Pierre Vaudaux

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, fuily compiemented the ciumping 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 aiiowed the weakiy ciumping 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 ceil 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. S. aureus, particularly in the N‐and C‐termini.

Fibronectin-binding protein acts as Staphylococcus aureus invasin via fibronectin bridging to integrin alpha5beta1

The ability of Staphylococcus aureus to invade mammalian cells may explain its capacity to colonize mucosa and to persist in tissues after bacteraemia. To date, the underlying molecular mechanisms of cellular invasion by S. aureus are unknown, despite its high prevalence and difficulties in treatment. Here, we show cellular invasion as a novel function for an S. aureus adhesin, previously implicated solely in attachment. S. aureus, but not S. epidermidis, invaded epithelial 293 cells in a temperature‐ and F‐actin‐dependent manner. Formaldehyde‐fixed and live bacteria were equally invasive, suggesting that no active bacterial process was involved. All clinical S. aureus isolates analysed, but only a subset of laboratory strains, were invasive. Fibronectin‐binding proteins (FnBPs) acted as S. aureus invasins, because: (i) FnBP deletion mutants of invasive laboratory strains lost invasiveness; (ii) expression of FnBPs in non‐invasive strains conferred invasiveness; and (iii) the soluble isolated fibronectin‐binding domain of FnBP (D1–D4) completely blocked invasion. Integrin α5β1 served as host cell receptor, which interacted with staphylococcal FnBPs through cellular or soluble fibronectin. FnBP‐deficient mutants lost invasiveness for epithelial cells, endothelial cells and fibroblasts. Thus, fibronectin‐dependent bridging between S. aureus FnBPs and host cell integrin α5β1 is a conserved mechanism for S. aureus invasion of human cells. This may prove useful in developing new therapeutic and vaccine strategies for S. aureus infections.

Clumping factor B (ClfB), a new surface‐located fibrinogen‐binding adhesin of Staphylococcus aureus

The surface‐located fibrinogen‐binding protein (clumping factor; ClfA) of Staphylococcus aureus has an unusual dipeptide repeat linking the ligand binding domain to the wall‐anchored region. Southern blotting experiments revealed several other loci in the S. aureus Newman genome that hybridized to a probe comprising DNA encoding the dipeptide repeat. One of these loci is analysed here. It also encodes a fibrinogen‐binding protein, which we have called ClfB. The overall organization of ClfB is very similar to that of ClfA, and the proteins have considerable sequence identity in the signal sequence and wall attachment domains. However, the A regions are only 26% identical. Recombinant biotinylated ClfB protein bound to fibrinogen in Western ligand blots. ClfB reacted with the α‐ and β‐chains of fibrinogen in the ligand blots in contrast to ClfA, which binds exclusively to the γ‐chain. Analysis of proteins released from the cell wall of S. aureus Newman by Western immunoblotting using antibody raised against the recombinant A region of ClfB identified a 124 kDa protein as the clfB gene product. This protein was detectable only on cells that were grown to the early exponential phase. It was absent from cells from late exponential phase or stationary phase cultures. Using a clfB mutant isolated by allelic replacement alone and in combination with a clfA mutation, the ClfB protein was shown to promote (i) clumping of exponential‐phase cells in a solution of fibrinogen, (ii) adherence of exponential‐phase bacteria to immobilized fibrinogen in vitro, and (iii) bacterial adherence to ex vivo human haemodialysis tubing, suggesting that it could contribute to the pathogenicity of biomaterial‐related infections. However, in wild‐type exponential‐phase S. aureus Newman cultures, ClfB activity was masked by the ClfA protein, and it did not contribute at all to interactions of cells from stationary‐phase cultures with fibrinogen. ClfB‐dependent bacterial adherence to immobilized fibrinogen was inhibited by millimolar concentrations of Ca2+ and Mn2+, which indicates that, like ClfA, ligand binding by ClfB is regulated by a low‐affinity inhibitory cation binding site.

