Full genome sequence analysis of African swine fever virus isolates from Cameroon

African swine fever (ASF) is a devastating disease of domestic pigs that has spread across the globe since its introduction into Georgia in 2007. The etiological agent is a large double-stranded DNA virus with a genome of 170 to 180 kb in length depending on the isolate. Much of the differences in genome length between isolates are due to variations in the copy number of five different multigene families that are encoded in repetitive regions that are towards the termini of the covalently closed ends of the genome. Molecular epidemiology of African swine fever virus (ASFV) is primarily based on Sanger sequencing of a few conserved and variable regions, but due to the stability of the dsDNA genome changes in the variable regions occur relatively slowly. Observations in Europe and Asia have shown that changes in other genetic loci can occur and that this could be useful in molecular tracking. ASFV has been circulating in Western Africa for at least forty years. It is therefore reasonable to assume that changes may have accumulated in regions of the genome other than the standard targets over the years. At present only one full genome sequence is available for an isolate from Western Africa, that of a highly virulent isolate collected from Benin during an outbreak in 1997. In Cameroon, ASFV was first reported in 1981 and outbreaks have been reported to the present day and is considered endemic. Here we report three full genome sequences from Cameroon isolates of 1982, 1994 and 2018 outbreaks and identify novel single nucleotide polymorphisms and insertion-deletions that may prove useful for molecular epidemiology studies in Western Africa and beyond.

The remarkably multifunctional fibronectin binding proteins of Staphylococcus aureus

Staphylococcus aureus expresses two distinct but closely related multifunctional cell wall-anchored (CWA) proteins that bind to the host glycoprotein fibronectin. The fibronectin binding proteins FnBPA and FnBPB comprise two distinct domains. The C-terminal domain comprises a tandem array of repeats that bind to the N-terminal type I modules of fibronectin by the tandem β-zipper mechanism. This causes allosteric activation of a cryptic integrin binding domain, allowing fibronectin to act as a bridge between bacterial cells and the α₅β₁ integrin on host cells, triggering bacterial uptake by endocytosis. Variants of FnBPA with polymorphisms in fibronectin binding repeats (FnBRs) that increase affinity for the ligand are associated with strains that infect cardiac devices and cause endocarditis, suggesting that binding affinity is particularly important in intravascular infections. The N-terminal A domains of FnBPA and FnBPB have diverged into seven antigenically distinct isoforms. Each binds fibrinogen by the 'dock, lock and latch' mechanism characteristic of clumping factor A. However, FnBPs can also bind to elastin, which is probably important in adhesion to connective tissue in vivo. In addition, they can capture plasminogen from plasma, which can be activated to plasmin by host and bacterial plasminogen activators. The bacterial cells become armed with a host protease which destroys opsonins, contributing to immune evasion and promotes spreading during skin infection. Finally, some methicillin-resistant S. aureus (MRSA) strains form biofilm that depends on the elaboration of FnBPs rather than polysaccharide. The A domains of the FnBPs can interact homophilically, allowing cells to bind together as the biofilm accumulates.

Staphylococcus epidermidis serine--aspartate repeat protein G (SdrG) binds to osteoblast integrin alpha V beta 3

Staphylococcus epidermidis is the leading etiologic agent of orthopaedic implant infection. Contamination of the implanted device during insertion allows bacteria gain entry into the sterile bone environment leading to condition known as osteomyelitis. Osteomyelitis is characterised by weakened bones associated with progressive bone loss. The mechanism through which S. epidermidis interacts with bone cells to cause osteomyelitis is poorly understood. We demonstrate here that S. epidermidis can bind to osteoblasts in the absence of matrix proteins. S. epidermidis strains lacking the cell wall protein SdrG had a significantly reduced ability to bind to osteoblasts. Consistent with this, expression of SdrG in Lactococcus lactis resulted in significantly increased binding to the osteoblasts. Protein analysis identified that SdrG contains a potential integrin recognition motif. αVβ3 is a major integrin expressed on osteoblasts and typically recognises RGD motifs in its ligands. Our results demonstrate that S. epidermidis binds to recombinant purified αVβ3, and that a mutant lacking SdrG failed to bind. Blocking αVβ3 on osteoblasts significantly reduced binding to S. epidermidis. These studies are the first to identify a mechanism through which S. epidermidis binds to osteoblasts and potentially offers a mechanism through which implant infection caused by S. epidermidis leads to osteomyelitis.

Elucidating the role of Staphylococcus epidermidis serine-aspartate repeat protein G in platelet activation

BACKGROUND

Staphylococcus epidermidis is a commensal of the human skin that has been implicated in infective endocarditis and infections involving implanted medical devices. S. epidermidis induces platelet aggregation by an unknown mechanism. The fibrinogen-binding protein serine-aspartate repeat protein G (SdrG) is present in 67-91% of clinical strains.

OBJECTIVES

To determine whether SdrG plays a role in platelet activation, and if so to investigate the role of fibrinogen in this mechanism.

METHODS

SdrG was expressed in a surrogate host, Lactococcus lactis, in order to investigate its role in the absence of other staphylococcal components. Platelet adhesion and platelet aggregation assays were employed.

RESULTS

L. lactis expressing SdrG stimulated platelet aggregation (lag time: 2.9 +/- 0.5 min), whereas the L. lactis control did not. L. lactis SdrG-induced aggregation was inhibited by alpha(IIb)beta3 antagonists and aspirin. Aggregation was dependent on both fibrinogen and IgG, and the platelet IgG receptor FcgammaRIIa. Preincubation of the bacteria with Bbeta-chain fibrinopeptide inhibited aggregation (delaying the lag time six-fold), suggesting that fibrinogen acts as a bridging molecule. Platelets adhered to L. lactis SdrG in the absence of fibrinogen. Adhesion was inhibited by alpha(IIb)beta3 antagonists, suggesting that this direct interaction involves alpha(IIb)beta3. Investigation using purified fragments of SdrG revealed a direct interaction with the B-domains. Adhesion to the A-domain involved both a fibrinogen and an IgG bridge.

CONCLUSION

SdrG alone is sufficient to support platelet adhesion and aggregation through both direct and indirect mechanisms.

Antibody response in patients with endocarditis caused by Staphylococcus aureus

BACKGROUND

Staphylococcus aureus expresses a variety of adhesins involved in the colonization of host tissues. This study aimed to evaluate the role of staphylococcal surface proteins in the aetiology of infective endocarditis (IE) and the host immune response to infection.

MATERIALS AND METHOD

The ELISA assays were used to assess the adherence of S. aureus isolates recovered from the blood cultures of 19 patients with IE (16 were drug abusers) to subendothelial matrix proteins. Anti-adhesin antibody titre was measured incubating surface-coated bacterial antigens with patients' IgG. S. aureus effects on platelet aggregation were evaluated with an aggregometer.

