Staphylococcus aureus coagulases are exploitable yet stable public goods in clinically relevant conditions.

Coagulation is an innate defense mechanism intended to limit blood loss and trap invading pathogens during infection. However, Staphylococcus aureus has the ability to hijack the coagulation cascade and generate clots via secretion of coagulases. Although many S. aureus have this characteristic, some do not. The population dynamics regarding this defining trait have yet to be explored. We report here that coagulases are public goods that confer protection against antimicrobials and immune factors within a local population or community, thus promoting growth and virulence. By utilizing variants of a methicillin-resistant S. aureus we infer that the secretion of coagulases is a cooperative trait, which is subject to exploitation by invading mutants that do not produce the public goods themselves. However, overexploitation, "tragedy of the commons," does not occur at clinically relevant conditions. Our micrographs indicate this is due to spatial segregation and population viscosity. These findings emphasize the critical role of coagulases in a social evolution context and provide a possible explanation as to why the secretion of these public goods is maintained in mixed S. aureus communities.

[Virulence factors of Staphylococcus aureus associated with intramammary infections in cows: relevance and role as immunogens]. / Factores de virulencia de Staphylococcus aureus asociados con infecciones mamarias en bovinos: relevancia y rol como agentes inmunógenos.

Staphylococcus aureus is the most prevalent mastitis pathogen in Argentina and worldwide. Lack of effectiveness of traditional control measures based on milking hygiene and antibiotic therapy against this organism has led to the development of alternatives directed to prevent the disease. Among them, the manipulation of host immune mechanisms through vaccination has been explored. The identification of virulence factors able to stimulate host immune defenses is key to developing a rational vaccine. S. aureus has multiple virulence factors that interact with the host at different stages of an intramammary infection. The use of some of these factors as immunogens has been shown to elicit protective responses in the host. The structure, function, and use as immunogens of S. aureus virulence factors considered to be relevant at different stages of intrammamary infections caused by this organism are reviewed in this article.

Factores de virulencia de Staphylococcus aureus asociados con infecciones mamarias en bovinos: relevancia y rol como agentes inmunógenos / [Virulence factors of Staphylococcus aureus associated with intramammary infections in cows: relevance and role as immunogens].

Staphylococcus aureus is the most prevalent mastitis pathogen in Argentina and worldwide. Lack of effectiveness of traditional control measures based on milking hygiene and antibiotic therapy against this organism has led to the development of alternatives directed to prevent the disease. Among them, the manipulation of host immune mechanisms through vaccination has been explored. The identification of virulence factors able to stimulate host immune defenses is key to developing a rational vaccine. S. aureus has multiple virulence factors that interact with the host at different stages of an intramammary infection. The use of some of these factors as immunogens has been shown to elicit protective responses in the host. The structure, function, and use as immunogens of S. aureus virulence factors considered to be relevant at different stages of intrammamary infections caused by this organism are reviewed in this article.

The role of staphylothrombin-mediated fibrin deposition in catheter-related Staphylococcus aureus infections.

Staphylococcus aureus (S. aureus) is a frequent cause of catheter-related infections. S. aureus secretes the coagulases staphylocoagulase and von Willebrand factor-binding protein, both of which form a staphylothrombin complex upon binding to prothrombin. Although fibrinogen and fibrin facilitate the adhesion of S. aureus to catheters, the contribution of staphylothrombin-mediated fibrin has not been examined. In this study, we use a S. aureus mutant lacking both coagulases (Δcoa/vwb) and dabigatran, a pharmacological inhibitor of both staphylothrombin and thrombin, to address this question. Genetic absence or chemical inhibition of pathogen-driven coagulation reduced both fibrin deposition and the retention of S. aureus on catheters in vitro. In a mouse model of jugular vein catheter infection, dabigatran reduced bacterial load on jugular vein catheters, as well as metastatic kidney infection. Importantly, inhibition of staphylothrombin improved the efficacy of vancomycin treatment both in vitro and in the mouse model.

Two distinct coagulase-dependent barriers protect Staphylococcus aureus from neutrophils in a three dimensional in vitro infection model.

