CF) lung. The multifaceted nature of iron acquisition by both bacterial and fungal pathogens encompasses a range of conserved and species-specific mechanisms, including secretion of iron-binding siderophores, utilization of siderophores from other species, release of iron from host iron-binding proteins and haemoproteins, and ferrous iron uptake. Pathogens adapt and deploy specific systems depending on iron availability, bioavailability of the iron pool, stage of infection and presence of competing pathogens. Understanding the dynamics of pathogen iron acquisition has the potential to unveil new avenues for therapeutic intervention to treat both acute and chronic CF infections. Here, we examine the range of strategies utilized by the primary CF pathogens to acquire iron and discuss the different approaches to targeting iron acquisition systems as an antimicrobial strategy.

Investigational drugs to treat methicillin-resistant Staphylococcus aureus

INTRODUCTION

Staphylococcus aureus remains one of the leading causes of morbidity and mortality worldwide. This is to a large extent due to antibiotic-resistant strains, in particular methicillin-resistant S. aureus (MRSA). While the toll of invasive MRSA infections appears to decrease in U.S. hospitals, the rate of community-associated MRSA infections remains constant and there is a surge of MRSA in many other countries, a situation that calls for continuing if not increased efforts to find novel strategies to combat MRSA infections.

AREAS COVERED

This review provides an overview of current investigational drugs and therapeutic antibodies against S. aureus in early clinical development (up to phase II clinical development). It includes a short description of the mechanism of action and a presentation of microbiological and clinical data.

EXPERT OPINION

Increased recent antibiotic development efforts and results from pathogenesis research have led to several new antibiotics and therapies, such as anti-virulence drugs, as well as a more informed selection of targets for vaccination efforts against MRSA. This developing portfolio of novel anti-staphylococcal drugs will hopefully provide us with additional and more efficient ways to combat MRSA infections in the near future and prevent us from running out of treatment options, even if new resistances arise.

MF59- and Al(OH)3-Adjuvanted Staphylococcus aureus (4C-Staph) Vaccines Induce Sustained Protective Humoral and Cellular Immune Responses, with a Critical Role for Effector CD4 T Cells at Low Antibody Titers

Staphylococcus aureus (S. aureus) is an important opportunistic pathogen that may cause invasive life-threatening infections, like sepsis and pneumonia. Due to the increasing antibiotic resistance, the development of an effective vaccine against S. aureus is needed. Although a correlate of protection against staphylococcal diseases is not yet established, several findings suggest that both antibodies and CD4 T cells might contribute to optimal immunity. In this study, we show that adjuvanting a multivalent vaccine (4C-Staph) with MF59, an oil-in-water emulsion licensed in human vaccines, further potentiated antigen-specific IgG titers and CD4 T-cell responses compared to alum and conferred protection in the peritonitis model of S. aureus infection. Moreover, we showed that MF59- and alum-adjuvanted 4C-Staph vaccines induced persistent antigen-specific humoral and T-cell responses, and protected mice from infection up to 4 months after immunization. Furthermore, 4C-Staph formulated with MF59 was used to investigate which immune compartment is involved in vaccine-induced protection. Using CD4 T cell-depleted mice or B cell-deficient mice, we demonstrated that both T and B-cell responses contributed to 4C-Staph vaccine-mediated protective immunity. However, the role of CD4 T cells seemed more evident in the presence of low-antibody responses. This study provides preclinical data further supporting the use of the adjuvanted 4C-Staph vaccines against S. aureus diseases, and provides critical insights on the correlates of protective immunity necessary to combat this pathogen.

Protein Crystallography in Vaccine Research and Development

The use of protein X-ray crystallography for structure-based design of small-molecule drugs is well-documented and includes several notable success stories. However, it is less well-known that structural biology has emerged as a major tool for the design of novel vaccine antigens. Here, we review the important contributions that protein crystallography has made so far to vaccine research and development. We discuss several examples of the crystallographic characterization of vaccine antigen structures, alone or in complexes with ligands or receptors. We cover the critical role of high-resolution epitope mapping by reviewing structures of complexes between antigens and their cognate neutralizing, or protective, antibody fragments. Most importantly, we provide recent examples where structural insights obtained via protein crystallography have been used to design novel optimized vaccine antigens. This review aims to illustrate the value of protein crystallography in the emerging discipline of structural vaccinology and its impact on the rational design of vaccines.