RESULTS

Staphylococcus aureus isolates, from the patients with IE, exhibited a high expression of several surface components recognizing extracellular matrix proteins: clumping factors A and B (ClfA and ClfB) and fibronectin-binding proteins (FnbpA and FnbpB), whereas only four strains expressed the collagen-binding protein CNA. Bacteria also interacted with platelets both in the absence or presence of fibronectin or fibrinogen and some strongly supported platelet aggregation. Almost all patients presented significantly higher antibody reactivity to ClfA, ClfB, FnbpA, CNA and MAP (MHC class II analogous protein) than in sera from healthy individuals. On the contrary, the reactivity to CNA was remarkable only in three patients. The IgG preparations weakly inhibited the binding of bacteria to fibronectin, whereas they exhibited considerable blocking activity on staphylococcal attachment to fibrinogen or collagen.

CONCLUSION

Adhesins ClfA, ClfB and FnbpA are produced in vivo and appear important factors both in valve colonization and in promoting host immune responses.

Global regulation of Staphylococcus aureus genes by Rot

Staphylococcus aureus produces a wide array of cell surface and extracellular proteins involved in virulence. Expression of these virulence factors is tightly controlled by numerous regulatory loci, including agr, sar, sigB, sae, and arl, as well as by a number of proteins with homology to SarA. Rot (repressor of toxins), a SarA homologue, was previously identified in a library of transposon-induced mutants created in an agr-negative strain by screening for restored protease and alpha-toxin. To date, all of the SarA homologues have been shown to act as global regulators of virulence genes. Therefore, we investigated the extent of transcriptional regulation of staphylococcal genes by Rot. We compared the transcriptional profile of a rot agr double mutant to that of its agr parental strain by using custom-made Affymetrix GeneChips. Our findings indicate that Rot is not only a repressor but a global regulator with both positive and negative effects on the expression of S. aureus genes. Our data also indicate that Rot and agr have opposing effects on select target genes. These results provide further insight into the role of Rot in the regulatory cascade of S. aureus virulence gene expression.

Further observations on the size, shape, and hydration of casein micelles from novel analytical ultracentrifuge and capillary viscometry approaches

The size, shape, and hydration of casein micelles were estimated using a combination of sedimentation velocity (time-derivative analysis) in the analytical ultracentrifuge and capillary viscometry applied to skimmed milk. On the basis of sedimentation time-derivative and Wales-van Holde analyses the casein micelles appear as large spherical molecules of s0T,b = 845S, Mw approximately 2.8 x 10(8), hydrodynamic radius approximately 77.8 nm, and ks/[eta] = 1.6. The molecular hydration (i.e., the extent of chemically bound and physically entrained solvent) was calculated to be 3.4 g/g. These results appear to be in good agreement with comparable results from electron microscopy and dynamic light scattering.

A molecular description of the gelation mechanism of konjac mannan

A molecular level description of the time course of the gelation of the polysaccharide konjac mannan (KM) is presented and the role of alkali addition is considered in detail. NMR relaxometry is utilized as a complementary methodology to mechanical spectroscopy in order to probe events occurring as a prelude to network formation, and high-resolution NMR is used to follow the deactetylation process. It is shown that the addition of alkali plays an important solubilizing role in addition to facilitating the deacetylation of the chain. Deacetylation is important both in reducing the inherent aqueous solubility of the polymer and in progressively negating the alkali-induced polyelectrolytic nature of the polysaccharide chain via reaction induced pH changes. It is proposed that observed induction periods following alkali addition (during which the elastic modulus does not rise) are not simply deacetylation delays but are related to the aggregation kinetics of the deacetylated material.

Structural organization of the fibrinogen-binding region of the clumping factor B MSCRAMM of Staphylococcus aureus

The clumping factor B (ClfB) of Staphylococcus aureus is a surface protein that binds to fibrinogen (Ni Eidhin, D., Perkins, S., Francois, P., Vaudaux, P., Hook, M., and Foster, T. J., 1998 Mol. Microbiol. 30, 245-257). The ligand-binding activity is located in the approximately 500-residue A-region (residues 44-542), which represents the N-terminal half of the MSCRAMM protein. We now hypothesize that the ClfB A-region is composed of three subdomains, which we have named N1, N2, and N3, respectively. To examine this hypothesis, we expressed recombinant forms of the individual putative subdomains, the tandem motifs N12 and N23, and the full-length A-region N123. Far UV circular dichroism spectra showed that each subdomain is composed mainly of beta-sheets with little or no discernible alpha-helices. Heat-induced unfolding of individual subdomains occurred with a single state transition and was reversible, indicating that the subdomains can fold as discreet units. Gel permeation chromatography indicated that N2, N3, and N23 are globular. In contrast, domain N1 appeared to be elongated and conferred a somewhat elongated structure on segments containing this subdomain (i.e. N12 or N123). N123, N12, and N23 all bound to fibrinogen, but N23 had a higher affinity for fibrinogen than that observed for the full-length A-region; N123 or for N12. However, an extended N terminus of N23 was required for ligand binding. A form of N23 that was generated by proteolytic processing and lacked the N-terminal extension was unable to bind fibrinogen. Recombinant forms of individual subdomains did not bind fibrinogen. The addition of recombinant N23 effectively inhibited ClfB-mediated bacterial adherence to fibrinogen, and N123 caused some reduction in bacterial attachment, whereas N12 was essentially inactive. Antibodies raised against the central N2 domain of the A-region were the most effective at inhibiting bacterial adhesion to immobilized fibrinogen, although anti-N3 or anti-N1 antibodies also caused some reduction in ClfB-mediated adherence to fibrinogen.

Influence of thermal history on the structural and mechanical properties of agarose gels

Using a multitechnique approach, two temperature domains have been identified in agarose gelation. Below 35 degrees C, fast gelation results in strong, homogeneous and weakly turbid networks. The correlation length, evaluated from the wavelength dependence of the turbidity, is close to values of pore size reported in the literature. Above 35 degrees C, gelation is much slower and is associated with the formation of large-scale heterogeneities that can be monitored by a marked change in the wavelength dependence of turbidity and visualised by transmission electron microscopy. Curing agarose gels at temperatures above 35 degrees C, and then cooling them to 20 degrees C, produces much weaker gels than those formed directly at 20 degrees C. Dramatic reductions in the elastic modulus and failure strain and stress are found in this case as a result of demixing during cure. An interpretation, based on the kinetic competition between osmotic forces (in favor of phase separation) and elastic forces (that prevent it) is proposed.

