Study of Staphylococcus aureus pathogenic genes by transfer and expression in the less virulent organism Streptococcus gordonii

Contribution of staphylococcal virulence factors in the pathogenesis of thrombosis

Staphylococci are responsible for many infections in humans, starting with skin and soft tissue infections and finishing with invasive diseases such as endocarditis, sepsis and pneumonia, which lead to high mortality. Patients with sepsis often demonstrate activated clotting pathways, decreased levels of anticoagulants, decreased fibrinolysis, activated endothelial surfaces and activated platelets. This results in disseminated intravascular coagulation and formation of a microthrombus, which can lead to a multiorgan failure. This review describes various staphylococcal virulence factors that contribute to vascular thrombosis, including deep vein thrombosis in infected patients. The article presents mechanisms of action of different factors released by bacteria in various host defense lines, which in turn can lead to formation of blood clots in the vessels.

Investigation on Potential ESKAPE Surrogates for 222 and 254 nm Irradiation Experiments

Background

Due to the increase in multidrug-resistant pathogens, it is important to investigate further antimicrobial options. In order not to have to work directly with pathogens, the investigation of possible surrogates is an important aspect. It is examined how suitable possible surrogate candidates for ESKAPE pathogens are for UVC applications. In addition, the inactivation sensitivities to 222 and 254 nm radiation are compared in relation.

Methods

Non-pathogenic members (Enterococcus mundtii, Staphylococcus carnosus, Acinetobacter kookii, Pseudomonas fluorescens and Escherichia coli) of genera of ESKAPE strains were photoinactivated in PBS with irradiation wavelengths of 222 and 254 nm (no non-pathogenic Klebsiella was available). Log reduction doses were determined and compared to published photoinactivation results on ESKAPE pathogens. It was assumed that non-pathogenic bacteria could be designated as surrogates for one wavelength and one ESKAPE strain, if the doses were between the 25 and 75% quantiles of published log reduction dose of the corresponding pathogen.

Results

For all non-pathogen relatives (except A. kookii), higher average log reduction doses were required for irradiation at 222 nm than at 254 nm. Comparison by boxplot revealed that five of eight determined log reduction doses of the possible surrogates were within the 25 and 75% quantiles of the data for ESKAPE pathogens. The measured log reduction dose for non-pathogenic E. coli was above the 75% quantile at 222 nm, and the log reduction dose for S. carnosus was below the 25% quantile at 254 nm.

Conclusion

For more than half of the studied cases, the examined ESKAPE relatives in this study can be applied as surrogates for ESKAPE pathogens. Because of lack of data, no clear statement could be made for Enterococcus faecalis at 222 nm and Acinetobacter baumannii at both wavelengths.

Marginal role of von Willebrand factor-binding protein and coagulase in the initiation of endocarditis in rats with catheter-induced aortic vegetations

Staphylococcus aureus is the leading cause of infective endocarditis (IE). While the role of S. aureus cell-wall associated protein clumping factor A (ClfA) in promoting IE has been already demonstrated, that of the secreted plasma-clotting factors staphylocoagulase (Coa) and von Willebrand factor-binding protein (vWbp) has not yet been elucidated. We investigated the role of Coa and vWbp in IE initiation in rats with catheter-induced aortic vegetations, using Lactococcus lactis expressing coa, vWbp, clfA or vWbp/clfA, and S. aureus Newman Δcoa, ΔvWbp, ΔclfA or Δcoa/ΔvWbp/ΔclfA mutants. vWbp-expression increased L. lactis valve infection compared to parent and coa-expressing strains (incidence: 62%, versus 0% and 13%, respectively; P < 0.01). Likewise, expression of clfA increased L. lactis infectivity (incidence: 80%), which was not further affected by co-expression of vWbp. In symmetry, deletion of the coa or vWbp genes in S. aureus did not decrease infectivity (incidence: 68 and 64%, respectively) whereas deletion of clfA did decrease valve infection (incidence: 45%; P = 0.03 versus parent), which was not further affected by the triple deletion Δcoa/ΔvWbp/ΔclfA (incidence: 36%; P > 0.05 versus ΔclfA mutant). Coa does not support the initial colonization of IE (in L. lactis) without other key virulence factors and vWbp contributes to initiation of IE (in L. lactis) but is marginal in the present of ClfA.

Staphylococcus aureus, master manipulator of the human hemostatic system

The coagulation system does not only offer protection against bleeding, but also aids in our defense against invading microorganisms. The hemostatic system and innate immunity are strongly entangled, which explains why so many infections are complicated by either bleeding or thrombosis. Staphylococcus aureus (S. aureus), currently the most deadly infectious agent in the developed world, causes devastating intravascular infections such as sepsis and infective endocarditis. During these infections S. aureus comes in close contact with the host hemostatic system and proves to be a master in manipulating coagulation. The coagulases of S. aureus directly induce coagulation by activating prothrombin. S. aureus also manipulates fibrinolysis by triggering plasminogen activation via staphylokinase. Furthermore, S. aureus binds and activates platelets and interacts with key coagulation proteins such as fibrin(ogen), fibronectin and von Willebrand factor. By manipulating the coagulation system S. aureus gains a significant advantage over the host defense mechanisms. Studying the interplay between S. aureus and the hemostatic system can therefore lead to new innovative therapies for battling S. aureus infections.

