Isolation of Staphylococcus aureus clumping factor

Staphylococcus aureus induces platelet aggregation via a fibrinogen-dependent mechanism which is independent of principal platelet glycoprotein IIb/IIIa fibrinogen-binding domains

Platelet aggregation by bacteria is felt to play an important role in the pathogenesis of infective endocarditis. However, the mechanisms involved in bacterium-induced platelet aggregation are not well-defined. In the present study, we examined the mechanisms by which Staphylococcus aureus causes rabbit platelet aggregation in vitro. In normal plasma, the kinetics of S. aureus-induced platelet aggregation were rapid and biphasic. The onset and magnitude of aggregation phase 1 varied with the bacterium-platelet ratio, with maximal aggregation observed at a ratio of 5:1. The onset of aggregation phase 2 was delayed in the presence of apyrase (an ADP hydrolase), suggesting that this later aggregation phase may be triggered by secreted ADP. The onset of aggregation phase 2 was delayed in the presence of prostaglandin I2-treated platelets, and this phase was absent when paraformaldehyde-fixed platelets were used, implicating platelet activation in this process. Platelet aggregation phase 2 was dependent on S. aureus viability and an intact bacterial cell wall, and it was mitigated by antibody directed against staphylococcal clumping factor (a fibrinogen-binding protein) and by the cyclooxygenase inhibitor indomethacin. Similarly, aggregation phase 2 was either delayed or absent in three distinct transposon-induced S. aureus mutants with reduced capacities to bind fibrinogen in vitro. In addition, a synthetic pentadecapeptide, corresponding to the staphylococcal binding domain in the C terminus of the fibrinogen delta-chain, blocked aggregation phase 2. However, phase 2 of aggregation was not inhibited by two synthetic peptides (alone or in combination) analogous to the two principal fibrinogen-binding domains on the platelet glycoprotein (GP) IIb/IIIa integrin receptor: (i) a recognition site on the IIIa molecule for the Arg-Gly-Asp (RGD) sequence of the fibrinogen alpha-chain and (ii) a recognition site on the IIb molecule for a dodecapeptide sequence of the fibrinogen delta-chain. This differs from ADP-induced platelet aggregation, which relies on an intact platelet GP IIb/IIIa receptor with an accessible RGD sequence and dodecapeptide recognition site for fibrinogen. Furthermore, a monoclonal antibody directed against the RGD recognition site on rabbit platelet GP IIb/IIIa receptors failed to inhibit rabbit platelet aggregation by S. aureus. Collectively, these data suggest that S. aureus-induced platelet aggregation requires bacterial binding to fibrinogen but is not principally dependent upon the two major fibrinogen-binding domains on the platelet GP IIb/IIIa integrin receptor, the RGD and dodecapeptide recognition sites.

Quantitative comparison of clumping factor- and coagulase-mediated Staphylococcus aureus adhesion to surface-bound fibrinogen under flow

The contributions of clumping factor and coagulase in mediating Staphylococcus aureus adhesion to surface-adsorbed fibrinogen have been quantified by using a new methodology and analysis. The attachment or detachment kinetics of bacteria were directly observed in a radial flow chamber with a well-defined laminar flow field and a spatially varying shear rate and were quantified by recursively scanning the chamber surface and counting cells via automated video microscopy and image analysis with a motorized stage and focus control. Intrinsic rate constants for attachment or detachment were estimated as functions of shear rate for the wild-type Newman strain of S. aureus and for mutants lacking clumping factor, coagulase, or both proteins on surfaces coated with plasma, fibrinogen, or albumin. Clumping factor, but not coagulase, increased the probability of attachment and decreased the probability of detachment of S. aureus on plasma-coated surfaces; however, both clumping factor and, to a lesser extent, coagulase increased the probability of attachment on the purified-fibrinogen-coated surface. All mutants were resistant to detachment on the purified-fibrinogen-coated surface, suggesting the possibility of an additional adhesion mechanism which was independent of coagulase or clumping factor and effective only for fully attached cells. Together, these results suggest that the presence of clumping factor plays the primary role in enhancing adhesion to surfaces with adsorbed fibrinogen, not only by enhancing the probability of cell attachment but also by increasing the strength of the resulting adhesion.