Simulation of endo-PG digest patterns and implications for the determination of pectin fine structure

Novel stepwise approaches to the calculation of enzyme digest patterns are described and used in the validation of a computer simulation. Results obtained using the simulation show that, while a previously proposed model of endo-PG action captures some of the salient features of this enzymes behaviour, it is not sufficient to successfully predict experimental digest patterns from pectic substrates. Subsequently, it has been shown that a modified model incorporating existing information regarding subsite architecture and speculative site tolerances for esterified residues, goes someway towards improving the situation.

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.

Gelation behaviour of konjac glucomannan with different molecular weights

The deacetylation and gelation of konjac glucomannan (KGM) following alkali addition was investigated by Fourier transform infrared, while the rheological properties of KGM with different molecular weights were studied by dynamic viscoelastic measurements in shear mode and penetration force tests. It was found that gelation occurred after significant deacetylation had taken place. Rheometrical studies revealed that KGM with different molecular weights exhibited different gelation characteristics in small amplitude oscillatory shear flow. For the samples of fractionated KGM with lower molecular weights, a decrease in both the storage shear modulus (G') and loss shear modulus (G") was observed during gelation at temperatures above 75 degrees C. It is suggested that the decrease results from the wall slip between sample and measuring geometry owing to a rapid gelation process with syneresis and/or disentanglement of molecular chains adsorbed on the surface of parallel plates from those located in the bulk. Penetration force tests were employed to confirm the occurrence of slippage and thereby no decreases in rigidity of samples were observed during gelation. For the native KGM samples decreases in G' and G" during gelation were not observed, and it is suggested that this is due to the effect of the higher molecular weight and increased solution viscosity of these samples on the gelation kinetics.

Expression of pls, a gene closely associated with the mecA gene of methicillin-resistant Staphylococcus aureus, prevents bacterial adhesion in vitro

The pls gene, coding for a large surface protein of methicillin-resistant Staphylococcus aureus, was cloned from a strain which adheres poorly to several mammalian proteins. The structure of pls revealed three distinct repeat regions, one of which was a serine-aspartate repeat characteristic of the Clf-Sdr family of surface proteins in staphylococci. The lengths of the repeat regions varied in different clinical strains and could be used as epidemiological markers. pls was found to be closely associated with the mecA gene by pulsed-field gel electrophoresis analysis of SmaI-digested DNA. A pls mutant constructed by allele replacement adhered well to immobilized fibronectin and immunoglobulin G, in contrast to the parental strain, suggesting that Pls could have a role in preventing adhesion at some stages during an infection.

Identification of residues in the Staphylococcus aureus fibrinogen-binding MSCRAMM clumping factor A (ClfA) that are important for ligand binding

Clumping factor A (ClfA) is a cell surface-associated protein of Staphylococcus aureus that promotes binding of this pathogen to both soluble and immobilized fibrinogen (Fg). Previous studies have localized the Fg-binding activity of ClfA to residues 221-559 within the A region of this protein. In addition, the C-terminal part of the A region (residues 484-550) has been implicated as being important for Fg binding. In this study, we further investigate the involvement of this part of ClfA in the interaction of this protein with Fg. Polyclonal antibodies generated against a recombinant protein encompassing residues 500-559 of the A region inhibited the interaction of both S. aureus and recombinant ClfA with immobilized Fg in a dose-dependent manner. Using site-directed mutagenesis, two adjacent residues, Glu(526) and Val(527), were identified as being important for the activity of ClfA. S. aureus expressing ClfA containing either the E526A or V527S substitution exhibited a reduced ability to bind to soluble Fg and to adhere to immobilized Fg. Furthermore, bacteria expressing ClfA containing both substitutions were almost completely defective in Fg binding. The E526A and V527S substitutions were also introduced into recombinant ClfA (rClfA-(221-559)) expressed in Escherichia coli. The single mutant rClfA-(221-559) proteins showed a significant reduction in affinity for both immobilized Fg and a synthetic fluorescein-labeled C-terminal gamma-chain peptide compared with the wild-type protein, whereas the double mutant rClfA-(221-559) protein was almost completely defective in binding to either species. Substitution of Glu(526) and/or Val(527) did not appear to alter the secondary structure of rClfA-(221-559) as determined by far-UV circular dichroism spectroscopy. These data suggest that the C terminus of the A region may contain at least part of the Fg-binding site of ClfA and that Glu(526) and Val(527) may be involved in ligand recognition.

Characterization of a putative pathogenicity island from bovine Staphylococcus aureus encoding multiple superantigens

Previous studies have demonstrated that a proportion of Staphylococcus aureus isolates from bovine mastitis coproduce toxic shock syndrome toxin (TSST) and staphylococcal enterotoxin C (SEC). In this study, molecular genetic analysis of one such strain, RF122, revealed the presence of a 15,891-bp putative pathogenicity island (SaPIbov) encoding the genes for TSST (tst), the SEC bovine variant (sec-bovine), and a gene (sel) which encodes an enterotoxin-like protein. The island contains 21 open reading frames specifying hypothetical proteins longer than 60 amino acids including an integrase-like gene. The element is bordered by 74-bp direct repeats at the left and right junctions, and the integration site lies adjacent to the 3' end of the GMP synthase gene (gmps) in the S. aureus chromosome. SaPIbov contains a central region of sequence identity with the previously characterized tst pathogenicity island SaPI1 (J. A. Lindsay et al., Mol. Microbiol. 29:527-543, 1998). A closely related strain, RF120, of the same multilocus enzyme electrophoretic type, random amplified polymorphic DNA type, and ribotype, does not contain the island, implying that the element is mobile and that a recent insertion/deletion event has taken place. TSST and TSST/SEC-deficient mutants of S. aureus strain RF122 were constructed by allele replacement. In vitro bovine Vbeta-specific lymphocyte expansion analysis by culture supernatants of wild-type strains and of tst and sec-bovine allele replacement mutants revealed that TSST stimulates BTB13-specific T cells whereas SEC-bovine stimulates BTB93-specific T cells. This suggests that the presence of SaPIbov may contribute to modulation of the bovine immune response.

Investigations towards an efficacious and safe strangles vaccine: submucosal vaccination with a live attenuated Streptococcus equi

As part of a search for a safe and efficacious strangles vaccine, several different vaccines and different vaccination routes were tested in foals. The degree of protection was evaluated after an intranasal challenge with virulent Streptococcus equi by clinical, postmortem and bacteriological examinations. Inactivated vaccines containing either native purified M-protein (500 microg per dose) or whole S equi cells (10(10) cells per dose) administered at least twice intramuscularly at intervals of four weeks, did not protect against challenge. Different live attenuated S equi mutants administered at least twice at intervals of four weeks by the intranasal route were either safe but not protective or caused strangles. In contrast, a live attenuated deletion mutant administered intramuscularly, induced complete protection but also induced unacceptable local reactions at the site of vaccination. Submucosal vaccination in the inner side of the upper lip with the live attenuated mutant at > or =10(8) colony-forming units per dose, appeared to be safe and efficacious in foals as young as four months of age. The submucosal vaccinations caused small transient swellings that resolved completely within two weeks, and postmortem no vaccine remnants or other abnormalities were found at the site of vaccination.