Vaccines and Immunotherapy for Staphylococcal Infections

Nosocomial or hospital-acquired infections are associated with prolonged hospitalizations and increased healthcare costs. Infections associated with surgical implants are becoming more difficult and more costly to manage, as they require repeated surgical procedures and a longer period of time to treat patients. Continued advances in the use of medical devices, an increase in the number of immunocompromised patients, and a steady rise in the prevalence of antibiotic-resistant organisms has renewed interest in the development of novel therapies that can be used to prevent and treat nosocomial infections. This review provides an overview of bacterial adhesins and focuses on novel immunological therapies developed to treat staphylococcal infections.

Pathogenic conversion of coagulase-negative staphylococci

Humans and animals are colonized by members of the genus Staphylococcus, however only some of these species evolved to cause invasive disease. The genetic basis for conversion of commensal staphylococci into pathogens is not known. We hypothesized that Staphylococcus aureus genes for coagulation and agglutination in vertebrate blood (coa, vwb and clfA) may support pathogenic conversion. Expression of coa and vwb in Staphylococcus epidermidis or Staphylococcus simulans supported a coagulase-positive phenotype but not the ability to cause disease in a mouse model of bloodstream infection. However, the simultaneous expression of coa, vwb and clfA in coagulase-negative staphylococci enabled bacterial agglutination in plasma and enhanced survival of S. simulans in human whole blood. Agglutination of S. simulans in the bloodstream of infected mice upon expression of coa, vwb and clfA provided also a mean for dissemination and replication in distal organs. Thus, the acquisition of genes for bacterial agglutination with fibrin appear sufficient for the conversion of commensal staphylococci into invasive pathogens.

Staphylococcus aureus Aggregation and Coagulation Mechanisms, and Their Function in Host-Pathogen Interactions

The human commensal bacterium Staphylococcus aureus can cause a wide range of infections ranging from skin and soft tissue infections to invasive diseases like septicemia, endocarditis, and pneumonia. Muticellular organization almost certainly contributes to S. aureus pathogenesis mechanisms. While there has been considerable focus on biofilm formation and its role in colonizing prosthetic joints and indwelling devices, less attention has been paid to nonsurface-attached group behavior like aggregation and clumping. S. aureus is unique in its ability to coagulate blood, and it also produces multiple fibrinogen-binding proteins that facilitate clumping. Formation of clumps, which are large, tightly packed groups of cells held together by fibrin(ogen), has been demonstrated to be important for S. aureus virulence and immune evasion. Clumps of cells are able to avoid detection by the host's immune system due to a fibrin(ogen) coat that acts as a shield, and the size of the clumps facilitates evasion of phagocytosis. In addition, clumping could be an important early step in establishing infections that involve tight clusters of cells embedded in host matrix proteins, such as soft tissue abscesses and endocarditis. In this review, we discuss clumping mechanisms and regulation, as well as what is known about how clumping contributes to immune evasion.

Two Dimensional Structural Analysis and Expression of a New Staphylococcus aureus Adhesin Based Fusion Protein

OBJECTIVES

Staphylococcus aureus is a foremost source of numerous nosocomial and community acquired infections. Antibiotic therapy for vancomycin resistant S. aureus (VRSA) can not promise the eradication of infections. Since adhesion is the major route of infections, adhesin based vaccine could suppress S. aureus infections. Fibronectin binding protein A (FnBPA) and clumping factor A (ClfA) are major responsible adhesions involved in S. aureus infections, so they could be candidate vaccine molecules against an extensive range of infections. This project intended to express a new fusion protein construct and analysis of biological activity regarding binding activity.

MATERIALS AND METHODS

pfnbA- ClfA construct was transformed to Escherichia coli BL21 (DE3). Transformant E. coli were grown in LB broth and induced with IPTG and cellular extracts were separated on SDS-PAGE. RT-PCR was performed to verify expression. Binding activity of fusion protein was studied using human gingival fibroblast (HGF) cell line. D1-D3 protein from unpublished study was used as control.

RESULTS

The expected fusion protein fragment showed by SDS-PAGE. RT-PCR verified the existence of mRNA relating to expressed fusion protein. Binding activity of S. aureus decreased after treatment of HGF cells with fusion protein.

CONCLUSION

In total, binding activity of fusion protein was approximately two fold lesser than D1-D3 protein. It is supposed that the fusion protein could not be attached to its ligand easily and would be more accessible to antigen presenting cells and consequently protective antibodies will be produced. This project is pending for in vivo infection study in animal model.