Cloning, expression, and nucleotide sequence of a Staphylococcus aureus gene (fbpA) encoding a fibrinogen-binding protein

Septicemia due to Staphylococcus aureus often begins as a focal infection (e.g., colonized wounds or catheters) from which the organism gains access to the bloodstream. On the basis of recent data from this laboratory, it is likely that S. aureus colonizes catheters and endothelium by using a fibrinogen-binding protein to mediate adhesion to fibrinogen-coated surfaces. To characterize the fibrinogen-reactive protein, we screened a lambda Zap library of S. aureus DB, a clinical isolate, for clones that were reactive with fibrinogen. Of 100,000 plaques screened, 3 were found to react with fibrinogen on immunoblots. Plasmid DNA prepared from clones 14, 30, and 36, upon digestion with EcoR1, which released the insert, revealed fragments of 4.6, 3.6, and 3.2 kb, respectively. To identify the cloned protein expressed in E. coli, cells were fractionated into periplasmic, membrane, and cytoplasmic fractions. Expression studies of clone 14, which comprised approximately two-thirds of the mature molecule, including the C terminus, revealed a 34-kDa fibrinogen-reactive protein in both the periplasmic and membrane fractions. This protein, designated FbpA, could be partially purified on a fibrinogen column. By using both clones 14 and 36 as templates, the complete DNA sequence of the fibrinogen-binding protein was obtained, yielding a molecule with a predicted size of 69,991 Da. Although sequence analysis revealed a high degree of homology with coagulase, there is a unique sequence of 11 amino acids that is not found in three known coagulases as well as two recently cloned fibrinogen-binding proteins. This unique sequence shares homology with a cell wall anchor motif found in other gram-positive surface proteins.

Cross-reactive antigens shared by Pseudomonas aeruginosa, Helicobacter pylori, Campylobacter jejuni, and Haemophilus influenzae may cause false-positive titers of antibody to H. pylori

Cystic fibrosis (CF) patients suffer from many of the gastrointestinal conditions which occur in non-CF individuals, e.g., dyspepsia and peptic ulceration. These symptoms may be caused by Helicobacter pylori but could also be due to either pancreatic insufficiency or the intensive antibiotic treatment used in CF patients. Since CF patients chronically infected with Pseudomonas aeruginosa produce antibodies against a wide range of antigens, including antigens common to many other bacteria, e.g., GroEL and lipopolysaccharide, we studied, by the Western blot (immunoblot) technique, the specificity of immunoglobulin G antibodies to H. pylori in Danish CF patients chronically infected with P. aeruginosa, CF patients without P. aeruginosa infection but with Haemophilus influenzae infection, patients with dyspeptic ulcers associated with H. pylori, and patients recovering from acute Campylobacter jejuni or Campylobacter coli infection. Sera from CF patients with chronic P. aeruginosa or H. influenzae infection and patients recovering from acute C. jejuni infection cross-reacted with H. pylori antigens. A strong cross-reacting protein antigen at approximately 14 kDa and minor cross-reactive antigens at approximately 27, 30, and 60 kDa (the heat shock protein GroEL is equivalent to the common antigen of P. aeruginosa) could be demonstrated. The results of this study show that high immunoglobulin G antibody titers against H. pylori in CF patients cannot be regarded as indicating present or past H. pylori infection unless their specificity is proven by absorption studies.

Characterization of fibrinogen-binding properties of Actinomyces pyogenes

This study was designed to further characterize the fibrinogen-binding properties of Actinomyces pyogenes. The fibrinogen-binding capacities of a selected A. pyogenes culture could be significantly enhanced by heat pretreatment (60 degrees C, 1 h) of the bacteria. The fibrinogen-binding site seemed to be a protein which was specific for fibrinogen. In phagocytosis studies, binding of fibrinogen to A. pyogenes significantly increased the phagocytic capacity of bovine polymorphonuclear leucocytes.