Cellular invasion by Staphylococcus aureus involves a fibronectin bridge between the bacterial fibronectin-binding MSCRAMMs and host cell beta1 integrins

Although Staphylococcus aureus is primarily considered an extracellular pathogen, recent evidence suggests that this bacterium can invade a variety of nonprofessional phagocytic cells. Here we investigate the early stages of cellular invasion by S. aureus and determine the bacterial and host components that are required for this process. S. aureus expresses two cell surface-associated fibronectin (FN)-binding proteins (FnbpA and FnbpB) that mediate the interaction of the bacteria with both soluble and solid-phase FN in vitro. Using a mutant of S. aureus that lacks the expression of both Fnbps, we show that the expression of either protein is necessary for efficient uptake by the mouse fibroblast line GD25beta1A. Invasion could be inhibited by soluble recombinant proteins encompassing either the FN-binding D repeat region or the A region (and B repeats) of FnbpA, suggesting that the activities of both regions are important in this process. We demonstrate that FN is also required for invasion of this cell line. In the presence of FN-depleted fetal bovine serum, the invasion level was reduced by approximately 40% compared to in the presence of whole fetal bovine serum. Invasion could be further reduced by the addition of anti-mouse FN antibodies to the assay. Finally, we utilize a mutant mouse fibroblast line, which lacks beta1 integrin expression, to demonstrate that host cell beta1 integrins are necessary for efficient cellular invasion. The level of invasion of the mutant cell line GD25 was reduced by approximately 97% compared to the beta1-expressing complemented cell line GD25beta1A. In addition, invasion of the GD25beta1A cell line could be inhibited by an RGD-containing peptide, further implicating a role for integrins in this process. Based on these observations, we put forward a model of S. aureus invasion in which host FN forms a bridge between the bacterial Fnbps and host cell beta1 integrins, leading to bacterial uptake.

Protein A is the von Willebrand factor binding protein on Staphylococcus aureus

Endovascular infection is a highly critical complication of invasive Staphylococcus aureus disease. For colonization, staphylococci must first adhere to adhesive endovascular foci. Von Willebrand factor (vWF) is a large, multimeric glycoprotein mediating platelet adhesion at sites of endothelial damage. Earlier it was demonstrated that vWF binds to and promotes the surface adhesion of S. aureus, prompting this effort to identify the vWF adhesin. In Western ligand assays of S. aureus lysates, staphylococcal protein A (SPA) was recognized by purified vWF. Surface plasmon resonance demonstrated the binding of soluble vWF to immobilized recombinant protein A with a K(d) of 1.49 x 10(-8) mol/L. Using flow cytometry, the binding of fluorescein isothiocyanate-labeled vWF to S. aureus was found to be saturable and inhibitable by unlabeled vWF, antiprotein-A antibodies, or IgG. Isogenic Deltaspa::Tc(r) mutants were constructed by the insertion of a tetracycline resistance cassette into spa using allelic replacement, and it exhibited decreased binding of soluble vWF and decreased adhesion to vWF-adsorbed surfaces. The interaction was restored on complementation of the mutants with spa-containing plasmid pSPA7235. In conclusion, protein A confers interaction of S. aureus with soluble and immobilized vWF in a newly discovered function characterizing protein A as a novel member of the staphylococcal surface protein adhesin superfamily and suggesting its potential role in the pathogenesis of endovascular staphylococcal disease.

Contribution of clumping factor B to pathogenesis of experimental endocarditis due to Staphylococcus aureus

Staphylococcus aureus Newman with an insertion mutation in clfB, the gene encoding clumping factor B, only marginally decreased infection rate (P>0.05) in rats with experimental endocarditis. In contrast, clfB complementation on a multicopy plasmid significantly increased infectivity (P<0.05) over the deleted mutants. Although clfB could affect endovascular infection, its importance in experimental endocarditis was limited.

Clinical isolates of Staphylococcus aureus exhibit diversity in fnb genes and adhesion to human fibronectin

OBJECTIVES

The fibronectin-binding proteins (FnBPs) of Staphylococcus aureus are involved in the pathogenesis of infection, but their characteristics in clinical isolates are incompletely defined. The aim of this study was to evaluate phenotypic and genotypic characteristics of the FnBPs of a large collection of recent isolates.

METHODS

The adherence of 163 S. aureus isolates to immobilized fibronectin was compared with that of S. aureus 8325-4 using a microtitre assay. The presence of the genes encoding the fibronectin-binding proteins FnBPA and FnBPB was evaluated by Southern dot blot using probes specific for region A of fnbA or fnbB.

RESULTS

The adherence of clinical isolates to fibronectin (expressed as a percentage of the mean adherence of S. aureus 8325-4) was 56%-125% for 155 isolates (95%), and less than 20% for eight isolates (5%). Adherence of the bacterial group associated with orthopaedic implant-associated infection was significantly greater than that for isolates associated with nasal carriage, endocarditis, or septic arthritis/osteomyelitis. Southern dot blot demonstrated that 126/163 isolates had two genes (77%) and 37/163 had one detectable gene (23%). There was no difference in adherence between isolates with one or two fnb, but isolates associated with invasive disease (endocarditis or primary septic arthritis and/or osteomyelitis) were more likely to have two genes.

CONCLUSIONS

These data demonstrate diversity in the FnBPs of clinical isolates of S. aureus. The findings suggest that the interplay between pathogenesis and a single virulence determinant is unlikely to be a uniform process across a spectrum of infections. This confirms the need to extend the study of staphylococcal pathogenesis from the laboratory to non-uniform populations of clinically relevant isolates.

Evidence for the existence of a multidrug efflux transporter distinct from NorA in Staphylococcus aureus

A Staphylococcus aureus norA disruption mutant was created by allelic replacement. Exposure of this mutant to norfloxacin produced SA K1748, a derivative with raised fluoroquinolone MICs, found to be the result of a grlA mutation, and raised organic cation MICs. Ethidium and enoxacin uptake was identical in SA K1748 and its parent, but pre-exposure of SA K1748 to organic cations caused a reduction in ethidium uptake as a result of increased efflux. Altered ethidium uptake and efflux, as well as increased MICs of other organic cations, suggest that SA K1748 possesses a non-NorA multidrug efflux transporter that is inducible by its substrates.