Staphylococcus aureus secretes coagulase and von Willebrand factor binding protein to modify the coagulation cascade and establish host infections

Clinical isolates of Staphylococcus aureus secrete coagulases, polypeptides that bind to and activate prothrombin, thereby converting fibrinogen to fibrin and promoting the clotting of plasma or blood. Two staphylococcal products, the canonical coagulase (Coa) as well as the recently identified von Willebrand factor binding protein (vWbp), promote similar modifications of the coagulation cascade during host infection. Staphylococcal binding to fibrinogen or fibrin is an important attribute of disease pathogenesis, which leads to the formation of abscesses and bacterial persistence in host tissues and also enables the pathogen to cause lethal sepsis. Circumstantial evidence suggests that the product of coagulase activity, staphylococci captured within a fibrin meshwork, enable this pathogen to disseminate as thromboembolic lesions and to resist opsonophagocytic clearance by host immune cells. In addition, the coagulation products of staphylococci appear to display discrete differences when compared to those of thrombin-mediated coagulation, the latter representing a key innate defense mechanism against many invading pathogens. Preclinical evidence suggests that inactivation or neutralization of coagulases may prevent the pathogenesis of staphylococcal infections, a strategy that could be used to combat the current epidemic of hospital-acquired infections with drug-resistant S. aureus isolates.

Inhibition of staphylothrombin by dabigatran reduces Staphylococcus aureus virulence

BACKGROUND

Staphylocoagulase and von Willebrand binding protein (VWbp) bind to prothrombin to form the staphylothrombin complex that converts fibrinogen into fibrin.

OBJECTIVES

To study the role of staphylothrombin and its inhibition by dabigatran on Staphylococcus aureus virulence.

METHODS

We studied the effect of staphylothrombin inhibition on bacterial attachment to polystyrene surfaces, leukocyte activation and bactericidal activity for S. aureus ATCC 25923, S. aureus Newman, and staphylocoagulase- and VWbp-negative S. aureus Newman mutants in the presence or absence of prothrombin and fibrinogen. We measured the abscess size after subcutaneous (s.c.) injection of S. aureus ATCC 25923 and S. aureus Newman, as well as an S. aureus Newman mutant strain lacking staphylocoagulase and VWbp, in mice treated with either dabigatran or placebo.

RESULTS

Staphylothrombin-mediated fibrin increased the association of S. aureus to polystyrene surfaces and reduced the bactericidal activity of leukocytes. The absence or inhibition of staphylothrombin decreased the bacterial association, enhanced leukocyte activation and reduced bacterial survival in vitro. Abscess size was smaller in mice treated with dabigatran or infected with a coagulase-negative mutant.

CONCLUSION

Inhibition or the absence of staphylothrombin reduced S. aureus virulence in in vitro and in vivo models.

Evaluation of clumping factor A binding region A in a subunit vaccine against Staphylococcus aureus-induced mastitis in mice

The present study evaluated the potential of recombinant binding region A of clumping factor A (rClfA-A) to be an effective component of a vaccine against mastitis induced by Staphylococcus aureus in the mouse. rClfA-A and inactivated S. aureus were each emulsified in Freund's adjuvant, mineral oil adjuvant, and Seppic adjuvant; phosphate-buffered saline was used as a control. Seven groups of 12 mice each were immunized intraperitoneally three times at 2-week intervals. The titers of IgG and subtypes thereof (IgG1 and IgG2a) in the rClfA-A-immunized group were more than 1,000-fold higher than those in the killed-bacteria-immunized group (P < 0.01). Of the three adjuvants used, mineral oil adjuvant induced the highest antibody levels for both antigens (P < 0.001). Furthermore, the anti-rClfA-A antibody capacities for bacterial adhesion and opsonizing phagocytosis were significantly greater in the rClfA-A-immunized group than in the killed-bacteria-immunized group (P < 0.05). Lactating mice immunized with either rClfA-A or inactivated vaccine were challenged with S. aureus via the intramammary route. The numbers of bacteria recovered from the murine mammary glands 24 h after inoculation were significantly lower in the rClfA-A group than in the killed-bacteria-immunized group (P < 0.001). Histologic examination of the mammary glands showed that rClfA-A immunization effectively preserved tissue integrity. Thus, rClfA-A emulsified in an oil adjuvant provides strong immune protection against S. aureus-induced mastitis in the mouse.

Outer membrane porin M35 of Moraxella catarrhalis mediates susceptibility to aminopenicillins

Background

The outer membrane protein M35 is a conserved porin of type 1 strains of the respiratory pathogen Moraxella catarrhalis. It was previously shown that M35 is involved in the uptake of essential nutrients required for bacterial growth and for nasal colonization in mice. The aim of this study was (i) to characterize the potential roles of M35 in the host-pathogen interactions considering the known multifunctionality of porins and (ii) to characterize the degree of conservation in the phylogenetic older subpopulation (type 2) of M. catarrhalis.

Results

Isogenic m35 mutants of the type 1 strains O35E, 300 and 415 were tested for their antimicrobial susceptibility against 15 different agents. Differences in the MIC (Minimum Inhibitory Concentration) between wild-type and mutant strains were found for eight antibiotics. For ampicillin and amoxicillin, we observed a statistically significant 2.5 to 2.9-fold MIC increase (p < 0.03) in the m35 mutants. Immunoblot analysis demonstrated that human saliva contains anti-M35 IgA. Wild-type strains and their respective m35 mutants were indistinguishable with respect to the phenotypes of autoagglutination, serum resistance, iron acquisition from human lactoferrin, adherence to and invasion of respiratory tract epithelial cells, and proinflammatory stimulation of human monocytes. DNA sequencing of m35 from the phylogenetic subpopulation type 2 strain 287 revealed 94.2% and 92.8% identity on the DNA and amino acid levels, respectively, in comparison with type 1 strains.