Effect of treatment with methicillin and gentamicin in a new experimental mouse model of foreign body infection

A new mouse model of foreign body infection has been developed. Intraperitoneal placement of a silicone catheter followed by injection of 10(8) Staphylococcus aureus organisms resulted in a reproducible, localized foreign body infection. The infection persisted as an intra-abdominal abscess surrounding the catheter for at least 30 days. Treatment with up to nine doses of methicillin or gentamicin or both was started 3 days after infection. The treatment showed a significant effect (P < 0.05), measured as reduction of bacteria on the foreign body, for all three regimens with a reduction of up to 2 log units, but no synergism was observed. The result of the treatment was poor, despite the facts that the local concentrations of methicillin were greater than the MIC for at least 72 h and that nine peak concentrations of gentamicin of > 13 micrograms/ml were obtained. The poor result of the treatment was not caused by development of antibiotic resistance or influenced by protein concentration, pH, or local presence in the pus of inhibitors of antibiotics. Both antibiotics showed good effects in time-kill studies in vitro on bacteria on catheters taken out of infected mice and catheters infected in vitro. During treatment, the proportion of intracellular bacteria increased in all treated mice to 60 to 75% compared with 20 to 30% in nontreated mice (P < 0.05). This indicates that intracellular survival of staphylococci may influence the outcome of the treatment in foreign body infections.

Molecular characterization of the clumping factor (fibrinogen receptor) of Staphylococcus aureus

Four mutants of Staphylococcus aureus strain Newman that were defective in the fibrinogen receptor (clumping factor) were isolated by transposon Tn917 mutagenesis. Southern hybridization analysis of the mutants identified transposon-host DNA junction fragments, one of which was cloned and used to generate a probe to identify and clone the wild-type clumping factor locus (clfA). The mutants failed to form clumps in soluble fibrinogen and adhered poorly to polymethylmethacrylate (PMMA) coverslips coated with fibrinogen. A single copy of the clfA gene, when introduced into the chromosome of the mutant strains, fully complemented the clumping deficiency of these strains and restored the ability of these mutants to adhere to fibrinogen-coated PMMA. In addition, the cloned clfA gene on a shuttle plasmid allowed the weakly clumping strain 8325-4 to form clumps with the same avidity as the wild-type strain Newman and also significantly enhanced the adherence of 8325-4 strains. Thus the formation of clumps in soluble fibrinogen correlated with adherence of bacteria to solid-phase fibrinogen. The clfA gene encodes a fibrinogen-binding protein with an apparent molecular mass of c. 130 kDa. The amino acid sequence of the protein was deduced from the DNA sequence; it was predicted that a 896 residue protein (molecular mass 92 kDa) would be expressed. The putative ClfA protein has features that suggest that it is associated with the cell surface. Furthermore it contains a novel 308 residue region comprising dipeptide repeats predominantly of Asp and Ser ending 28 residues upstream from the LPXTG motif common to wall-associated proteins. Significant homology was found between the ClfA protein and the fibronectin-binding proteins of S. aureus, particularly in the N- and C-termini.

Immunoglobulin G antibodies to Helicobacter pylori in patients with dyspeptic symptoms investigated by the western immunoblot technique

Helicobacter pylori is a gram-negative, curved, rod-shaped bacterium known to cause gastritis and to be an important factor in the pathogenesis of peptic ulcers. Serological testing has recently been proposed as an aid in diagnosis of H. pylori infections. In this study, we used the Western immunoblot technique to evaluate the possibility of using one or more of the antigens from H. pylori for this purpose. Thirteen major bands and about 30 minor bands could be identified by Western blotting when sera from 53 consecutive dyspeptic patients, 27 healthy children, and 25 blood donors were evaluated. Antibodies against most of the major bands were found significantly more frequently in patients with H. pylori infections than in patients without such infections. However, antibodies against a single polypeptide were not produced by all patients with H. pylori infection. Polypeptides with molecular masses of 120, 50, and between 19 and 36 kDa seem to be the most specific polypeptides for the diagnosis of H. pylori infections. This study showed only minor differences in antigenic composition between different clinical isolates of H. pylori, and serological cross-reactions with other bacteria were limited. Major serological cross-reactions were found only with Campylobacter jejuni and with bacterial lipopolysaccharide. However, one band at 60 kDa reacted with antiserum to the Legionella micdadei common antigen, which may indicate a cross-reaction with common antigen from several other bacteria. This study demonstrates that a number of bands may be useful as antigens in serological tests after isolation and purification.