Rapid Detection of Methicillin-Resistant Staphylococcus aureus Directly from Sterile or Nonsterile Clinical Samples by a New Molecular Assay

ABSTRACT A rapid procedure was developed for detection and identification of methicillin-resistant Staphylococcus aureus (MRSA) directly from sterile sites or mixed flora samples (e.g., nose or inguinal swabs). After a rapid conditioning of samples, the method consists of two main steps: (i) immunomagnetic enrichment in S. aureus and (ii) amplification-detection profile on DNA extracts using multiplex quantitative PCR (5′-exonuclease qPCR, TaqMan). The triplex qPCR assay measures simultaneously the following targets: (i) mecA gene, conferring methicillin resistance, common to both S. aureus and Staphylococcus epidermidis; (ii) femA gene from S. aureus; and (iii) femA gene from S. epidermidis. This quantitative approach allows discrimination of the origin of the measured mecA signal. qPCR data were calibrated using two reference strains (MRSA and methicillin-resistant S. epidermidis) processed in parallel to clinical samples. This 96-well format assay allowed analysis of 30 swab samples per run and detection of the presence of MRSA with exquisite sensitivity compared to optimal culture-based techniques. The complete protocol may provide results in less than 6 h (while standard procedure needs 2 to 3 days), thus allowing prompt and cost-effective implementation of contact precautions.

Adhesion properties of mutants of Staphylococcus aureus defective in fibronectin-binding proteins and studies on the expression of fnb genes

Staphylococcus aureus 8325‐4 has the potential to express two distinct cell wall‐associated fibronectin‐binding proteins called FnBPA and FnBPB. In order to test if both proteins are expressed in S. aureus and if both are required for promoting bacterial adhesion to fibronectin‐coated surfaces, insertion mutations were isolated in each gene. A DNA fragment encoding tetracycline resistance was inserted into fnbA and a fragment encoding erythromycin resistance was inserted into fnbB. A double fnbA fnbB mutant was also constructed. The fnbA and fnbB single mutants showed no significant reduction in their adhesion to polymethylmethacrylate coverslips that had been coated in vitro with fibronectin. However, the double mutant was completely defective in adhesion. Monospecific antibodies directed against the non‐conserved N‐terminal regions of both proteins confirmed the lack of expression of FnBPs in the mutant strains. Wild‐type fnbA and fnbB genes cloned seperately on a multicopy plasmid were each able to restore fully the adhesion‐defective phenotype of the 8325‐4 fnbA fnbB mutant. This demonstrates that both fnb genes are expressed in S. aureus and that both contribute to the ability of strain 8325‐4 to adhere to fibronectin‐coated surfaces. The double mutant was also defective in adhesion to coverslips that had been removed from tissue cages implanted subcutaneously in guinea‐pigs, which suggests that fibronectin is important in promoting attachment of S. aureus to biomaterial in vivo.

Identification of the ligand-binding domain of the surface-located fibrinogen receptor (clumping factor) of Staphylococcus aureus

The ability of Staphylococcus aureus to bind to fibrinogen and fibrin is believed to be an important factor in the initiation of foreign‐body and wound Infections. Recently, we reported the cloning and sequencing of the gene clfA encoding the fibrinogen receptor (clumping factor, ClfA) of S. aureus strain Newman and showed that the gene product was responsible for the clumping of bacteria in soluble fibrinogen and for the adherence of bacteria to solid‐phase fibrinogen. This was confirmed here by showing that antibodies raised against purified Region A inhibited both of these properties. Also, immunofluorescent microscopic analysis of wild‐type Newman and a clfA::Tn917 mutant of Newman with anti‐ClfA Region A sera confirmed that Region A is exposed on the bacterial cell surface. Furthermore, polystyrene beads coated with the Region A protein formed clumps in soluble fibrinogen showing that the ClfA protein alone is sufficient for the clumping phenotype. Western immunoblotting with anti‐ClfA Region A antibodies identified the native ClfA receptor as a 185 kDa protein that was released from the cell wall of S. aureus by lysostaphin treatment. A single extensive ligand‐binding site was located within Region A of the ClfA protein. Truncated ClfA proteins were expressed in Escherichia coli. Lysates of E. coli and proteins that had been purified by affinity chromatography were tested for (i) their ability to bind fibrinogen in Western ligand blotting experiments, (ii) for their ability to inhibit clumping of bacteria in fibrinogen solution and adherence of bacteria to solid‐phase fibrinogen, and (iii) for their ability to neutralize the blocking activity of anti‐ClfA Region A antibody. These tests allowed the ligand‐binding domain to be localized to a 218‐residue segment (residues 332‐550) within Region A.