Staphylococcus aureus is a pyogenic abscess-forming facultative pathogenic microorganism expressing a large set of virulence-associated factors. Among these, secreted proteins with binding capacity to plasma proteins (e.g. fibrinogen binding proteins Eap and Emp) and prothrombin activators such as Coagulase (Coa) and vWbp are involved in abscess formation. By using a three-dimensional collagen gel (3D-CoG) supplemented with fibrinogen (Fib) we studied the growth behavior of S. aureus strain Newman and a set of mutants as well as their interaction with mouse neutrophils by real-time confocal microscopy. In 3D-CoG/Fib, S. aureus forms microcolonies which are surrounded by an inner pseudocapsule and an extended outer dense microcolony-associated meshwork (MAM) containing fibrin. Coa is involved in formation of the pseudocapsule whereas MAM formation depends on vWbp. Moreover, agr-dependent dispersal of late stage microcolonies could be observed. Furthermore, we demonstrate that the pseudocapsule and the MAM act as mechanical barriers against neutrophils attracted to the microcolony. The thrombin inhibitor argatroban is able to prevent formation of both pseudocapsule and MAM and supports access of neutrophils to staphylococci. Taken together, this model can simulate specific stages of S. aureus abscess formation by temporal dissection of bacterial growth and recruitment of immune cells. It can complement established animal infection models in the development of new treatment options.

Adhesion mechanisms of staphylococci.

Staphylococcal adherence to an either biotic or abiotic surface is the critical first event in the establishment of an infection with these serious pathogens. Especially Staphylococcus aureus harbours a variety of proteinaceous and non-proteinaceous adhesins that mediate attachment to a multitude of host factors, such as extracellular matrix and plasma proteins and human host cells, or intercellular adhesion, which is essential for biofilm accumulation. Proteinaceous adhesins may be classified in covalently surface-anchored proteins of the MSCRAMM (microbial surface components recognizing adhesive matrix molecules) family or in proteins that are surface-associated by different means, such as ionic or hydrophobic interactions. Non-covalently surface-associated proteins include the autolysin/adhesins, proteins of the SERAM (secretable expanded repertoire adhesive molecules) family, or membrane-spanning proteins. Non-proteinaceous adhesins comprise the polysaccharide PIA (polysaccharide intercellular adhesin) and wall teichoic and lipoteichoic acids. The features and functions of surface and surface-associated protein adhesins as well as of non-proteinaceous adhesins are discussed.

Key role for clumping factor B in Staphylococcus aureus nasal colonization of humans.

BACKGROUND: Staphylococcus aureus permanently colonizes the vestibulum nasi of one-fifth of the human population, which is a risk factor for autoinfection. The precise mechanisms whereby S. aureus colonizes the nose are still unknown. The staphylococcal cell-wall protein clumping factor B (ClfB) promotes adhesion to squamous epithelial cells in vitro and might be a physiologically relevant colonization factor. METHODS AND FINDINGS: We define the role of the staphylococcal cytokeratin-binding protein ClfB in the colonization process by artificial inoculation of human volunteers with a wild-type strain and its single locus ClfB knock-out mutant. The wild-type strain adhered to immobilized recombinant human cytokeratin 10 (CK10) in a dose-dependent manner, whereas the ClfB(-) mutant did not. The wild-type strain, when grown to the stationary phase in a poor growth medium, adhered better to CK10, than when the same strain was grown in a nutrient-rich environment. Nasal cultures show that the mutant strain is eliminated from the nares significantly faster than the wild-type strain, with a median of 3 +/- 1 d versus 7 +/- 4 d (p = 0.006). Furthermore, the wild-type strain was still present in the nares of 3/16 volunteers at the end of follow-up, and the mutant strain was not. CONCLUSIONS: The human colonization model, in combination with in vitro data, shows that the ClfB protein is a major determinant of nasal-persistent S. aureus carriage and is a candidate target molecule for decolonization strategies.

Molecular basis for Staphylococcus aureus-mediated platelet aggregate formation under arterial shear in vitro.

OBJECTIVE: Staphylococcus aureus is the most frequent causative organism of infective endocarditis (IE) and is characterized by thrombus formation on a cardiac valve that can embolize to a distant site. Previously, we showed that S. aureus clumping factor A (ClfA) and fibronectin-binding protein A (FnBPA) can stimulate rapid platelet aggregation. METHODS AND RESULTS: In this study we investigate their relative roles in mediating aggregate formation under physiological shear conditions. Platelets failed to interact with immobilized wild-type S. aureus (Newman) at shear rates <500 s(-1) but rapidly formed an aggregate at shear rates >800 s(-1). Inactivation of the ClfA gene eliminated aggregate formation at any shear rate. Using surrogate hosts that do not interact with platelets bacteria overexpressing ClfA supported rapid aggregate formation under high shear with a similar profile to Newman whereas bacteria overexpressing FnBPA did not. Fibrinogen binding to ClfA was found to be essential for aggregate formation although fibrinogen-coated surfaces only allowed single-platelets to adhere under all shear conditions. Blockade of the platelet immunoglobulin receptor Fc gammaRIIa inhibited aggregate formation. CONCLUSIONS: Thus, fibrinogen and IgG binding to ClfA is essential for aggregate formation under arterial shear conditions and may explain why S. aureus is the major cause of IE.