Molecular analysis of the tagF gene, encoding CDP-Glycerol:Poly(glycerophosphate) glycerophosphotransferase of Staphylococcus epidermidis ATCC 14990

Staphylococcus epidermidis ATCC 14990 produces a wall-associated glycerol teichoic acid which is chemically identical to the major wall-associated teichoic acid of Bacillus subtilis 168. The S. epidermidis tagF gene was cloned from genomic DNA and sequenced. When introduced on a plasmid vector into B. subtilis 1A486 carrying the conditionally lethal temperature-sensitive mutation tagF1 (rodC1), it expressed an 85-kDa protein which allowed colonies to grow at the restrictive temperature. This showed that the cloned S. epidermidis gene encodes a functional CDP-glycerol:poly(glycerophosphate) glycerophosphotransferase. An amino acid substitution at residue 616 in the recombinant TagF protein eliminated complementation. Unlike B. subtilis, where the tagF gene is part of the tagDEF operon, the tagF gene of S. epidermidis is not linked to any other tag genes. We attempted to disrupt the chromosomal tagF gene in S. epidermidis TU3298 by directed integration of a temperature-sensitive plasmid but this failed, whereas a control plasmid containing the 5' end of tagF on a similarly sized DNA fragment was able to integrate. This suggests that the tagF gene is essential and that the TagF and other enzymes involved in teichoic acid biosynthesis could be targets for new antistaphylococcal drugs.

Identification of plasma proteins adsorbed on hemodialysis tubing that promote Staphylococcus aureus adhesion

Risk factors for Staphylococcus aureus infections in patients undergoing hemodialysis include underlying disease, material-induced host defects, and the presence of vascular access catheters. To determine the specific contribution of various potentially adsorbed plasma components in promoting S aureus adhesion to shunt tubing during chronic hemodialysis, we quantified their respective amounts by Western immunoblotting and densitometry and estimated their individual adhesion-promoting activities with specific adhesion-modified bacterial mutants. Fibrinogen, which was the only component consistently present in tubing protein extracts from all patients, was adsorbed in significantly higher amounts on predialyzer than on postdialyzer tubing segments. In contrast, fibronectin and von Willebrand factor were irregularly present in patients' tubing, whereas vitronectin or thrombospondin remained undetectable in all samples. The contribution of fibrinogen in promoting S aureus attachment to hemodialysis tubing was demonstrated by (1) the significantly lower adhesion of a cIfA mutant of strain Newman compared with its parent; (2) the increased attachment of strain 8325-4 after complementation with the cloned cIfA gene on the multicopy plasmid pCF4; and (3) the general tendency for strains Newman and 8325-4(pCF4) to express higher attachment on predialyzer compared with postdialyzer tubing segments in relationship with the higher content of fibrinogen on the former material. However, the specific S aureus attachment-promoting activity of both prefilter and postfilter tubing-adsorbed fibrinogen were much lower than that of the native in vitro-adsorbed protein and may reflect masking or inactivation of the in vivo-adsorbed protein by unknown mechanisms.

A molecular model for the formation and properties of fluid gels

The effects of biopolymer gelation in a shear field are discussed. Gel particles are produced if the gelation mechanism involves an aggregation step. Particular attention is paid to the molecular events of ordering and aggregation upon cooling, investigating the differences in such processes as a result of shearing during gelation. A model is proposed which follows the conception of the particles, their growth, physical properties and stability.

Alpha-toxin damages the air-blood barrier of the lung in a rat model of Staphylococcus aureus-induced pneumonia

We have shown that injury to alveolar epithelial type I cells may account, in part, for damage to the air-blood barrier of the lung in a rat model of Staphylococcus aureus pneumonia. We have also shown that alpha-toxin is an important cause of damage to the air-blood barrier; however, our data suggest that the toxin is not acting directly on alveolar type I cells.

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 noninvasive strains conferred invasiveness; and (iii) the soluble isolated fibronectin-binding domain of FnBP (D1-D4) completely blocked invasion. Integrin alpha5beta1 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 alpha5beta1 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.

Introduction of a norA promoter region mutation into the chromosome of a fluoroquinolone-susceptible strain of Staphylococcus aureus using plasmid integration

It has been postulated that a mutation 11 bp 3' to the -10 motif of the norA promoter is involved in the increased expression of the gene observed in some strains of Staphylococcus aureus exhibiting efflux-related fluoroquinolone resistance. Introduction of this mutation into the chromosome of a fluoroquinolone-susceptible strain by plasmid integration resulted in the minimum inhibitory concentrations of NorA substrates being increased, fluoroquinolone uptake being reduced, and norA expression being enhanced. Diffuse hybridization of norA and integrating vector probes at a similar molecular weight range, higher than that of the norA transcript, was observed in the integrant, suggesting the possibility of a plasmid-based promoter contributing to norA expression. The ratio of the quantity of this transcript, which was also observed in the parent strain of the integrant, to the quantity of primary norA transcript was 0.14, demonstrating that it was unlikely that this mRNA species contributed significantly to the results observed. It is more likely that the introduced promoter region mutation does affect the expression of norA.

Staphylococcus schleiferi subsp. schleiferi expresses a fibronectin-binding protein

Staphylococcus schleiferi subsp. schleiferi is associated with a range of nosocomial infections, but the pathogenic mechanisms by which these occur are poorly understood. This study provides phenotypic and genotypic evidence for the expression of a cell wall-anchored fibronectin-binding protein by this species.

Crystallization of ClfA and ClfB fragments: the fibrinogen-binding surface proteins of Staphylococcus aureus

Recombinant constructs encoding the fibrinogen-binding domains of ClfA and ClfB from Staphylococcus aureus have been crystallized. ClfA was crystallized in the orthorhombic space group P212121 with unit-cell parameters a = 39.58, b = 81.39 and c = 112.65 A. A complete data set was recorded to 2.1 A resolution and had a Vm of 2. 3 A3 Da-1 with 46.5% solvent, suggesting one molecule per asymmetric unit. Co-crystals of ClfA with the 17 amino-acid C-terminal peptide of fibrinogen gamma-chain diffracted to 2.1 A resolution and had unit-cell parameters a = 39.11, b = 81.39 and c = 109.51 A in the space group P212121. ClfB was crystallized in the tetragonal space group P41212 or P43212 with unit-cell parameters a = 96.31, b = 96. 31 and c = 84.13 A and diffracted to 2.45 A resolution. The estimated Vm of 2.6 A3 Da-1 with 53% solvent indicated one molecule in the asymmetric unit.

