Purification and characterization of Campylobacter rectus surface layer proteins

The Clinical Importance of Campylobacter concisus and Other Human Hosted Campylobacter Species

Historically, Campylobacteriosis has been considered to be zoonotic; the Campylobacter species that cause human acute intestinal disease such as Campylobacter jejuni and Campylobacter coli originate from animals. Over the past decade, studies on human hosted Campylobacter species strongly suggest that Campylobacter concisus plays a role in the development of inflammatory bowel disease (IBD). C. concisus primarily colonizes the human oral cavity and some strains can be translocated to the intestinal tract. Genome analysis of C. concisus strains isolated from saliva samples has identified a bacterial marker that is associated with active Crohn's disease (one major form of IBD). In addition to C. concisus, humans are also colonized by a number of other Campylobacter species, most of which are in the oral cavity. Here we review the most recent advancements on C. concisus and other human hosted Campylobacter species including their clinical relevance, transmission, virulence factors, disease associated genes, interactions with the human immune system and pathogenic mechanisms.

The bacterial surface layer provides protection against antimicrobial peptides

This report describes a previously unrecognized role for bacterial surface layers as barriers that confer protection against antimicrobial peptides. As antimicrobial peptides exist in natural environments, S-layers may provide a bacterial survival mechanism that has been selected for through evolution.

CccS and CccP are involved in construction of cell surface components in the cyanobacterium Synechocystis sp. strain PCC 6803

We have previously identified two target genes (slr1667 and slr1668) for transcriptional regulation by a cAMP receptor protein, SYCRP1, in a cAMP-dependent manner. For this study we investigated the localizations of products of slr1667 and slr1668 (designated cccS and cccP, respectively) biochemically and immunocytochemically, and examined the phenotypes of their disruptants. CccS protein was detected in the culture medium and the acid-soluble fraction containing proteins derived from outside the outer membrane. Disruptants of cccS and cccP showed a more or less similar pleiotropic phenotype. Several proteins secreted into the culture medium or retained on the outside of the outer membrane were greatly reduced in both disruptants compared with the wild type. Electron microscopy revealed that the cccS disruptant lacked the thick pili responsible for motility and that the cccP disruptant had almost no discernible thick pili on its cell surface. Both disruptants largely secreted far greater amounts of yellow pigments into the culture medium than did the wild type. Furthermore, the disruptions reduced the amount of UV-absorbing compound(s) extractable from the exopolysaccharide layer. These results suggest that the cccS and cccP genes are involved in the construction of cell surface components in Synechocystis sp. strain PCC 6803.

Identification of P18, a surface protein produced by the fish pathogen Flavobacterium psychrophilum

AIMS

This study was focused on the identification of associated outer membrane proteins which may play a role in the specific interactions between Flavobacterium psychrophilum (the aetiological agent of cold-water disease and rainbow trout fry syndrome in salmonid fish worldwide) and the fish tissues.

METHODS AND RESULTS

The surface protein interactions with the outer membrane being mainly ionic, different methods were used for the detachment of proteins from the cell surface of Fl. psychrophilum involving detergent-free buffers or solutions known to perturb the ionic interactions. Such treatments led to the isolation of a surface protein, named P18 in accordance with its relative molecular mass. The expression of P18 was not related to the growth conditions (liquid or solid medium, temperature and aeration) or the strains of Fl. psychrophilum tested here.

CONCLUSIONS

Preliminary characterization indicated that P18 is a surface antigen which is not sugar-modified and might be a subunit of a surface layer (i.e. S-layer), one of the most common surface structures on bacteria.

SIGNIFICANCE AND IMPACT OF THE STUDY

Data reported here should be used as the basis for further works involving the purification and characterization of P18 to identify the specific roles of such a surface protein, especially the interaction between this protein and the host surface.