Clumping factor A of Staphylococcus aureus inhibits phagocytosis by human polymorphonuclear leucocytes.

Staphylococcus aureus is a major cause of nosocomial and community-acquired infection. It expresses several factors that promote avoidance of phagocytosis by polymorphonuclear leucocytes. Clumping factor A (ClfA) is a fibrinogen-binding surface protein of S. aureus that is an important virulence factor in several infection models. This study investigated whether ClfA is an antiphagocytic factor, and whether its antiphagocytic properties were based on its ability to bind fibrinogen. In S. aureus, ClfA was shown to be of equal importance to protein A, the antiphagocytic properties of which are well established. ClfA expressed in a surrogate Gram-positive host was also found to be antiphagocytic. A ClfA mutant that was unable to bind fibrinogen had a similar antiphagocytic effect to native ClfA in the absence of fibrinogen. ClfA inhibited phagocytosis in the absence of fibrinogen, and showed enhanced inhibition in the presence of fibrinogen.

The role of Staphylococcus aureus adhesins in the pathogenesis of ventricular assist device-related infections.

Ventricular assist devices (VADs) are an important form of therapy for end-stage congestive heart failure. However, infection of the VAD, which is often caused by Staphylococcus aureus, poses a major threat to survival. Using a novel in vitro binding assay with VAD membranes and a heterologous lactococcal system of expression, we identify 3 S. aureus proteins--clumping factor A (ClfA) and fibronectin binding proteins A and B (FnBPA and FnBPB) as the main factors involved in adherence to VAD polyurethane membranes. Adherence is greatly diminished by long implantation times, reflecting a change in topological features of the VAD membrane, and is primarily mediated by the FnBPA domains in the staphylococcal proteins. We also compare the adherence of S. aureus mutant strains and show that other staphylococcal components appear to be involved in adherence to VAD membranes. Finally, we demonstrate that ClfA, FnBPA, and FnBPB mediate bacterial infection of implanted murine intra-aortic polyurethane patches.

ClfA(221-550), a fibrinogen-binding segment of Staphylococcus aureus clumping factor A, disrupts fibrinogen function.

Clumping factor A (ClfA) is a surface protein of Staphylococcus aureus bacteria known for its ability to bind the C-terminus of plasma fibrinogen gamma chain, which participates in mediating fibrinogen-platelet interaction and fibrin cross-linking, resulting in thrombus formation. With an aim to develop agents that block fibrinogen gamma chain C-terminus, the fibrinogen-binding segment of ClfA locating at residues 221-550 was produced by recombinant technology and tested for its ability to inhibit platelet functions and fibrin clot formation. Recombinant ClfA(221-550) bound fibrinogen and blocked fibrinogen-platelet interaction, resulting in the inhibition of both ADP- and collagen-induced platelet aggregations. ClfA(221-550) also affected fibrin clot formation, in which factor XIIIa-mediated cross-linking of fibrinogen gamma chains was abrogated by ClfA(221-550) leaving the release of fibrinopeptides A and B from fibrinogen by thrombin unaltered, indicating that ClfA(221-550) interfered with fibrin clot formation without affecting thrombin's catalytic activity. Platelet-mediated clot retraction depends on both platelet-fibrinogen interaction and fibrin clot formation, which makes platelet thrombus less susceptible to fibrinolysis. At the concentration that reduced platelet aggregation by 40%, ClfA(221-550) prevented platelet-mediated clot retraction, whereas the glycoprotein IIb/IIIa antagonist tirofiban needed a higher concentration in inhibiting clot retraction than inhibiting platelet aggregation. By virtue of the multiple effects of ClfA(221-550) on platelet aggregation, fibrin clot formation and platelet-mediated clot retraction, the binding of ClfA(221-550) to fibrinogen merits further investigation for its potential as a new antithrombotic agent.

Fibronectin-binding proteins and fibrinogen-binding clumping factors play distinct roles in staphylococcal arthritis and systemic inflammation.