Genetic analysis of the cap5 locus of Staphylococcus aureus

Staphylococcus aureus expresses at least eight distinct serotypes of capsular polysaccharide (CP). Gene clusters involved in the expression of serotypes 1, 5 and 8 have been cloned and sequenced. In this report we describe the isolation and analysis of serotype 5 capsular polysaccharide-defective mutants. A naturally occurring cap mutation in the laboratory strains 8325-4 and RN4220 was mapped to the cap5E gene by genetic complementation. The cap5H-K genes were shown to be responsible for CP5 serotype specificity by transduction and complementation.

Surface protein adhesins of Staphylococcus aureus

Staphylococcus aureus can colonize the host to initiate infection by adhering to components of the extracellular matrix. Adherence is mediated by surface protein adhesins (MSCRAMMs). Ligand binding by these fibronectin-, fibrinogen- and collagen-binding proteins occurs by distinct mechanisms that are being investigated at the molecular level.

The binding of calcium to the B-repeat segment of SdrD, a cell surface protein of Staphylococcus aureus

In the Sdr family of Staphylococcus aureus cell surface proteins, three recently cloned members (Josefsson, E., McCrea, K., Ni Eidhin, D., O'Connell, D., Cox, J. A., Hook, M., and Foster, T. (1998) Microbiology, in press) display variable numbers of B-repeats, i.e. segments of 110-113 residues that probably make up one folding unit. Each B-repeat contains one conserved EF-hand motif and two acidic stretches. Equilibrium dialysis revealed that segment B1-B5 of SrdD contains 14 Ca2+-binding sites with high affinity ([Ca2+]0.5, 4 microM), whereas flow dialysis yielded 5 sites of high affinity (class I) and 10 of low affinity (class II). The discrepancy could be explained by the slow induction of high affinity in the class II sites. Kinetic experiments using fluorescent Ca2+ indicators corroborated slow binding of Ca2+ at the latter sites. Circular dichroism and Trp fluorescence showed that, whereas the Ca2+ form is well folded, the metal-free form seems strongly disorganized. The Ca2+-induced conformational changes comprise both fast and slow steps, giving thus a structural support for the induction of class II Ca2+-binding sites. The B-repeats may act as rulers or springs that modulate the distance between the interactive A region and the bacterial cell surface.

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 alpha- and beta-chains of fibrinogen in the ligand blots in contrast to ClfA, which binds exclusively to the gamma-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.

Potency testing of a live, genetically attenuated vaccine for salmonids

Studies have been performed on the use of a live vaccine for immunization of salmonids against the bacterial disease furunculosis. The protection elicited by a kanamycin-resistant aroA mutant of A. salmonicida (Brivax I) and an unmarked aroA deletion mutant (Brivax II) has been examined, and data compared with protection seen using a freeze-dried Brivax II preparation and a commercial, oil-adjuvanted killed vaccine for furunculosis. Whilst high relative percent survival (RPS) values were seen in fish vaccinated with broth-grown Brivax I after a natural exposure to furunculosis (70-100%), much lower RPS values (30-40%) were seen with Brivax II vaccinated fish after an experimental challenge. Nevertheless, the freeze-dried Brivax II formulation performed as well as the broth-grown Brivax II formulation and a commercial vaccine in these studies. In addition, the environmental impact in terms of bacterial shedding into the tank water has been estimated, and shown to approximately 0.03% of the total inoculum used. Lastly, the freeze-dried formulation has been tested for its ability to infect fish and prime for lymphocyte proliferation and antibody production, relative to broth-grown preparations. In all three experiments no significant differences were seen between fish given the broth-grown and freeze-dried formulations. Such data, together with observations that the freeze-dried live preparation had an extended shelf life with the same potency as freshly grown bacteria, show that the potential exists for a commercially viable live vaccine to be produced for use in aquaculture.

Assessment of the role of gamma-toxin in experimental endophthalmitis using a hlg-deficient mutant of Staphylococcus aureus

Purified gamma-toxin is known to have a proinflammatory effect in the rabbit vitreous humor. To assess the biological role of the gamma-toxin, when expressed in vivo by Staphylococcus aureus strain Newman, the vitreous humor of rabbit eye was used as an infection model. A gamma-toxin-deficient mutant of strain Newman was constructed by allelic replacement. S. aureus Newman wild-type, its hlg-deficient derivative strain (N65) and the strain N65 complemented with the wild-type hlg+ gene were injected into the vitreous humor of rabbit eye. All three strains produced a strong proinflammatory effect in the eye conjunctiva, posterior and anterior chambers, suggesting a role for another unidentified proinflammatory component of strain Newman distinct from the gamma-toxin. These components are not the leucocidin of Panton-Valentine, beta-toxin or alpha-toxin which are not produced by this strain. Only the hlg-deficient mutant lacked the ability to cause inflammation in the eyelid, whereas the two Hlg-producing strains gave strong inflammation. These data suggest that in vivo, strain Newman produces as yet unidentified proinflammatory molecules and that the in vivo-produced HlgA, HlgB and HlgC molecules expressed by the gamma-toxin locus, contribute in part to the inflammatory process observed in vivo in the rabbit eye.

The fibrinogen-binding MSCRAMM (clumping factor) of Staphylococcus aureus has a Ca2+-dependent inhibitory site

The clumping factor (ClfA) is a cell surface-associated protein of Staphylococcus aureus that promotes binding of fibrinogen or fibrin to the bacterial cell. Previous studies have shown that ClfA and the platelet integrin alphaIIbbeta3 recognize the same domain at the extreme C terminus of the fibrinogen gamma-chain. alphaIIbbeta3 interaction with this domain is known to occur in close proximity to a Ca2+-binding EF-hand structure in the alpha-subunit. Analysis of the primary structure of ClfA indicated the presence of a potential Ca2+-binding EF-hand-like motif at residues 310-321 within the fibrinogen-binding domain. Deletion mutagenesis and site-directed mutagenesis of this EF-hand in recombinant truncated ClfA proteins (Clf40, residues 40-559; and Clf41, residues 221-559) resulted in a significant reduction of affinity for native fibrinogen and a fibrinogen gamma-chain peptide. Furthermore, Ca2+ (or Mn2+) could inhibit the binding of the fibrinogen gamma-chain peptide to Clf40-(40-559) and the adhesion of S. aureus cells to immobilized fibrinogen with an IC50 of 2-3 mM. In contrast, Mg2+ (or Na+) at similar concentrations had no effect on the ClfA-fibrinogen interaction. Far-UV CD analysis of Clf40-(40-559) and Clf41-(221-559) in the presence of metal ions indicated Ca2+- and Mn2+-induced differences in secondary structure. These data suggest that Ca2+ binds to an inhibitory site(s) within ClfA and induces a conformational change that is incompatible with binding to the C terminus of the gamma-chain of fibrinogen. Mutagenesis studies indicate that the Ca2+-dependent inhibitory site is located within the EF-hand motif at residues 310-321.