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.

Vaccine composition formulated with a novel TLR7-dependent adjuvant induces high and broad protection against Staphylococcus aureus

Both active and passive immunization strategies against Staphylococcus aureus have thus far failed to show efficacy in humans. With the attempt to develop an effective S. aureus vaccine, we selected five conserved antigens known to have different roles in S. aureus pathogenesis. They include the secreted factors α-hemolysin (Hla), ess extracellular A (EsxA), and ess extracellular B (EsxB) and the two surface proteins ferric hydroxamate uptake D2 and conserved staphylococcal antigen 1A. The combined vaccine antigens formulated with aluminum hydroxide induced antibodies with opsonophagocytic and functional activities and provided consistent protection in four mouse models when challenged with a panel of epidemiologically relevant S. aureus strains. The importance of antibodies in protection was demonstrated by passive transfer experiments. Furthermore, when formulated with a toll-like receptor 7-dependent (TLR7) agonist recently designed and developed in our laboratories (SMIP.7-10) adsorbed to alum, the five antigens provided close to 100% protection against four different staphylococcal strains. The new formulation induced not only high antibody titers but also a Th1 skewed immune response as judged by antibody isotype and cytokine profiles. In addition, low frequencies of IL-17-secreting T cells were also observed. Altogether, our data demonstrate that the rational selection of mixtures of conserved antigens combined with Th1/Th17 adjuvants can lead to promising vaccine formulations against S. aureus.

Apo, Zn2+-bound and Mn2+-bound structures reveal ligand-binding properties of SitA from the pathogen Staphylococcus pseudintermedius

The Gram-positive bacterium Staphylococcus pseudintermedius is a leading cause of canine bacterial pyoderma, resulting in worldwide morbidity in dogs. S. pseudintermedius also causes life-threatening human infections. Furthermore, methicillin-resistant S. pseudintermedius is emerging, resembling the human health threat of methicillin-resistant Staphylococcus aureus. Therefore it is increasingly important to characterize targets for intervention strategies to counteract S. pseudintermedius infections. Here we used biophysical methods, mutagenesis, and X-ray crystallography, to define the ligand-binding properties and structure of SitA, an S. pseudintermedius surface lipoprotein. SitA was strongly and specifically stabilized by Mn2+ and Zn2+ ions. Crystal structures of SitA complexed with Mn2+ and Zn2+ revealed a canonical class III solute-binding protein with the metal cation bound in a cavity between N- and C-terminal lobes. Unexpectedly, one crystal contained both apo- and holo-forms of SitA, revealing a large side-chain reorientation of His64, and associated structural differences accompanying ligand binding. Such conformational changes may regulate fruitful engagement of the cognate ABC (ATP-binding cassette) transporter system (SitBC) required for metal uptake. These results provide the first detailed characterization and mechanistic insights for a potential therapeutic target of the major canine pathogen S. pseudintermedius, and also shed light on homologous structures in related staphylococcal pathogens afflicting humans.

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

The online version of this article (doi:10.1186/s13073-014-0093-3) contains supplementary material, which is available to authorized users.

Protectome analysis: a new selective bioinformatics tool for bacterial vaccine candidate discovery

New generation vaccines are in demand to include only the key antigens sufficient to confer protective immunity among the plethora of pathogen molecules. In the last decade, large-scale genomics-based technologies have emerged. Among them, the Reverse Vaccinology approach was successfully applied to the development of an innovative vaccine against Neisseria meningitidis serogroup B, now available on the market with the commercial name BEXSERO® (Novartis Vaccines). The limiting step of such approaches is the number of antigens to be tested in in vivo models. Several laboratories have been trying to refine the original approach in order to get to the identification of the relevant antigens straight from the genome. Here we report a new bioinformatics tool that moves a first step in this direction. The tool has been developed by identifying structural/functional features recurring in known bacterial protective antigens, the so called "Protectome space," and using such "protective signatures" for protective antigen discovery. In particular, we applied this new approach to Staphylococcus aureus and Group B Streptococcus and we show that not only already known protective antigens were re-discovered, but also two new protective antigens were identified.