Staphylococcus aureus is a commonly encountered pathogen in humans, and it has the potential to cause destructive and life-threatening conditions, including septic arthritis. The pathogenicity of staphylococci depends on the expression of virulence factors. Among these, staphylococcal cell-surface proteins with tissue-adhesive functions have been suggested to mediate the colonization of host tissues, a crucial step in the establishment of infection. We investigated the relative contribution of the fibronectin-binding proteins (FnBPs) and fibrinogen-binding clumping factors (Clfs) to staphylococcal virulence in an experimental model of septic arthritis. The results show that these 2 sets of proteins play distinctly different roles in the development and progression of septic arthritis. Although Clfs significantly contributed to the arthritogenicity of S. aureus, FnBPs had no effect on the development of arthritis. Conversely, FnBPs played an important role in the induction of systemic inflammation, characterized by interleukin-6 secretion, severe weight loss, and mortality.

Soluble fibrin is the main mediator of Staphylococcus aureus adhesion to platelets.

BACKGROUND: Infective endocarditis (IE) caused by Staphylococcus aureus is associated with significant morbidity and mortality rates. Platelets play a dual role as adhesive cells forming associates with bacteria as well as specialized inflammatory cells. The specific role of the various factors involved in bacteria-platelet association has not yet been fully elucidated. METHODS AND RESULTS: We observed a dramatic increase in the capability to bind S aureus when platelets were activated with thrombin (from 5% to 30%, P<0.001). To pinpoint platelet-binding sites involved in the interaction, platelets from knockout mice and from patients with selective inherited deficiency of membrane proteins or of granules were used. CD36, GPIIb/IIIa, and P-selectin were excluded as receptors for S aureus. Platelets from patients with alpha-delta-storage pool disease and Gray platelet syndrome indicate the requirement of alpha-granule contents. Platelet activation by ADP did not promote platelet-S aureus associate formation, although these platelets were covered with bound fibrinogen. Only small numbers of associates between fibrinogen-covered bacteria and ADP-activated platelets were observed. Formation of fibrin alone was also not sufficient to induce association. Only when fibrin formation and platelet activation occurred together were large numbers of associates formed (P<0.001). A potential receptor for fibrin on S aureus is clumping factor A. Addition of thrombospondin-1 to control platelets increased the number of associates (P=0.02). CONCLUSIONS: Soluble fibrin but not fibrinogen is the main mediator of platelet-S aureus association. In addition, platelet activation and the release of alpha-granule contents, particularly thrombospondin-1, is a requirement for platelet-S aureus association.

Clumping factor A-mediated virulence during Staphylococcus aureus infection is retained despite fibrinogen depletion.

Clumping factor A (ClfA), a fibrinogen-binding protein expressed on the Staphylococcus aureus cell surface, has previously been shown to act as a virulence factor in experimental septic arthritis. Although the interaction between ClfA and fibrinogen is assumed to be of importance for the virulence of S. aureus, this has not been demonstrated in any in vivo model of infection. Therefore, the objective of this study was to investigate the contribution of this interaction to ClfA-mediated virulence in murine S. aureus-induced arthritis. Ancrod, a serine protease with thrombin-like activity, was used to induce in vivo depletion of fibrinogen in mice. Ancrod treatment significantly aggravated septic arthritis following inoculation with a ClfA-expressing strain (Newman) compared to control treatment. Also, ancrod treatment tended to enhance the arthritis induced by a clfA mutant strain (DU5876), indicating that fibrinogen depletion exacerbates septic arthritis in a ClfA-independent manner. Most importantly, the ClfA-expressing strain was much more arthritogenic than the isogenic clfA mutant, following inoculation of fibrinogen-depleted mice. This finding indicates that the interaction between ClfA and free fibrinogen is not required for ClfA-mediated functions contributing to S. aureus virulence. It is conceivable that ClfA contributes to the virulence of S. aureus through interactions with other host ligands than fibrinogen.

Susceptibility of Gram-positive bacteria from ICU patients in UK hospitals to antimicrobial agents.