Fibrinogen acts as a bridging molecule in the adherence of Staphylococcus aureus to cultured human endothelial cells

The propensity of Staphylococcus aureus to cause acute endovascular infections during transient bacteremia is poorly understood. To examine the events leading to the attachment of staphylococci to endothelium, adherence assays were developed to study the role of blood factors in the mediation of staphylococcal adherence to cultured human umbilical vein endothelium in vitro. Results indicate that the preferential attachment of S. aureus to endothelial cells is mediated by fibrinogen adsorbed from plasma onto the endothelial surface. The binding is apparently specific because it could be blocked by goat anti-human fibrinogen antibody in a dose-dependent fashion while nonimmune goat IgG, mouse MAb against AG-1 (a platelet antigen found on the endothelial cell surface), nonspecific mouse MAb and rabbit antibodies to human vitronectin and fibronectin were not inhibitory. Our data also indicate that fibrinogen is a necessary but not the only blood constituent in the mediation of binding of S. aureus to endothelium. This was supported by the finding that fibrinogen alone, at a concentration equivalent to that in plasma, did not potentiate staphylococcal adherence as much as plasma while afibrinogenemic plasma reconstituted with fibrinogen did. Because fibrinogen is known to bind to endothelial cells, our data is consistent with the hypothesis that fibrinogen and additional plasma factor(s), possibly activated during inflammation, promote staphylococcal adherence to endothelium. In addition, the role of the fibrinogen binding receptor of S. aureus as an adherence factor to native endothelium is also suggested.

Expression of clumping and fibrinogen-binding activities of Staphylococcus aureus at various growth stages

Clumping and fibrinogen-binding activities of 4 Staphylococcus aureus strains (Cowan I, Newman D2C, Wood 46 and NCTC 5655) were assayed with a semiquantitative clumping test and an enzyme-linked immunosorbent assay (ELISA), respectively. Distinct positive clumping was detected with whole cells of the 3 strains except Wood 46. Amounts of fibrinogen required for a definite clumping depended greatly on strains as well as on their growth phases. On the other hand, fibrinogen-binding activities were detected both in culture supernatants and in cell lysates of all the 4 strains, and the levels were rather comparable with one another and relatively steady through their growth cycles. No significant correlation was thus found among expression behavior of clumping and fibrinogen-binding activities.

Potential for vaccination against infections caused by Staphylococcus aureus

Surface polysaccharides and proteins from S. aureus which could serve as components of a future subunit vaccine against staphylococcal disease in man and animals have recently been characterized. The majority of bovine mastitis and human clinical isolates of S. aureus produce a thin polysaccharide capsule which probably impairs phagocytosis. Protective immunity to S. aureus infections in laboratory animals has been induced by immunization with polysaccharide, and immune serum promotes phagocytosis of bacteria in vitro. S. aureus expresses several surface-exposed proteins that bind host plasma proteins to the bacterial cell or promote adherence of bacteria to host cells or to tissues. These activities may help bacteria avoid host defences and stimulate adherence and colonization to form foci of infection. In this article the properties of S. aureus surface polysaccharides and proteins are reviewed. Their contribution to virulence and the possibility that they could be used as components of new vaccine to combat mastitis in ruminants and nosocomial infections is discussed.

The role of fibrinogen in mediating staphylococcal adherence to fibers

The use of tampons and surgical gauze pads and colonization with Staphylococcus aureus have been established as risk factors for the development of toxic shock syndrome. To elucidate the role of blood factors in the mediation of staphylococcal adherence to fibers used in tampons and surgical packing, an adherence assay with cotton fibers was developed. Results demonstrated that cotton disks precoated with fibrinogen in the presence of human serum albumin bound a significant percentage of the inoculum for both staphylococcal strains tested when compared to human serum albumin controls. Likewise, fibers pretreated with plasma or defibrinated blood containing a small amount of fibrin revealed comparable staphylococcal adherence to that of fibrinogen. In contrast, fibers pretreated with serum, fibronectin, or vitronectin did not exhibit significant augmentation in staphylococcal attachment in comparison to human serum albumin controls. The attachment of staphylococci to fibrinogen and/or fibrin appeared to be specific and is blocked by goat anti-human fibrinogen antibody, but not fibronectin, vitronectin, or nonimmune goat IgG. Thus, our data indicate that fibrinogen/fibrin is the dominant blood component in the mediation of staphylococcal adherence to fibers used in tampons and surgical gauze pads.