Conclusion

The increase in MIC for ampicillin and amoxicillin, respectively, in the M35-deficient mutants indicates that this porin affects the outer membrane permeability for aminopenicillins in a clinically relevant manner. The presence of IgA antibodies in healthy human donors indicates that M35 is expressed in vivo and recognized as a mucosal antigen by the human host. However, immunoblot analysis of human saliva suggests the possibility of antigenic variation of immunoreactive epitopes, which warrants further analysis before M35 can be considered a potential vaccine candidate.

Infectivity of Lactobacillus rhamnosus and Lactobacillus paracasei isolates in a rat model of experimental endocarditis

The potential pathogenicity of selected (potentially) probiotic and clinical isolates of Lactobacillus rhamnosus and Lactobacillus paracasei was investigated in a rat model of experimental endocarditis. In addition, adhesion properties of the lactobacilli for fibrinogen, fibronectin, collagen and laminin, as well as the killing activity of the platelet-microbicidal proteins fibrinopeptide A (FP-A) and connective tissue activating peptide 3 (CTAP-3), were assessed. The 90 % infective dose (ID(90)) of the L. rhamnosus endocarditis isolates varied between 10(6) and 10(7) c.f.u., whereas four of the six (potentially) probiotic L. rhamnosus isolates showed an ID(90) that was at least 10-fold higher (10(8) c.f.u.) (P<0.001). In contrast, the two other probiotic L. rhamnosus isolates exhibited an ID(90) (10(6) and 10(7) c.f.u.) comparable to the ID(90) of the clinical isolates of this species investigated (P>0.05). Importantly, these two probiotic isolates shared the same fluorescent amplified fragment length polymorphism cluster type as the clinical isolate showing the lowest ID(90) (10(6) c.f.u.). L. paracasei tended to have a lower infectivity than L. rhamnosus (ID(90) of 10(7) to > or =10(8) c.f.u.). All isolates had comparable bacterial counts in cardiac vegetations (P>0.05). Except for one L. paracasei strain adhering to all substrates, all tested lactobacilli adhered only weakly or not at all. The platelet peptide FP-A did not show any microbicidal activity against the tested lactobacilli, whereas CTAP-3 killed the majority of the isolates. In general, these results indicate that probiotic lactobacilli display a lower infectivity in experimental endocarditis compared with true endocarditis pathogens. However, the difference in infectivity between L. rhamnosus endocarditis and (potentially) probiotic isolates could not be explained by differences in adherence or platelet microbicidal protein susceptibility. Other disease-promoting factors may exist in these organisms and warrant further investigation.

Effects of alpha-phosphoglucomutase deficiency on cell wall properties and fitness in Streptococcus gordonii

Streptococcus gordonii alpha-phosphoglucomutase, which converts glucose 6-phosphate to glucose 1-phosphate, is encoded by pgm. The pgm transcript is monocistronic and is initiated from a sigma(A)-like promoter. Mutants with a gene disruption in pgm exhibited an altered cell wall muropeptide pattern and a lower teichoic acid content, and had reduced fitness both in vitro and in vivo. In vitro, the reduced fitness included reduced growth, reduced viability in the stationary phase and increased autolytic activity. In vivo, the pgm-deficient strain had a lower virulence in a rat model of experimental endocarditis.

Lack of in vitro biofilm formation does not attenuate the virulence of Streptococcus gordonii in experimental endocarditis

The ability to induce experimental endocarditis of biofilm-deficient mutants of Streptococcus gordonii was studied in an isogenic background. Strains were inactivated in either comD, fruK or pbp2b genes, which are involved in biofilm formation. These strains were clearly impaired (>75% reduction) in biofilm production in vitro. However, this did not result in a decreased severity of infection in vivo.

Clumping factor A ofStaphylococcus aureusinhibits 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.

Characterization of a humanized monoclonal antibody recognizing clumping factor A expressed by Staphylococcus aureus

We report the humanization and characterization of monoclonal antibody (MAb) T1-2 or tefibazumab, a monoclonal antibody that recognizes clumping factor A expressed on the surface of Staphylococcus aureus. We demonstrate that the binding kinetics of MAb T1-2 is indistinguishable compared to that of its murine parent. Furthermore, MAb T1-2 is shown to enhance the opsonophagocytic uptake of ClfA-coated latex beads, protect against an intravenous challenge in a prophylactic model of rabbit infective endocarditis, and enhance the efficacy of vancomycin therapy in a therapeutic model of established infective endocarditis.

Fibrinogen and fibronectin binding cooperate for valve infection and invasion in Staphylococcus aureus experimental endocarditis

The expression of Staphylococcus aureus adhesins in Lactococcus lactis identified clumping factor A (ClfA) and fibronectin-binding protein A (FnBPA) as critical for valve colonization in rats with experimental endocarditis. This study further analyzed their role in disease evolution. Infected animals were followed for 3 d. ClfA-positive lactococci successfully colonized damaged valves, but were spontaneously eradicated over 48 h. In contrast, FnBPA-positive lactococci progressively increased bacterial titers in vegetations and spleens. At imaging, ClfA-positive lactococci were restricted to the vegetations, whereas FnBPA-positive lactococci also invaded the adjacent endothelium. This reflected the capacity of FnBPA to trigger cell internalization in vitro. Because FnBPA carries both fibrinogen- and fibronectin-binding domains, we tested the role of these functionalities by deleting the fibrinogen-binding domain of FnBPA and supplementing it with the fibrinogen-binding domain of ClfA in cis or in trans. Deletion of the fibrinogen-binding domain of FnBPA did not alter fibronectin binding and cell internalization in vitro. However, it totally abrogated valve infectivity in vivo. This ability was restored in cis by inserting the fibrinogen-binding domain of ClfA into truncated FnBPA, and in trans by coexpressing full-length ClfA and truncated FnBPA on two separate plasmids. Thus, fibrinogen and fibronectin binding could cooperate for S. aureus valve colonization and endothelial invasion in vivo.