Adult vaccination

Vaccination of children has had a major impact on the morbidity and mortality of many infectious diseases globally. However, with age, immune responses to vaccines can be less robust, which can be further enhanced by underlying diseases that are common in the older adult. In many countries around the globe booster vaccinations against diphtheria, tetanus, and pertussis are recommended for adults. For the older adult, vaccination against pneumococcal diseases, influenza and herpes zoster are also recommended. Despite these recommendations, the widespread use of these vaccines in the adult population clearly lags behind the vaccine uptake and successes documented for pediatric vaccination programs. Furthermore, extensive and sometimes inappropriate use of antibiotics have fostered the emergence of antibiotic-resistant bacteria (e.g., methicillin resistant Staphylococcus aureus (MRSA)) as well as increased susceptibility in the elderly to bacterial species such as Clostridium difficile. Infectious diseases remain an important unmet medical need and new concepts to successfully implement vaccination of adults are urgently needed.

Deferoxamine mesylate enhances virulence of community-associated methicillin resistant Staphylococcus aureus

Staphylococcus aureus is a leading cause of bacterial infections. Strains of community-associated methicillin-resistant S. aureus (CA-MRSA), such as USA300, display enhanced virulence and fitness. Patients suffering from iron overload diseases often undergo iron chelation therapy with deferoxamine mesylate (DFO). Here, we show that USA300 uses this drug to acquire iron. We further demonstrate that mice administered DFO I.P., versus those not administered DFO, had significantly higher bacterial burden in livers and kidneys after I.V. challenge with USA300, associated with increased abscess formation and tissue destruction. The virulence of USA300 mutants defective for DFO uptake was not affected by DFO treatment.

Crystal and solution structure analysis of FhuD2 from Staphylococcus aureus in multiple unliganded conformations and bound to ferrioxamine-B

Iron acquisition is a central process for virtually all organisms. In Staphylococcus aureus, FhuD2 is a lipoprotein that is a high-affinity receptor for iron-bound hydroxamate siderophores. In this study, FhuD2 was crystallized bound to ferrioxamine-B (FXB), and also in its ligand-free state; the latter structures are the first for hydroxamate-binding receptors within this protein family. The structure of the FhuD2-FXB conformation shows that residues W197 and R199 from the C-terminal domain donate hydrogen bonds to the hydroxamate oxygens, and a ring of aromatic residues cradles the aliphatic arms connecting the hydroxamate moieties of the siderophore. The available ligand-bound structures of FhuD from Escherichia coli and YfiY from Bacillus cereus show that, despite a high degree of structural conservation, three protein families have evolved with critical siderophore binding residues on either the C-terminal domain (S. aureus), the N-terminal domain (E. coli), or both (B. cereus). Unliganded FhuD2 was crystallized in five conformations related by rigid body movements of the N- and C-terminal domains. Small-angle X-ray scattering (SAXS) indicates that the solution conformation of unliganded FhuD2 is more compact than the conformations observed in crystals. The ligand-induced conformational changes for FhuD2 in solution are relatively modest and depend on the identity of the siderophore. The crystallographic and SAXS results are used to discuss roles for the liganded and unliganded forms of FhuD2 in the siderophore transport mechanism.