Staphylococcal Small Colony Variants Have Novel Mechanisms for Antibiotic Resistance

Over the past 4 years, a variant subpopulation of Staphylococcus aureus has been characterized that is defective in electron transport. These organisms grow slowly and are typical of the previously described small colony variants (SCVs). Indeed, many earlier papers included data that are consistent with defective respiratory activity in SCVs. We present a hypothesis that serves as biochemical basis for the development of SCVs. These variants are particularly interesting because they have been associated with very persistent infections, and they are more resistant to many antibiotics than normal S. aureus. Because of their slow growth, atypical colonial morphology, and unusual biochemical profile, they are easily missed or misidentified in the clinical laboratory. This is of some significance, as this subpopulation is more resistant to antibiotics than the parent population from which they arose. When an infection is particularly resistant to therapy, persists for a long period, or fails to respond to apparently adequate antimicrobial therapy, clinicians and clinical laboratory personnel should consider special efforts to search for SCVs.

Evidence of an Intracellular Reservoir in the Nasal Mucosa of Patients with Recurrent Staphylococcus aureus Rhinosinusitis

Severe infections due to Staphylococcus aureus require prolonged therapy for cure, and relapse may occur even years after the first episode. Persistence of S. aureus may be explained, in part, by nasal carriage of S. aureus, which occurs in a large percentage of healthy humans and represents a major source of systemic infection. However, the persistence of internalized S. aureus within mucosal cells has not been evaluated in humans. Here, we provide the first in vivo evidence of intracellular reservoirs of S. aureus in humans, which were assessed in endonasal mucosa specimens from patients suffering from recurrent S. aureus rhinosinusitis due to unique, patient-specific bacterial clonotypes. Heavily infected foci of intracellular bacteria located in nasal epithelium, glandular, and myofibroblastic cells were revealed by inverted confocal laser scan fluorescence and electron microscopic examination of posttherapy intranasal biopsy specimens from symptom-free patients undergoing surgery on the sinuses. Intracellular residence may provide a sanctuary for pathogenic bacteria by protecting them from host defense mechanisms and antibiotic treatment during acute, recurrent S. aureus rhinosinusitis.

Reassessing the Role of Staphylococcus aureus Clumping Factor and Fibronectin-Binding Protein by Expression in Lactococcus lactis

ABSTRACT 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, bothclfA and fnbA increased the adherence of lactococci to their specific ligands to a similar extent as theS. aureus gene donor. In vivo, the minimum inoculum size producing endocarditis in ≥80% of the rats (80% infective dose [ID80]) with the parent lactococcus was ≥107CFU. In contrast, clfA-expressing andfnbA-expressing lactococci required only 105CFU 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 = 104 to 105 CFU) in this model. The results confirmed the role ofclfA 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 theclfA and fnbA products should be blocked for the therapy to be effective.

Increased Expression of Clumping Factor and Fibronectin-Binding Proteins by hemB Mutants of Staphylococcus aureus Expressing Small Colony Variant Phenotypes

ABSTRACT Small colony variants (SCVs) of Staphylococcus aureus are slow-growing subpopulations that cause persistent and relapsing infections. The altered phenotype of SCV can arise from defects in menadione or hemin biosynthesis, which disrupt the electron transport chain and decrease ATP concentrations. With SCVs, virulence is altered by a decrease in exotoxin production and susceptibility to various antibiotics, allowing their intracellular survival. The expression of bacterial adhesins by SCVs is poorly documented. We tested fibrinogen- and fibronectin-mediated adhesion of a hemB mutant of S. aureus 8325-4 that is defective for hemin biosynthesis and exhibits a complete SCV phenotype. In this strain, adhesion to fibrinogen and fibronectin was significantly higher than that of its isogenic, normally growing parent and correlated with the increased surface display of these adhesins as assessed by flow cytometry. Real-time quantitative reverse transcription-PCR demonstrated increased expression of clfA and fnb genes by the hemB mutant compared to its isogenic parent. The influence of the hemB mutation on altered adhesin expression was confirmed by showing complete restoration of the wild-type adhesive phenotype in the hemB mutant, either by complementing with intact hemB or by supplementing the growth medium with hemin. Increased surface display of fibrinogen and fibronectin adhesins by the hemB mutation occurred independently from agr, a major regulatory locus of virulence factors in S. aureus. Both agr-positive and agr-lacking hemB mutants were also more efficiently internalized by human embryonic kidney cells than were their isogenic controls, presumably because of increased surface display of their fibronectin adhesins.