Open-label, dose escalation study of the safety and pharmacokinetic profile of tefibazumab in healthy volunteers

Tefibazumab (Aurexis) is a humanized monoclonal antibody being evaluated as adjunctive therapy for the treatment of Staphylococcus aureus infections. This open-label, dose escalation study evaluated the safety and pharmacokinetics of tefibazumab in 19 healthy volunteers aged 18 to 69 years. Each subject received a single administration of tefibazumab at a dose of 2, 5, 10, or 20 mg/kg of body weight infused over 15 min. Plasma samples for pharmacokinetic assessments were obtained before infusion as well as 1, 6, 12, and 24 h and 3, 4, 7, 21, 28, 42, and 56 days after dosing. Plasma concentrations of tefibazumab were detected 1 h after the end of the infusion, with a mean maximum concentration of drug in serum (C(max)) of 59, 127, 252, and 492 microg/ml following doses of 2, 5, 10, and 20 mg/kg, respectively. The median time to maximum concentration of drug in serum (T(max)) was 1.0 h for each dose. The mean elimination half-life (t(1/2)) was approximately 22 days. The volume of distribution (V) was 4.7, 6.7, 7.2, and 7.2 liters after doses of 2, 5, 10, and 20 mg/kg, respectively. Clearance (CL) was 6.0, 9.2, 10.2, and 9.9 ml/hr, respectively. At the highest dose, plasma levels of tefibazumab were >100 microg/ml for 21 days. On day 56, the mean plasma concentrations were 6.3, 10.0, 16.4, and 30.5 microg/ml for the 2, 5, 10, and 20 mg/kg doses, respectively. Tefibazumab exhibited linear kinetics across doses of 5, 10, and 20 mg/kg. No anti-tefibazumab antibodies were detected after dosing in any subject. There were no serious adverse events, and tefibazumab was well tolerated over the entire dose range.

Role of sigmaB in the expression of Staphylococcus aureus cell wall adhesins ClfA and FnbA and contribution to infectivity in a rat model of experimental endocarditis

Isogenic Staphylococcus aureus strains with different capacities to produce sigma(B) activity were analyzed for their ability to attach to fibrinogen- or fibronectin-coated surfaces or platelet-fibrin clots and to cause endocarditis in rats. In comparison to the sigma(B)-deficient strain, BB255, which harbors an rsbU mutation, both rsbU-complemented and sigma(B)-overproducing derivatives exhibited at least five times greater attachment to fibrinogen- and fibronectin-coated surfaces and showed increased adherence to platelet-fibrin clots. No differences in adherence were seen between BB255 and a DeltarsbUVWsigB isogen. Northern blotting analyses revealed that transcription of clfA, encoding fibrinogen-binding protein clumping factor A, and fnbA, encoding fibronectin-binding protein A, were positively influenced by sigma(B). Sigma(B) overproduction resulted in a statistically significant increase in positive spleen cultures and enhanced bacterial densities in both the aortic vegetations and spleens at 16 h postinoculation. In contrast, at 72 h postinoculation, tissues infected with the sigma(B) overproducer had lower bacterial densities than did those infected with BB255. These results suggest that although sigma(B) appears to increase the adhesion of S. aureus to various host cell-matrix proteins in vitro, it has limited effect on pathogenesis in the rat endocarditis model. Sigma(B) appears to have a transient enhancing effect on bacterial density in the early stages of infection that is lost during progression.

A humanized monoclonal antibody targeting Staphylococcus aureus

This current presentation describes the in vitro and in vivo characterization of Aurexis (tefibazumab), a humanized monoclonal antibody that exhibits a high affinity and specificity and for the Staphylococcus aureus MSCRAMM (Microbial Surface Components Recognizing Adhesive Matrix Molecules) protein ClfA. Aurexis inhibited ClfA binding to human fibrinogen, and enhanced the opsonophagocytic uptake of ClfA-coated beads. Preclinical in vivo testing revealed that a single administration of Aurexis significantly protected against an IV challenge with a methicillin resistant S. aureus (MRSA) strain in murine septicemia and rabbit infective endocarditis (IE) models. Safety and pharmacokinetic data from a 19-patient phase I study support continued evaluation of Aurexis in phase II studies.