Secretory delivery of recombinant proteins in attenuated Salmonella strains: potential and limitations of Type I protein transporters

Live attenuated Salmonella strains have been extensively explored as oral delivery systems for recombinant vaccine antigens and effector proteins with immunoadjuvant and immunomodulatory potential. The feasibility of this approach was demonstrated in human vaccination trials for various antigens. However, immunization efficiencies with live vaccines are generally significantly lower compared to those monitored in parenteral immunizations with the same vaccine antigen. This is, at least partly, due to the lack of secretory expression systems, enabling large-scale extracellular delivery of vaccine and effector proteins by these strains. Because of their low complexity and the terminal location of the secretion signal in the secreted protein, Type I (ATP-binding cassette) secretion systems appear to be particularly suited for development of such recombinant extracellular expression systems. So far, the Escherichia coli hemolysin system is the only Type I secretion system, which has been adapted to recombinant protein secretion in Salmonella. However, this system has a number of disadvantages, including low secretion capacity, complex genetic regulation, and structural restriction to the secreted protein, which eventually hinder high-level in vivo delivery of recombinant vaccines and effector proteins. Thus, the development of more efficient recombinant protein secretion systems, based on Type I exporters can help to improve efficacies of live recombinant Salmonella vaccines. Type I secretion systems, mediating secretion of bacterial surface layer proteins, such as RsaA in Caulobacter crescentus, are discussed as promising candidates for improved secretory delivery systems.

Campylobacter surface-layers (S-layers) and immune evasion

Many pathogenic bacteria have evolved mechanisms for evading host immune systems. One evasion mechanism is manifest by the surface layer (S-layer), a paracrystalline protein structure composed of S-layer proteins (SLPs). The S-layer, possessed by 2 Campylobacter species (C. fetus and C. rectus), is external to the bacterial outer membrane and can have multiple functions in immune avoidance. C. fetus is a pathogen of ungulates and immunocompromised humans, in whom it causes disseminated bloodstream disease. In C. fetus, the S-layer is required for dissemination and is involved in 2 mechanisms of evasion. First, the S-layer confers resistance to complement-mediated killing in non-immune serum by preventing the binding of complement factor C3b to the C. fetus cell surface. S-layer expressing C. fetus strains remain susceptible to complement-independent killing, utilizing opsonic antibodies directed against the S-layer. However, C. fetus has also evolved a mechanism for avoiding antibody-mediated killing by high-frequency antigenic variation of SLPs. Antigenic variation is accomplished by complex DNA inversion events involving a family of multiple SLP-encoding genes and a single SLP promoter. Inversion events result in the expression of antigenically variant S-layers, which require distinct antibody responses for killing. C. rectus is implicated in the pathogenesis of periodontal disease and also possesses an S-layer that appears to be involved in evading the human system. Although studied less extensively than its C. fetus counterpart, the C. rectus S-layer appears to confer resistance to complement-mediated killing and to cause the down-regulation of proinflammatory cytokines.

Antigenic properties of the GroEL-like protein of Campylobacter rectus

The purpose of this study was to clarify the antigenic properties of the GroEL-like protein of Campylobacter rectus using a specific polyclonal antibody directed to the purified 64-kDa GroEL-like protein (pAb-CrGroEL), a polyclonal antibody directed to the Actinobacillus actinomycetemcomitans GroEL-like protein (pAb-AaGroEL) and a monoclonal antibody against the recombinant human HSP60 (mAb-HuHSP60). In SDS-PAGE/Western immunoblotting analysis, mAb-HuHSP60, pAb-CrGroEL and pAb-AaGroEL were found to react with the GroEL-like protein (64-kDa) present in all C. rectus strains. A 150-kDa protein in C. rectus ATCC 33238 also reacted strongly with pAb-CrGroEL. This 150-kDa protein was found to be present on the surface-associated material of bacterial cells, as determined by transmission electron microscopy and immunogold labelling of cells with pAb-CrGroEL. Analysis of the first 20 N-terminal amino acids of the sequence of the 150-kDa protein revealed a strong homology (80%) with the C. rectus surface layer (S-layer) protein. Investigation of the biochemical nature of antigenic determinants using periodic acid and proteolytic enzymes showed that the C. rectus GroEL-like protein possessed immunodominant epitopes in both peptide and carbohydrate chains, and that the immunoreactive determinants of the 150-kDa protein belonged to carbohydrate. These results suggest that the GroEL-like protein and the S-layer protein of C. rectus may share the same carbohydrate epitopes.