Microbiologists in 25 sentinel laboratories were each asked to refer up to 100 clinically-significant Gram-positive bacteria isolated from consecutive intensive care unit (ICU) patients. A total of 1595 isolates were collected from patients in 23 hospitals; these included Staphylococcus aureus (47.6%), coagulase-negative staphylococci (CNS) (30.6%), enterococci (14.3%), pneumococci (2.8%) and other streptococci (3.5%). A few coryneforms, other bacilli and a Nocardia sp. were also collected. Rates of oxacillin resistance among S. aureus and CNS isolates were 59.3 and 78.5%, respectively. Vancomycin-resistant S. aureus were not detected, although two isolates (0.3%) were resistant to teicoplanin [minimum inhibitory concentrations (MICs) 8 mg/L]. In contrast, 13.7% of CNS were teicoplanin resistant (MICs 8-32 mg/L) and 1.2% were resistant to vancomycin. Among the enterococci, 72.5% were Enterococcus faecalis and 24.5% were Enterococcus faecium, the remainder including isolates of Enterococcus casseliflavus or Enterococcus gallinarum. Eighteen percent of E. faecium isolates were vancomycin-resistant, compared with only 3% of E. faecalis isolates. Rates of high-level gentamicin resistance in E. faecalis and E. faecium were 40 and 25%, respectively. Nine percent of pneumococci and streptococci were resistant to penicillin, with 7 and 11%, respectively, resistant to erythromycin. None of the isolates showed resistance to linezolid, with the MICs for the entire study population falling in the range of 0.5-4 mg/L.

Development and validation of a molecular beacon probe-based real-time polymerase chain reaction assay for rapid detection of methicillin resistance in Staphylococcus aureus.

CONTEXT: A rapid, real-time, duplex, fluorescent molecular beacon probe-based polymerase chain reaction (PCR) assay was recently developed for the detection of methicillin-resistant Staphylococcus aureus. OBJECTIVE: To describe the development and validation of this unique assay. DESIGN: Prospective laboratory analysis. SETTING: Urban health region/centralized diagnostic microbiology laboratory. BACTERIAL STRAINS: One hundred eighty-one previously characterized clinical and American Type Culture Collection isolates, including 50 strains each of methicillin-resistant and methicillin-sensitive S aureus, plus 50 strains of coagulase-negative staphylococci and 31 nonstaphylococcal isolates to ensure assay specificity. INTERVENTION: Assays were performed on purified genomic DNA extracted from growing bacterial colonies. Two sets of oligonucleotide primers were used to specifically amplify the mecA and nuc genes, followed by detection of amplicons using fluorophore-labeled molecular beacon probes. Assays were performed on the Mx4000 Multiplex Quantitative PCR System (Stratagene Inc, La Jolla, Calif). MAIN OUTCOME MEASURES: (1) Assay sensitivity and specificity, and (2) analytical sensitivity. RESULTS: The assay demonstrated 100% sensitivity and 100% specificity, and accurately characterized isolates as methicillin-resistant S aureus, methicillin-sensitive S aureus, or methicillin-resistant coagulase-negative staphylococci, with test results available in 2.5 hours. The analytical sensitivity of the assay was determined to be between 6 and 60 genomic equivalents. CONCLUSIONS: This assay is rapid, accurate, easy to perform, and is compatible with other real-time PCR instruments, making it a suitable alternative to conventional PCR methodologies.

Reassessing the role of Staphylococcus aureus clumping factor and fibronectin-binding protein by expression in Lactococcus lactis.

Since Staphylococcus aureus expresses multiple pathogenic factors, studying their individual roles in single-gene-knockout mutants is difficult. To circumvent this problem, S. aureus clumping factor A (clfA) and fibronectin-binding protein A (fnbA) genes were constitutively expressed in poorly pathogenic Lactococcus lactis using the recently described pOri23 vector. The recombinant organisms were tested in vitro for their adherence to immobilized fibrinogen and fibronectin and in vivo for their ability to infect rats with catheter-induced aortic vegetations. In vitro, both clfA and fnbA increased the adherence of lactococci to their specific ligands to a similar extent as the S. aureus gene donor. In vivo, the minimum inoculum size producing endocarditis in > or =80% of the rats (80% infective dose [ID80]) with the parent lactococcus was > or =10(7) CFU. In contrast, clfA-expressing and fnbA-expressing lactococci required only 10(5) CFU to infect the majority of the animals (P < 0.00005). This was comparable to the infectivities of classical endocarditis pathogens such as S. aureus and streptococci (ID80 = 10(4) to 10(5) CFU) in this model. The results confirmed the role of clfA in endovascular infection, but with a much higher degree of confidence than with single-gene-inactivated staphylococci. Moreover, they identified fnbA as a critical virulence factor of equivalent importance. This was in contrast to previous studies that produced controversial results regarding this very determinant. Taken together, the present observations suggest that if antiadhesin therapy were to be developed, at least both of the clfA and fnbA products should be blocked for the therapy to be effective.

