Staphylococcus aureus infections are associated with abscess formation and bacterial persistence; however, the genes that enable this lifestyle are not known. We show here that following intravenous infection of mice, S. aureus disseminates rapidly into organ tissues and elicits abscess lesions that develop over weeks but cannot be cleared by the host. Staphylococci grow as communities at the center of abscess lesions and are enclosed by pseudocapsules, separating the pathogen from immune cells. By testing insertional variants in genes for cell wall-anchored surface proteins, we are able to infer the stage at which these molecules function. Fibrinogen-binding proteins ClfA and ClfB are required during the early phase of staphylococcal dissemination. The heme scavenging factors IsdA and IsdB, as well as SdrD and protein A, are necessary for abscess formation. Envelope-associated proteins, Emp and Eap, are either required for abscess formation or contribute to persistence. Fluorescence microscopy revealed Eap deposition within the pseudocapsule, whereas Emp was localized within staphylococcal abscess communities. Antibodies directed against envelope-associated proteins generated vaccine protection against staphylococcal abscess formation. Thus, staphylococci employ envelope proteins at discrete stages of a developmental program that enables abscess formation and bacterial persistence in host tissues.

Staphylococcus aureus, a bacterial commensal of the human nares and skin, is a frequent cause of soft tissue and bloodstream infections. A hallmark of staphylococcal infections is their frequent recurrence, even when treated with antibiotics and surgical intervention, which demonstrates the bacterium’s ability to manipulate innate and adaptive immune responses. In this Review, we highlight how S. aureus virulence factors inhibit complement activation, block and destroy phagocytic cells and modify host B and T cell responses, and we discuss how these insights might be useful for the development of novel therapies against infections with antibiotic resistant strains such as methicillin-resistant S. aureus.

The bacterial pathogen Staphylococcus aureus seeds abscesses in host tissues to replicate at the center of these lesions, protected from host immune cells via a pseudocapsule. Using histochemical staining, we identified prothrombin and fibrin within abscesses and pseudocapsules. S. aureus secretes two clotting factors, coagulase (Coa) and von Willebrand factor binding protein (vWbp). We report here that Coa and vWbp together are required for the formation of abscesses. Coa and vWbp promote the non-proteolytic activation of prothrombin and cleavage of fibrinogen, reactions that are inhibited with specific antibody against each of these molecules. Coa and vWbp specific antibodies confer protection against abscess formation and S. aureus lethal bacteremia, suggesting that coagulases function as protective antigens for a staphylococcal vaccine.

Staphylococcus aureus infection is a frequent cause of sepsis in humans, a disease associated with high mortality and without specific intervention. When suspended in human or animal plasma, staphylococci are known to agglutinate, however the bacterial factors responsible for agglutination and their possible contribution to disease pathogenesis have not yet been revealed. Using a mouse model for S. aureus sepsis, we report here that staphylococcal agglutination in blood was associated with a lethal outcome of this disease. Three secreted products of staphylococci - coagulase (Coa), von Willebrand factor binding protein (vWbp) and clumping factor (ClfA) – were required for agglutination. Coa and vWbp activate prothrombin to cleave fibrinogen, whereas ClfA allowed staphylococci to associate with the resulting fibrin cables. All three virulence genes promoted the formation of thromboembolic lesions in heart tissues. S. aureus agglutination could be disrupted and the lethal outcome of sepsis could be prevented by combining dabigatran-etexilate treatment, which blocked Coa and vWbp activity, with antibodies specific for ClfA. Together these results suggest that the combined administration of direct thrombin inhibitors and ClfA-antibodies that block S. aureus agglutination with fibrin may be useful for the prevention of staphylococcal sepsis in humans.

The current epidemic of hospital- and community-acquired methicillin-resistant Staphylococcus aureus (MRSA) infections has caused significant human morbidity, but a protective vaccine is not yet available. Prior infection with S. aureus is not associated with protective immunity. This phenomenon involves staphylococcal protein A (SpA), an S. aureus surface molecule that binds to Fcγ of immunoglobulin (Ig) and to the Fab portion of VH3-type B cell receptors, thereby interfering with opsonophagocytic clearance of the pathogen and ablating adaptive immune responses. We show that mutation of each of the five Ig-binding domains of SpA with amino acid substitutions abolished the ability of the resulting variant SpAKKAA to bind Fcγ or Fab VH3 and promote B cell apoptosis. Immunization of mice with SpAKKAA raised antibodies that blocked the virulence of staphylococci, promoted opsonophagocytic clearance, and protected mice against challenge with highly virulent MRSA strains. Furthermore, SpAKKAA immunization enabled MRSA-challenged mice to mount antibody responses to many different staphylococcal antigens.

Heritable defects in human B cell/antibody development are not associated with increased susceptibility to Staphylococcus aureus infection. Protein A (SpA), a surface molecule of S. aureus, binds the Fcγ domain of immunoglobulin (Ig) and cross-links the Fab domain of VH3-type B cell receptors (IgM). Here we generated S. aureus spa variants harboring amino acid substitutions at four key residues in each of the five Ig-binding domains of SpA. Wild-type S. aureus required SpA binding to Ig to resist phagocytosis and SpA-mediated B cell receptor cross-linking to block antibody development in mice. The spaKKAA mutant, which cannot bind Ig or IgM, was phagocytosed and elicited B cell responses to key virulence antigens that protected animals against lethal S. aureus challenge. The immune evasive attributes of S. aureus SpA were abolished in µMT mice lacking mature B cells and antibodies. Thus, while wild-type S. aureus escapes host immune surveillance, the spaKKAA variant elicits adaptive responses that protect against recurrent infection.

Staphylococcus aureus is a leading cause of bacteremia and sepsis. The interaction of S. aureus with the endothelium is central to bloodstream infection pathophysiology yet remains ill-understood. We show herein that staphylococcal a-hemolysin, a pore-forming cytotoxin, is required for full virulence in a murine sepsis model. The a-hemolysin binding to its receptor A-disintegrin and metalloprotease 10 (ADAM10) upregulates the receptor's metalloprotease activity on endothelial cells, causing vascular endothelialcadherin cleavage and concomitant loss of endothelial barrier function. These cellular injuries and sepsis severity can be mitigated by ADAM10 inhibition. This study therefore provides mechanistic insight into toxin-mediated endothelial injury and suggests new therapeutic approaches for staphylococcal sepsis.

Staphylococcus aureus is the most frequent cause of bacteremia and hospital-acquired infection, however a vaccine that prevents staphylococcal disease is currently not available. Two sortase-anchored surface proteins, IsdA and IsdB, have been identified as subunit vaccines that, following active immunization, protect experimental animals against intravenous challenge with staphylococci. Here we investigate the molecular basis of this immunity and report that, when passively transferred to naïve mice, purified antibodies directed against IsdA or IsdB protected against staphylococcal abscess formation and lethal intravenous challenge. When added to mouse blood, IsdA- or IsdB-specific antibodies did not promote rapid opsonophagocytic killing of wild-type staphylococci. Antibodies directed against IsdA interfered with heme-binding and IsdB antibodies perturbed the ability of this surface protein to bind hemoglobin. As the structural genes for isdA and isdB are required for heme-iron scavenging during the pathogenesis of infection, we hypothesize that IsdA and IsdB antibodies may at least in part provide protection against staphylococci by interfering with the pathogen's heme-iron scavenging mechanisms.