Introduction of the mec element (methicillin resistance) into Staphylococcus aureus alters in vitro functional activities of fibrinogen and fibronectin adhesins

Some methicillin-resistant strains of Staphylococcus aureus are defective in the production of major surface components such as protein A, clumping factor, or other important adhesins to extracellular matrix components which may play a role in bacterial colonization and infection. To evaluate the impact of methicillin resistance (mec) determinants on bacterial adhesion mediated by fibrinogen or fibronectin adhesins, we compared the in vitro attachment of two genetically distinct susceptible strains (NCTC8325 and Newman) to protein-coated surfaces with that of isogenic methicillin-resistant derivatives. All strains containing an intact mec element in their chromosomes were found to be defective in adhesion to fibrinogen and fibronectin immobilized on polymethylmethacrylate coverslips, regardless of the presence or absence of additional mutations in the femA, femB, or femC gene, known to decrease expression of methicillin resistance in S. aureus. Western ligand affinity blotting or immunoblotting of cell wall-associated adhesins revealed similar contents of fibrinogen- or fibronectin-binding proteins in methicillin-resistant strains compared to those of their methicillin-susceptible counterparts. In contrast to methicillin-resistant strains carrying a mec element in their genomes, methicillin-resistant strains constructed in vitro, by introducing the mecA gene on a plasmid, retained their adhesion phenotypes. In conclusion, the chromosomal insertion of the mec element into genetically defined strains of S. aureus impairs the in vitro functional activities of fibrinogen or fibronectin adhesins without altering their production. This effect is unrelated to the activity of the mecA gene.

The dipeptide repeat region of the fibrinogen-binding protein (clumping factor) is required for functional expression of the fibrinogen-binding domain on the Staphylococcus aureus cell surface

Clumping factor of Staphylococcus aureus is a fibrinogen-binding protein that is located on the bacterial cell surface. The protein has an unusual repeat domain (region R) comprising mainly the dipeptide aspartate and serine. To determine if region R has a role in the surface display of the fibrinogen-binding region A domain, deletions lacking the region R encoding region of the clfA gene were generated. To determine the minimum length of region R required for wild-type levels of ClfA expression, variants with truncated region R domains were constructed. S. aureus cells expressing mutated clfA genes were tested for (i) proteins released by lysostaphin treatment that reacted with antisera specific for region A, (ii) clumping in soluble fibrinogen, (iii) adherence to immobilized fibrinogen and (iv) expression of the ClfA antigen on the cell surface by fluorescent activated cell sorting analysis. Each construct expressed three major immunoreactive proteins, two of which were putative N-terminal degradation products. Region R residues greater than 40 were required between region A and W (72 residues between region A and the LPDTG sorting signal) for wild-type levels of clumping in fibrinogen. A stepwise decrease in clumping titre was observed as the distance between region A and LPDTG was decreased from 72 to 4 residues. Similarly, a decrease in binding of anti-ClfA serum and in binding to fibrinogen-coated plastic surfaces was observed with cells expressing ClfA with 40 region R residues or less. Nevertheless, low levels of adherence to fibrinogen and binding to anti-ClfA serum occurred with ClfA derivatives that lacked region R altogether. This indicates that a small proportion of the ClfA molecules are linked to peptidoglycan very close to the cell surface but that residues greater than 72 are needed to allow sufficient ClfA molecules to span the entire cell wall and to display the biologically active A domain in a form that can participate fully in fibrinogen binding.

Specific roles of alpha-toxin and beta-toxin during Staphylococcus aureus corneal infection

Staphylococcus aureus corneal infection results in extensive inflammation and tissue damage. Our previous studies of bacterial mutants have demonstrated a role for alpha-toxin in corneal virulence. This study analyzes, by genetic rescue experiments, the virulence of mutants affecting alpha-toxin and beta-toxin activity and demonstrates the ocular toxicity of these purified staphylococcal proteins. Three types of isogenic mutants were analyzed: (i) mutants specifically deficient in alpha-toxin (Hla) or beta-toxin (Hlb), (ii) a mutant deficient in both Hla and Hlb, and (iii) a regulatory mutant, deficient in the accessory gene regulator (agr), that produces reduced quantities of multiple exoproteins, including alpha- and beta-toxins. Plasmids coding for Hla and Hlb (pDU1212 and pCU1hlb, respectively) were used to restore toxin activity to mutants specifically deficient in each of these toxins. Either corneas were injected intrastromally with logarithmic-phase S. aureus or purified alpha- or beta-toxins were administered to normal eyes. Ocular pathology was evaluated by slit lamp examination and myeloperoxidase activity of infiltrating polymorphonuclear leukocytes. Corneal homogenates were cultured to determine the CFU per cornea. Eyes infected with the wild-type strain developed significantly greater corneal damage than eyes infected with Agr-, Hlb-, or Hla- strains. Epithelial erosions produced by parent strains were not produced by Agr- or Hla- strains. Hlb+ strains, unlike Hlb- strains, caused scleral edema. Plasmid pDU1212 restored corneal virulence to strain DU1090 (Hla-), and plasmid pCU1hlb restored corneal virulence to strain DU5719 (Hlb-). Application of purified alpha-toxin produced corneal epithelial erosions and iritis, while application of beta-toxin caused scleral inflammation. These studies confirm the role of alpha-toxin as a major virulence factor during S. aureus keratitis and implicate beta-toxin, a mediator of edema, as a lesser contributor to ocular damage.

Chorismate synthase from Staphylococcus aureus

The aroC gene encoding chorismate synthase and the ndk gene encoding nucleoside diphosphate kinase (Ndk) were cloned from Staphylococcus aureus. DNA sequencing suggests that aroC is located in an operon with aroB and aroA and encodes a protein of 388 amino acids with 61% identity to the aroF gene product of Bacillus subtilis. The ndk gene of S. aureus encodes a protein of 149 amino acids which exhibits a high degree of identity to other bacterial Ndk proteins. The 3' end of the S. aureus gerCC gene was also identified by sequencing and was located immediately upstream of ndk. The gerCA and gerCB genes were found to be located upstream of gerCC by Southern hybridization analysis. This observed linkage of the gerC genes with the ndk, aroC and aroB genes has been similarly observed in B. subtilis. The S. aureus chorismate synthase was overexpressed to a high level in Escherichia coli using a T7 promoter plasmid construct, the enzyme was purified to near homogeneity in two steps and found to be a homotetramer with a subunit molecular mass, estimated by electrospray mass spectrometry, of 43024 Da. The properties of S. aureus chorismate synthase are compared with those of the B. subtilis and E. coli enzymes.