FbsA, a fibrinogen-binding protein from Streptococcus agalactiae, mediates platelet aggregation

The bacterium Streptococcus agalactiae is an etiologic agent in the pathogenesis of endocarditis in humans. FbsA, a fibrinogen-binding protein produced by this pathogen, is considered an important virulence factor. In the present study we provide evidence that S agalactiae clinical isolates bearing FbsA attach to fibrinogen and elicit a fibrinogen-dependent aggregation of platelets. Mutants of S agalactiae lacking the fbsA gene lost the ability to attach to fibrinogen and to aggregate platelets. Plasmid-mediated expression of fbsA restored the capability for fibrinogen binding and platelet aggregation in S agalactiae fbsA mutants, and allowed Lactococcus lactis to interact with fibrinogen and to aggregate human platelets. Moreover, a monoclonal anti-FbsA antibody inhibited bacterial adherence to fibrinogen and S agalactiae-induced platelet aggregation. Platelet aggregation was inhibited by aspirin, prostaglandin E(1,) the peptide RGDS, and the antibody abciximab, demonstrating the specificity of platelet aggregation by S agalactiae and indicating an involvement of integrin glycoprotein IIb/IIIa in the induction of platelet aggregation. Aggregation was also dependent on anti-FbsA IgG and could be inhibited by an antibody against the platelet FcgammaRIIA receptor. These findings indicate that FbsA is a crucial factor in S agalactiae-induced platelet aggregation and may therefore play an important role in S agalactiae-induced endocarditis.

Antibiotic-dependent correlation between drug-induced killing and loss of luminescence in Streptococcus gordonii expressing luciferase

Measuring antibiotic-induced killing relies on time-consuming biological tests. The firefly luciferase gene (luc) was successfully used as a reporter gene to assess antibiotic efficacy rapidly in slow-growing Mycobacterium tuberculosis. We tested whether luc expression could also provide a rapid evaluation of bactericidal drugs in Streptococcus gordonii. The suicide vectors pFW5luc and a modified version of pJDC9 carrying a promoterless luc gene were used to construct transcriptional-fusion mutants. One mutant susceptible to penicillin-induced killing (LMI2) and three penicillin-tolerant derivatives (LMI103, LMI104, and LMI105) producing luciferase under independent streptococcal promoters were tested. The correlation between antibiotic-induced killing and luminescence was determined with mechanistically unrelated drugs. Chloramphenicol (20 times the MIC) inhibited bacterial growth. In parallel, luciferase stopped increasing and remained stable, as determined by luminescence and Western blots. Ciprofloxacin (200 times the MIC) rapidly killed 1.5 log10 CFU/ml in 2-4 hr. Luminescence decreased simultaneously by 10-fold. In contrast, penicillin (200 times the MIC) gave discordant results. Although killing was slow (< or = 0.5 log10 CFU/ml in 2 hr), luminescence dropped abruptly by 50-100-times in the same time. Inactivating penicillin with penicillinase restored luminescence, irrespective of viable counts. This was not due to altered luciferase expression or stability, suggesting some kind of post-translational modification. Luciferase shares homology with aminoacyl-tRNA synthetase and acyl-CoA ligase, which might be regulated by macromolecule synthesis and hence affected in penicillin-inhibited cells. Because of resemblance, luciferase might be down-regulated simultaneously. Luminescence cannot be universally used to predict antibiotic-induced killing. Thus, introducing reporter enzymes sharing mechanistic similarities with normal metabolic reactions might reveal other effects than those expected.

Characterization of a protective monoclonal antibody recognizing Staphylococcus aureus MSCRAMM protein clumping factor A

The Staphylococcus aureus MSCRAMM (microbial surface components recognizing adhesive matrix molecules) protein clumping factor A (ClfA) has been shown to be a critical virulence factor in several experimental models of infection. This report describes the generation, characterization, and in vivo evaluation of a murine monoclonal antibody (MAb) against ClfA. Flow cytometric analysis revealed that MAb 12-9 recognized ClfA protein expressed by all of the clinical S. aureus strains obtained from a variety of sources. In assays measuring whole-cell S. aureus binding to human fibrinogen, MAb 12-9 inhibited S. aureus binding by over 90% and displaced up to 35% of the previously adherent S. aureus bacteria. Furthermore, a single infusion of MAb 12-9 was protective against an intravenous challenge with a methicillin-resistant strain of S. aureus in a murine sepsis model (P < 0.0001). These data suggest that anti-ClfA MAb 12-9 should be further investigated as a novel immunotherapy for the treatment and prevention of life-threatening S. aureus infections.

Infective endocarditis

Despite improvements in health care, the incidence of infective endocarditis has not decreased over the past decades. This apparent paradox is explained by a progressive evolution in risk factors; while classic predisposing conditions such as rheumatic heart disease have been all but eradicated, new risk factors for infective endocarditis have emerged. These include intravenous drug use, sclerotic valve disease in elderly patients, use of prosthetic valves, and nosocomial disease. Newly identified pathogens, which are difficult to cultivate--eg, Bartonella spp and Tropheryma whipplei--are present in selected individuals, and resistant organisms are challenging conventional antimicrobial therapy. Keeping up with these changes depends on a comprehensive approach, allying understanding of the pathogenesis of disease with the development of new drugs for infective endocarditis. Infection by staphylococci and streptococci is being dissected at the molecular level. New ideas for antimicrobial agents are being developed. These novel insights should help redefine preventive and therapeutic strategies against infective endocarditis.