Fibrinogen-binding protein/clumping factor from Staphylococcus aureus

The binding of staphylococcal components to fibrinogen was studied. Fibrinogen-binding material from lysed staphylococcal cells or culture supernatants was affinity purified on fibrinogen-Sepharose and analyzed on Western (immuno-) blots by the use of fibrinogen and antifibrinogen antibodies. Two main bands of 87 and 19 kilodaltons (kDa) and a weaker band of 35 kDa bound specifically to fibrinogen. A monoclonal antibody bound to all three bands, indicating that these were of the same origin. The yield of these components was much higher in the culture supernatant than on washed cells, suggesting that these molecules are essentially extracellular products. In a plasma coagulase test, the 87-kDa band, but not the 19-kDa band, clotted rabbit plasma, demonstrating that the 87-kDa molecule is coagulase. This was further confirmed by the fact that the 87-kDa band binds specifically to prothrombin. It was shown that the 87- and the 19-kDa molecules were present on the cell surface by surface labeling the cells with 125I. In addition, the fact that killed and washed cells could induce plasma clotting demonstrates that staphylococci have coagulase exposed on the surface. It was concluded that cell-bound coagulase has affinity for fibrinogen also in the absence of prothrombin and thus is responsible for the clumping of staphylococci in fibrinogen.

Antibody response to Staphylococcus aureus surface proteins in rabbits with persistent osteomyelitis after treatment with demineralized bone implants

A rabbit model was used to study the effect of allogeneic demineralized bone powder (DBP) implants on the persistence of Staphylococcus aureus osteomyelitis. Thirty-one rabbits with chronic osteomyelitis of the tibia established by day 21, were started on systemic antibiotics followed by either no additional treatment or debridement plus either DBP (with or without supplemental antibiotics) or supplemental antibiotics only. On day 21, cultures showed a mean of 2 x 10(4) CFU/mg of debrided osseous material. By day 70, the treatment most effective in clearing infection was found in animals treated with supplemental antibiotics only (mean of 142 +/- 116 CFU/mg). In contrast, infection persisted at a 7- to 10-fold-higher level in animals receiving DBP with and without supplemental antibiotics; these results suggest that DBP contributed to persistence of infection. Longitudinal sera were tested again staphylococcal sonic extracts by immunoblot. Detection of numerous probe-positive bands indicated complex but remarkably similar antibody responses among infected animals. Antibodies attached directly to the cell surfaces of staphylococci as shown by immunogold and blocked the binding of organisms to HEp-2 and human fetal lung cells in a radioadherence assay. Antibodies could be absorbed out by intact organisms and were unreactive by immunoblot against antigens derived from cells pretreated with pronase, proteinase K, or lysostaphin. These results indicate that the major response was directed against staphylococcal cell surface proteins. Surprisingly, only one major band (molecular weight, approximately 12,000) was detected when a homologous in vivo antigen preparation was studied by immunoblot. Antibody reactive against this peptide did not appear to react with staphylococci grown in vitro.

Detection of coagulase activity in Erysipelothrix rhusiopathiae

Coagulase activity was detected in 99% of 225 strains of Erysipelothrix rhusiopathiae. These strains included isolates from a variety of animal and environmental sources. Activity could be detected by the tube or slide technique, with the tube reactions being easier to interpret. Coagulation of rabbit and/or bovine plasma was observed, with most strains reacting in both. The activity appeared to be a common characteristic of the species and may be useful in differentiating E. rhusiopathiae from Listeria and Corynebacterium species, which fail to demonstrate the activity. There was no correlation noted between coagulase activity and the serotype, source, geographic origin, or virulence (as detected by mouse pathogenicity tests) of the isolates.

Antibody response to whole cells and teichoic acid of Staphylococcus aureus strain E 1369 in human sera

A whole cell Staphylococcus aureus enzyme-linked immunosorbent assay (ELISA) using the protein A-deficient strain E 1369 as antigen was compared to an ELISA with cell-wall teichoic acid, extracted from the same strain, as antigen for the detection of IgG antibodies against S. aureus in human sera. A total of 178 serum samples from 119 patients with S. aureus endocarditis, complicated and uncomplicated septicaemia, non-S. aureus endocarditis and septicaemia were investigated together with sera from 155 healthy controls. The sensitivity in diagnosing S. aureus endocarditis was better for the whole cell-ELISA (83%) as compared to the teichoic acid-ELISA (61%), and the same was true for complicated septicaemia (41% vs 29%). The whole cell ELISA was also more specific for S. aureus infections with only 3.6% false positive test values compared to 7.1% for the teichoic acid-ELISA. The differences were however, not statistically significant. None of the assays were able to distinguish between complicated and uncomplicated S. aureus septicaemia.