A live (delta aroA) Aeromonas salmonicida vaccine for furunculosis preferentially stimulates T-cell responses relative to B-cell responses in rainbow trout (Oncorhynchus mykiss)

We have previously described (L. M. Vaughan, P. R. Smith, and T. J. Foster, Infect. Immun. 61:2172-2181, 1993) the construction of a kanamycin-resistant aromatic-dependent mutant of Aeromonas salmonicida, the causative agent of furunculosis, and its use as a live vaccine. Here we describe the construction of an unmarked aroA deletion mutant and examine the nature and magnitude of immune responses in rainbow trout (Oncorhynchus mykiss) to this vaccine strain. Good proliferative and antibody responses were elicited by using a range of vaccine doses from 2 x 10(6) to 2 x 10(9) live bacteria per fish, and a clear vaccine dose effect was apparent. A significant positive effect of using live bacteria to prime for lymphocyte proliferation and antibody production was apparent. However, the problem of directly comparing the vaccine doses of live and killed bacterial preparations is discussed, since some replication of live bacteria in vivo is expected. Most importantly, the live vaccine preferentially stimulated enhanced T-cell responsiveness, as evidenced by significantly greater increases in T-cell proliferation than in B-cell proliferation, compared with responses by the respective cell populations from fish given a killed vaccine. The manner in which live vaccines elicit strong cell-mediated immune responses and the relevance to fish vaccine design are discussed.

A low-fibronectin-binding mutant of Staphylococcus aureus 879R4S has Tn918 inserted into its single fnb gene

A low-fibronectin-binding mutant of Staphylococcus aureus strain 879R4SSp generated by transposon Tn918 mutagenesis is attenuated in a rat endocarditis model (J.M. Kuypers & R.A. Procter, 1989, Infect Immun 57, 2306-2312). PCR and Southern hybridization analysis with primers and probes, respectively, for the fnbA and fnbB genes of strains 8325-4 showed that strain 879R4SSp possesses a single fnb gene which is homologous to fnbA. This was confirmed by sequencing 41 bp of 5' non-coding and 237 bp of 5' coding DNA, which showed 97% base identity to fnbA. Southern hybridization and sequencing showed that Tn918 was inserted 41 bp 5' to fnbA in the mutant 879R4SSp/1536, between the promoter and initiation codon. Reduced adherence of the mutant to surface-bound fibronectin correlated with lower expression of a 180 kDa wall-associated fibronectin-binding protein.

Influence of agr on fibrinogen binding in Staphylococcus aureus Newman

The ability of Staphylococcus aureus to bind fibrinogen is believed to be important in promoting bacterial adherence to both intravascular catheters and host tissues during infection. We investigated the influence of the global regulator agr on the fibrinogen binding capacity and its relationship to the expression of coagulase (encoded by coa) and clumping factor (encoded by clfA) in strain Newman. Strains were obtained by transducing site-specific mutations of clfA, coa, and agr into strain Newman to obtain single, double, and triple mutants of the respective genes. As expected, the clfA mutant bound less soluble 125I-labeled fibrinogen than the corresponding coa mutant in agr+ strains; however, with agr mutant strains, the upregulation in fibrinogen binding capacity correlated mostly with the increased expression and transcription of coagulase as shown by Western (immunoblot) and Northern (RNA) blot analysis. In particular, the coa agr double mutant resulted in a significant reduction in fibrinogen binding compared with that of the agr mutant. The contribution of clfA to fibrinogen binding in agr-negative strains was less than that of coa (32,740 +/- 1,189 versus 18,141 +/- 334 and 38,919 +/- 1,021 cpm for clfA agr, coa agr, and the single agr mutant, respectively). Thus, coagulase is a major binding protein for soluble fibrinogen in the agr-negative background. In in vitro microtiter and catheter adherence assays with solid-phase fibrinogen, clumping factor, but not coagulase, plays a major role in binding to immobilized fibrinogen. coa transcription was negatively modulated by agr and occurred mainly during the exponential growth phase. In contrast, clfA transcription was agr independent and was strongest during the postexponential phase. Although an agr coa clfA triple mutant bound less soluble fibrinogen than the agr coa double mutant (8,504 +/- 831 versus 18,141 +/- 334 cpm), significant residual fibrinogen binding capacity remained in the triple mutant, thus suggesting an additional fibrinogen binding component. By using direct ligand affinity blotting with 125I-fibrinogen, we could identify coagulase and an additional unidentified 52-kDa protein as a fibrinogen binding component in cell extracts. This band was absent in the extract of the coa clfA double mutant.

Host-bacteria interactions in foreign body infections

Persistent staphylococcal infections are a major medical problem, especially when they occur on implanted materials or intravascular catheters. This review describes some of the recently discovered molecular mechanisms of Staphylococcus aureus attachment to host proteins coating biomedical implants. These interactions involve specific surface proteins, called bacterial adhesins, that recognize specific domains of host proteins deposited on indwelling devices, such as fibronectin, fibrinogen, or fibrin. Elucidation of molecular mechanisms of S aureus adhesion to the different host proteins may lead to the development of specific inhibitors blocking attachment of S aureus, which may decrease the risk of bacterial colonization of indwelling devices.

Role of Staphylococcus aureus coagulase and clumping factor in pathogenesis of experimental endocarditis

The pathogenic role of staphylococcal coagulase and clumping factor was investigated in the rat model of endocarditis. The coagulase-producing and clumping factor-producing parent strain Staphylococcus aureus Newman and a series of mutants defective in either coagulase, clumping factor, or both were tested for their ability (i) to attach in vitro to either rat fibrinogen or platelet-fibrin clots and (ii) to produce endocarditis in rats with catheter-induced aortic vegetations. In vitro, the clumping factor-defective mutants were up to 100 times less able than the wild type strain to attach to fibrinogen and also significantly less adherent than the parents to platelet-fibrin clots. Coagulase-defective mutants, in contrast, were not altered in their in vitro adherence phenotype. The rate of in vivo infection was inoculum dependent. Clumping factor-defective mutants produced ca. 50% less endocarditis than the parent organisms when injected at inoculum sizes infecting, respectively, 40 and 80% (ID40 and ID80, respectively) of rats with the wild-type strain. This was a trend at the ID40 but was statistically significant at the ID80 (P < 0.05). Coagulase-defective bacteria were not affected in their infectivity. Complementation of a clumping factor-defective mutant with a copy of the wild-type clumping factor gene restored both its in vitro adherence and its in vivo infectivity. These results show that clumping factor plays a specific role in the pathogenesis of S. aureus endocarditis. Nevertheless, the rate of endocarditis with clumping factor-defective mutants increased with larger inocula, indicating the contribution of additional pathogenic determinants in the infective process.