Anti-clumping factor A immunoglobulin reduces the duration of methicillin-resistant Staphylococcus aureus bacteremia in an experimental model of infective endocarditis

SA-IGIV is a human polyclonal immunoglobulin containing elevated levels of antibodies specific for the fibrinogen-binding MSCRAMM protein clumping factor A (ClfA). In vitro, SA-IGIV specifically recognized ClfA that was expressed on the surface of Staphylococcus aureus and inhibited bacterial adherence to immobilized human fibrinogen by >95%. Moreover, SA-IGIV efficiently opsonized ClfA-coated fluorescent beads and facilitated phagocytosis by human polymorphonuclear leukocytes. To determine its potential therapeutic efficacy, SA-IGIV was evaluated in combination with vancomycin in a rabbit model of catheter-induced aortic valve infective endocarditis (IE) caused by methicillin-resistant S. aureus (MRSA). The combination therapy was more effective than vancomycin alone in sterilizing all valvular vegetations when used therapeutically during early (12-h) IE. The combination therapy resulted in clearance of bacteremia that was significantly faster than that of vancomycin alone in animals with well-established (24-h) IE. Therefore, in both early and well-established MRSA IE, the addition of SA-IGIV to a standard antibiotic regimen (vancomycin) increased bacterial clearance from the bloodstream and/or vegetations.

Analysis of Ebh, a 1.1-megadalton cell wall-associated fibronectin-binding protein of Staphylococcus aureus

In order for Staphylococcus aureus to adhere to host extracellular matrix (ECM) substrates, it elicits a wide range of surface proteins. We have characterized a novel approximately 1.1-MDa protein in S. aureus, termed Ebh (for ECM-binding protein homologue), which has homology to other ECM-binding proteins. Ebh consists of several domains, including a large central region with 44 imperfect repeats of 126 amino acids. Expression analysis revealed ebh to be growth phase regulated and repressed by agr. A fragment of the central repeat region of Ebh was cloned, overexpressed, and used in ligand-binding studies to determine Ebh function. The recombinant protein was found to specifically bind human fibronectin. Ebh is produced during human infection since serum samples taken from patients with confirmed S. aureus infections were found to contain anti-Ebh antibodies. Localization studies revealed Ebh to be cell envelope associated and is proposed to form a specialized surface structure involved in cellular adhesion.

Functional blocking of Staphylococcus aureus adhesins following growth in ex vivo media

Defining the role of Staphylococcus aureus adhesins in disease pathogenesis may depend on the use of bacteria grown in culture media that more closely reflect the human milieu than conventional broth. This study examined the functional effect on S. aureus adhesins following growth in an ex vivo medium containing a complex mixture of human proteins (used peritoneal dialysate) relative to growth in Todd-Hewitt broth. The adherence of S. aureus, cultured in dialysate, to fibronectin and fibrinogen was markedly reduced despite the expresion of full-length ClfA, ClfB, and fibronectin-binding proteins. Growth in dialysate resulted in the acquisition of a surface coat, as visualized by transmission electron microscopy, which was shown to contain fibronectin, fibrinogen, and immunoglobulins. Adherence of S. aureus to fibrinogen following growth in dialysate was significantly reduced by expression of protein A but was restored following growth in immunoglobulin-depleted dialysate. We conclude that bacterial adherence to solid-phase protein is critically dependent on the culture medium, that S. aureus adhesins may become saturated with target protein prior to contact with solid surfaces, and that there is an interaction between fibrinogen-binding proteins and immunoglobulin bound to protein A following contact with host proteins. These findings have important implications for future studies of S. aureus adhesins.

Pathogenesis of streptococcal and staphylococcal endocarditis

Although streptococcal and S. aureus IE share the same primary site of infection, their pathogenesis and clinical evolution present several major differences. Streptococci adhere to cardiac valves with pre-existing endothelial lesions. In contrast, S. aureus can colonize either damaged endothelium or invade physically intact endothelial cells. These interactions are mediated by multiple surface adhesins, some of which have been only partially characterized. Streptococci produce surface glucans (gtf and ftf), ECM adhesins (e.g., fibronectin-binding proteins, FimA), and platelet aggregating factors (phase I and phase II antigens, pblA, pblB, and pblT), all of which have been.

Transcription of clumping factor A in attached and unattached Staphylococcus aureus in vitro and during device-related infection

Staphylococcus aureus is one of the pathogens most frequently isolated in device-related infections. S. aureus is equipped with surface-associated proteins promoting specific binding to matrix molecules. Clumping factor A (ClfA, encoded by clfA) mediates adhesion to fibrinogen. Whereas the contribution of ClfA to pathogenicity is well documented, the influence of different growth and host parameters on gene activity is unclear. To elucidate this question, we investigated clfA transcript levels in an animal model of device-related infection and in planktonic and sessile bacteria grown in vitro. Specific mRNA from the S. aureus strains Newman, Reynolds, and RN6390 was quantified by LightCycler reverse transcription-PCR. In vitro, clfA transcript levels were low in the early logarithmic growth phase, but a clear increase was observed after the late logarithmic phase. Quantities of clfA transcripts were four to six times higher in the planktonic than in the sessile bacterial subpopulations grown to the stationary phase. During infection, in strains Newman and Reynolds levels of clfA transcripts in exudates accumulating in the infected devices were lower than those in the bacteria grown in vitro to stationary phase. clfA mRNA levels in the exudates increased during the initial phase of infection and remained constant after 96 h postinoculation. In contrast to the in vitro results, quantities of clfA transcripts in the unattached bacteria of the exudates never exceeded the level of clfA transcripts in the sessile bacteria attached to glass beads. However, a clear increase in clfA quantities in the sessile bacteria was observed late in infection after 144 h. In conclusion, maximal clfA transcript levels are reached late during growth in vitro and in vivo.