Structure and protective efficacy of the Staphylococcus aureus autocleaving protease EpiP

Despite the global medical needs associated with Staphylococcus aureus infections, no licensed vaccines are currently available. We identified and characterized a protein annotated as an epidermin leader peptide processing serine protease (EpiP), as a novel S. aureus vaccine candidate. In addition, we determined the structure of the recombinant protein (rEpiP) by X-ray crystallography. The crystal structure revealed that rEpiP was cleaved somewhere between residues 95 and 100, and we found that the cleavage occurs through an autocatalytic intramolecular mechanism. The protein expressed by S. aureus cells also appeared to undergo a similar processing event. To determine whether the protein acts as a serine protease, we mutated the hypothesized catalytic serine 393 residue to alanine, generating rEpiP-S393A. The crystal structure of this mutant protein showed that the polypeptide chain was not cleaved and was not interacting stably with the active site. Indeed, rEpiP-S393A was shown to be impaired in its protease activity. Mice vaccinated with rEpiP were protected from S. aureus infection (34% survival, P=0.0054). Moreover, the protective efficacy generated by rEpiP and rEpiP-S393A was comparable, implying that the noncleaving mutant could be used for vaccination purposes.

Protective activity of the CnaBE3 domain conserved among Staphylococcus aureus Sdr proteins

Staphylococcus aureus is an opportunistic pathogen, commensal of the human skin and nares, but also responsible for invasive nosocomial as well as community acquired infections. Staphylococcus aureus adheres to the host tissues by means of surface adhesins, such as SdrC, SdrD, and SdrE proteins. The Sdr family of proteins together with a functional A domain, contain respectively two, three or five repeated sequences called B motifs which comprise the CnaB domains. SdrD and SdrE proteins were reported to be protective in animal models against invasive diseases or lethal challenge with human clinical S. aureus isolates. In this study we identified a 126 amino acid sequence containing a CnaB domain, conserved among the three Sdr proteins. The three fragments defined here as CnaBC2, D5 and E3 domains even though belonging to phylogenetically distinct strains, displayed high sequence similarity. Based on the sequence conservation data, we selected the CnaBE3 domain for further analysis and characterization. Polyclonal antibodies raised against the recombinant CnaBE3 domain recognized SdrE, SdrC and SdrD proteins of different S. aureus lineages. Moreover, we demonstrated that the CnaBE3 domain was expressed in vivo during S. aureus infections, and that immunization of this domain alone significantly reduces the bacterial load in mice challenged with S. aureus. Furthermore, we show that the reduction of bacteria by CnaBE3 vaccination is due to functional antibodies. Finally, we demonstrated that the region of the SdrE protein containing the CnaBE3 domain was resistant to trypsin digestion, a characteristic often associated with the presence of an isopeptide bond.

Surfomics: shaving live organisms for a fast proteomic identification of surface proteins

Surface proteins play a critical role in the interaction between cells and their environment, as they take part in processes like signaling, adhesion, transport, etc. In pathogenic microorganisms, they can also participate in virulence or cytotoxicity. As these proteins have the highest chances to be recognized by the immune system, they are often the targets for the discovery of new vaccines. In addition, they can serve for the development of serological-based tools to diagnose infectious diseases. First-generation proteomic strategies for the identification of surface proteins rely on the biochemical fractionation and/or enrichment of this group of molecules or organelles containing them. However, in the last years, a novel second-generation approach has been developed, consisting of the digestion of live, intact cells with proteases, so that surface-exposed moieties (i.e. the "surfome" of a cell) are "shaved" and analyzed by LC/MS/MS. Here we review such a strategy, firstly set up and developed in Gram-positive bacteria, and further applied to Gram-negative bacteria, unicellular fungi, and also pluricellular organisms. We also discuss the advantages and inconvenients of the approach, and the still unresolved question about the intriguing presence of proteins predicted as cytoplasmic in the surfomes. This article is part of a Special Issue entitled: Trends in Microbial Proteomics.