Loss of clumping factor B fibrinogen binding activity by Staphylococcus aureus involves cessation of transcription, shedding and cleavage by metalloprotease.

The fibrinogen-binding protein clumping factor B (ClfB) of Staphylococcus aureus is present on the surface of cells from the early exponential phase of growth in greater amounts than on cells from late exponential phase and is barely detectable on cells from stationary phase. Expression of a clfB-lacZ fusion indicated that transcription stopped before the end of exponential phase. Mutations in the global regulators agr and sar had no effect on clfB transcription. The loss of ClfB protein from cells in stationary phase was due to expression ending before cells stopped growing, combined with shedding of some of the protein into the growth medium and dilution of those molecules remaining on the cell surface during the two to three cell division events leading to stationary phase. Two forms of the protein occurred on the cell surface, the smaller of which was generated by loss of a domain from the N terminus. The proportion of the smaller form increased as the cultures grew. The metalloprotease aureolysin was shown to be responsible for cleavage of ClfB. Cleavage was inhibited by EDTA and o-phenanthroline and did not occur in an aureolysin-deficient mutant. Purified aureolysin promoted cleavage of cell surface-located ClfB as well as the recombinant A domain of ClfB. Cleavage was detected at two sites, one located between residues Ser(197) and Leu(198) and the other between Ala(199) and Val(200). The truncated form of ClfB did not bind fibrinogen.

Clumping factor A mediates binding of Staphylococcus aureus to human platelets.

The direct binding of bacteria to platelets may be an important virulence mechanism in the pathogenesis of infective endocarditis. We have previously described Staphylococcus aureus strain PS12, a Tn551-derived mutant of strain ISP479, with reduced ability to bind human platelets in vitro. When tested in an animal model of endocarditis, the PS12 strain was less virulent than its parental strain, as measured by bacterial densities in endocardial vegetations and incidence of systemic embolization. We have now characterized the gene disrupted in PS12 and its function in platelet binding. DNA sequencing, Southern blotting, and PCR analysis indicate that PS12 contained two Tn551 insertions within the clumping factor A (ClfA) locus (clfA). The first copy was upstream from the clfA start codon and appeared to have no effect on ClfA production. The second insertion was within the region encoding the serine aspartate repeat of ClfA and resulted in the production of a truncated ClfA protein that was secreted from the cell. A purified, recombinant form of the ClfA A region, encompassing amino acids 40 through 559, significantly reduced the binding of ISP479C to human platelets by 44% (P = 0.0001). Immunoprecipitation of recombinant ClfA that had been incubated with solubilized platelet membranes coprecipitated a 118-kDa platelet membrane protein. This protein does not appear to be glycoprotein IIb. These results indicate that platelet binding by S. aureus is mediated in part by the direct binding of ClfA to a novel 118-kDa platelet membrane receptor.

Role of coagulase in a murine model of hematogenous pulmonary infection induced by intravenous injection of Staphylococcus aureus enmeshed in agar beads.

We describe a novel mouse model of acute staphylococcal pneumonia induced by intravenous injection of Staphylococcus aureus enmeshed in agar beads. For comparison, we also used various strains of bacteria, including three strains of S. aureus, two strains of Staphylococcus epidermidis, one strain of Streptococcus pyogenes, three strains of Pseudomonas aeruginosa, and one strain of Klebsiella pneumoniae. All except two strains of S. aureus were cleared rapidly from the lungs. When S. aureus NUMR1 enmeshed in agar beads was injected intravenously, the organisms concentrated and remained in the lung for a period longer than several weeks. Multiple lung abscesses were evident macroscopically, and histological examination of the infected lung showed multiple lung abscesses around the pulmonary arterioles, consisting of bacterial colonies encircled with fibrin filaments and surrounded by inflammatory cells of neutrophils and macrophages. When 14 strains of clinically isolated S. aureus were injected intravenously, the number of bacteria recovered from the lung tissue 7 days after infection correlated with the titer of staphylocoagulase (P < 0.01) but not with the titer of clumping factor. Injection of coagulase-deficient mutant strain DU5843 was associated with a markedly reduced number of viable bacteria isolated from the lung, compared with its coagulase-positive parental strain DU5789. Our results suggest that coagulase may play a role in the development of blood-borne staphylococcal pneumonia in our model. Our animal model is simple and reproducible and resembles blood-borne staphylococcal pneumonia in humans, and it could be useful for investigating the pathogenicity or treatment of staphylococcal pulmonary infection, including infections with methicillin-resistant S. aureus.