DNA immunization against the clumping factor A (ClfA) of Staphylococcus aureus

Adhesins are considered the most important virulence factors during early phases Staphylococcus aureus infection. Antibodies induced by vaccination toward an adhesin should reduce the adherence of the pathogen and augment its phagocytosis. The present report describes the immune response of mice to a DNA vaccine directed against one of these adhesins, clumping factor A (ClfA). Injection of plasmids expressing the fibrinogen-binding region A of ClfA induced a strong and specific antibody response to ClfA in mice. In addition, splenocyte proliferation was provoked by in vitro stimulation with recombinant ClfA, thus, indicating direct implication of these cells in the immune response. Pre-incubation of S. aureus with sera of vaccinated mice reduced the pathogen's ability to bind fibrinogen by up to 92%. These pre-incubated bacteria were phagocytosed by macrophages at an increased level in vitro and were less virulent in vivo in a mouse mastitis model. However, DNA-immunized mice were not protected against an intraperitoneal challenge. Overall, the results suggest that DNA immunization against adhesins represents a new and valuable approach to combat S. aureus infections.

Identification and characterization of a novel 38.5-kilodalton cell surface protein of Staphylococcus aureus with extended-spectrum binding activity for extracellular matrix and plasma proteins

The ability to attach to host ligands is a well-established pathogenic factor in invasive Staphylococcus aureus disease. In addition to the family of adhesive proteins bound to the cell wall via the sortase A (srtA) mechanism, secreted proteins such as the fibrinogen-binding protein Efb, the extracellular adhesion protein Eap, or coagulase have been found to interact with various extracellular host molecules. Here we describe a novel protein, the extracellular matrix protein-binding protein (Emp) initially identified in Western ligand blots as a 40-kDa protein due to its broad-spectrum recognition of fibronectin, fibrinogen, collagen, and vitronectin. Emp is expressed in the stationary growth phase and is closely associated with the cell surface and yet is extractable by sodium dodecyl sulfate. The conferring gene emp (1,023 nucleotides) encodes a signal peptide of 26 amino acids and a mature protein of a calculated molecular mass of 35.5 kDa. Using PCR, emp was demonstrated in all 240 S. aureus isolates of a defined clinical strain collection as well as in 6 S. aureus laboratory strains, whereas it is lacking in all 10 S. epidermidis strains tested. Construction of an allelic replacement mutant (mEmp50) revealed the absence of Emp in mEmp50, a significantly decreased adhesion of mEmp50 to immobilized fibronectin and fibrinogen, and restoration of these characteristics upon complementation of mEmp50. Emp expression was also demonstrable upon heterologous complementation of S. carnosus. rEmp expressed in Escherichia coli interacted with fibronectin, fibrinogen, and vitronectin in surface plasmon resonance experiments at a K(d) of 21 nM, 91 nM, and 122 pM, respectively. In conclusion, the biologic characterization of Emp suggests that it is a member of the group of secreted S. aureus molecules that interact with an extended spectrum of host ligands and thereby contribute to S. aureus pathogenicity.

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.

SdrG, a fibrinogen-binding bacterial adhesin of the microbial surface components recognizing adhesive matrix molecules subfamily from Staphylococcus epidermidis, targets the thrombin cleavage site in the Bbeta chain

Staphylococcus epidermidis is an important opportunistic pathogen and is a major cause of foreign body infections. We have characterized the ligand binding activity of SdrG, a fibrinogen-binding microbial surface component recognizing adhesive matrix molecules from S. epidermidis. Western ligand blot analysis showed that a recombinant form of the N-terminal A region of SdrG bound to the native Bbeta chain of fibrinogen (Fg) and to a recombinant form of the Bbeta chain expressed in Escherichia coli. By analyzing recombinant truncates and synthetic peptide mimetics of the Fg Bbeta chain, the binding site for SdrG was localized to residues 6-20 of this polypeptide. Recombinant SdrG bound to a synthetic 25-amino acid peptide (beta1-25) representing the N terminus of the Fg Bbeta chain with a KD of 1.4 x 10(-7) m as determined by fluorescence polarization experiments. This was similar to the apparent K(D) (0.9 x 10(-7) m) calculated from an enzyme-linked immunosorbent assay where SdrG bound immobilized Fg in a concentration-dependent manner. SdrG could recognize fibrinopeptide B (residues 1-14), but with a substantially lower affinity than that observed for SdrG binding to synthetic peptides beta1-25 and beta6-20. However, SdrG does not bind to thrombin-digested Fg. Thus, SdrG appears to target the thrombin cleavage site in the Fg Bbeta chain. In fact, SdrG was found to inhibit thrombin-induced fibrinogen clotting by interfering with fibrinopeptide B release.