Vaccine review: "Staphyloccocus aureus vaccines: problems and prospects"

Staphylococcus aureus is a leading cause of both healthcare- and community-associated infections globally. S. aureus exhibits diverse clinical presentations, ranging from benign carriage and superficial skin and soft tissue infections to deep wound and organ/space infections, biofilm-related prosthesis infections, life-threatening bacteremia and sepsis. This broad clinical spectrum, together with the high incidence of these disease manifestations and magnitude of the diverse populations at risk, presents a high unmet medical need and a substantial burden to the healthcare system. With the increasing propensity of S. aureus to develop resistance to essentially all classes of antibiotics, alternative strategies, such as prophylactic vaccination to prevent S. aureus infections, are actively being pursued in healthcare settings. Within the last decade, the S. aureus vaccine field has witnessed two major vaccine failures in phase 3 clinical trials designed to prevent S. aureus infections in either patients undergoing cardiothoracic surgery or patients with end-stage renal disease undergoing hemodialysis. This review summarizes the potential underlying reasons why these two approaches may have failed, and proposes avenues that may provide successful vaccine approaches to prevent S. aureus disease in the future.

Structural and functional characterization of the Staphylococcus aureus virulence factor and vaccine candidate FhuD2

Staphylococcus aureus is a human pathogen causing globally significant morbidity and mortality. The development of antibiotic resistance in S. aureus highlights the need for a preventive vaccine. In the present paper we explore the structure and function of FhuD2 (ferric-hydroxamate uptake D2), a staphylococcal surface lipoprotein mediating iron uptake during invasive infection, recently described as a promising vaccine candidate. Differential scanning fluorimetry and calorimetry studies revealed that FhuD2 is stabilized by hydroxamate siderophores. The FhuD2-ferrichrome interaction was of nanomolar affinity in surface plasmon resonance experiments and fully iron(III)-dependent. We determined the X-ray crystallographic structure of ligand-bound FhuD2 at 1.9 Å (1 Å=0.1 nm) resolution, revealing the bilobate fold of class III SBPs (solute-binding proteins). The ligand, ferrichrome, occupies a cleft between the FhuD2 N- and C-terminal lobes. Many FhuD2-siderophore interactions enable the specific recognition of ferrichrome. Biochemical data suggest that FhuD2 does not undergo significant conformational changes upon siderophore binding, supporting the hypothesis that the ligand-bound complex is essential for receptor engagement and uptake. Finally, immunizations with FhuD2 alone or FhuD2 formulated with hydroxamate siderophores were equally protective in a murine staphylococcal infection model, confirming the suitability and efficacy of apo-FhuD2 as a protective antigen, and suggesting that other class III SBPs might also be exploited as vaccine candidates.

Capsular polysaccharides are an important immune evasion mechanism for Staphylococcus aureus

Staphylococcus aureus can cause severe life threatening invasive diseases. The principal immune effector mechanism by which humans are protected from Gram positive bacteria such as S. aureus is antigen specific antibody- and complement-dependent opsonophagocytosis. This process can be measured in vitro using the opsonophagocytic antibody assay (OPA), which is a complex assay composed of live S. aureus bacteria, a complement source, phagocytic effector cells such as differentiated HL-60 cells, and test serum. In this report, we investigated the impact on the OPA of S. aureus surface antigens capsular polysaccharides (CP) and protein A (SpA). We demonstrated that higher CP expression renders bacteria more resistant to non-specific opsonophagocytic killing than increased SpA expression, suggesting that the expression of capsular polysaccharides may be the more important immune evasion strategy for S. aureus. Bacteria that were not fully encapsulated were highly susceptible to non-specific killing in the assay in the absence of immune serum. This non-specific killing was prevented by growing the bacteria under conditions that increased capsular polysaccharide levels on the surface of the bacteria. In contrast, the level of SpA expression had no detectable effect on non-specific killing in OPA. Using anti-CP antibodies we demonstrated type-specific killing in OPA of both MRSA and MSSA clinical isolates. SpA expression on the cell surface did not interfere with OPA activity, providing evidence that despite the role of SpA in sequestering antibodies by their Fc region, killing is easily accomplished in the presence of high titered anti-capsular polysaccharide antibodies. This highlights the role of CP as an important immune evasion mechanism and supports the inclusion of capsular polysaccharide antigens in the formulation of multi-component prophylactic vaccines against S. aureus.