Characterization of surface layer proteins from different Clostridium difficile clinical isolates

In a previous study we suggested that two surface proteins of a Clostridium difficile strain were involved in the formation of a regularly assembled surface layer (S-layer) external to the cell wall. In the present paper six C. difficile strains isolated from cases and healthy carriers were studied. By using freeze-etching and negative staining techniques two superimposed structurally different lattices were detected on the cell surface of the different C. difficile strains. In each strain, the outer S-layer lattice was arranged in a square symmetry and the inner S-layer lattice in hexagonal symmetry. The S-layer proteins from the different strains were isolated and characterized. Each strain showed two distinct S-layer glycoproteins ranging in molecular mass 36-56 kDa. Antigenic cross-reactivity among the S-layer proteins of higher molecular masses extracted from each strain was demonstrated whereas no antigenic relationship was observed among the different S-layer proteins of lower molecular masses. N-terminal sequence analysis showed the presence of common structural motifs conserved among the high S-layer proteins as well as among the low S-layer proteins. These data indicate that the presence of S-layer on C. difficile strains is common and that its glycoprotein subunits show a certain degree of heterogeneity.

Use of defined mutants to assess the role of the Campylobacter rectus S-layer in bacterium-epithelial cell interactions

Campylobacter rectus is a periodontal pathogen with a 150-kDa protein on its cell surface. This protein forms a paracrystalline lattice, called the S-layer, surrounding the outer membrane of this gram-negative bacterium. To initiate a genetic analysis of the possible role of the S-layer in the initial interaction of C. rectus with host epithelial cells, C. rectus strains lacking the S-layer protein gene (crsA) were constructed by allelic exchange mutagenesis. Surprisingly, the lack of the S-layer had only a minor effect on the interaction of C. rectus with HEp-2 epithelial cells; CrsA(+) cells were 30 to 50% more adherent than were CrsA(-) bacteria. Since the host cell expression of cytokines appears to play an important role in the pathogenesis of periodontal diseases, the effect of the S-layer on the epithelial cell cytokine response was also examined by quantitative reverse transcriptase PCR and enzyme-linked immunosorbent assay. Although there were no changes in the mRNA levels for the anti-inflammatory cytokines interleukin-1 receptor agonist (IL-1ra), IL-13, and transforming growth factor beta, the expression and secretion of the proinflammatory cytokines IL-6, IL-8, and tumor necrosis factor alpha (TNF-alpha) were significantly induced by both wild-type C. rectus and CrsA(-) bacteria. Interestingly, the kinetics of cytokine induction differed for the CrsA(+) and CrsA(-) bacteria. At early time points, the HEp-2 cells challenged with CrsA(-) bacteria produced higher levels of IL-6, IL-8, and TNF-alpha mRNA and protein than did cells challenged with CrsA(+) bacteria. We conclude that C. rectus may help initiate periodontitis by increasing the expression of proinflammatory cytokines and that the S-layer may temper this response to facilitate the survival of C. rectus at the site of infection.

The Treponema denticola major sheath protein is predominantly periplasmic and has only limited surface exposure

The recent discovery that the Treponema pallidum genome encodes 12 orthologs of the Treponema denticola major sheath protein (Msp) prompted us to reexamine the cellular location and topology of the T. denticola polypeptide. Experiments initially were conducted to ascertain whether Msp forms an array on or within the T. denticola outer membrane. Transmission electron microscopy (EM) of negatively stained and ultrathin-sectioned organisms failed to identify a typical surface layer, whereas freeze-fracture EM revealed that the T. denticola outer membrane contains heterogeneous transmembrane proteins but no array. In contrast, a lattice-like structure was observed in vesicles released from mildly sonicated treponemes; combined EM and biochemical analyses demonstrated that this structure was the peptidoglycan sacculus. Immunoelectron microscopy (IEM) subsequently was performed to localize Msp in T. denticola. Examination of negatively stained whole mounts identified substantial amounts of Msp in sonicated organisms. IEM of ultrathin-sectioned, intact treponemes also demonstrated that the preponderance of antigen was unassociated with the outer membrane. Lastly, immunofluorescence analysis of treponemes embedded in agarose gel microdroplets revealed that only minor portions of Msp are surface exposed. Taken as a whole, our findings challenge the widely held belief that Msp forms an array within the T. denticola outer membrane and demonstrate, instead, that it is predominantly periplasmic with only limited surface exposure. These findings also have implications for our evolving understanding of the contribution(s) of Msp/Tpr orthologs to treponemal physiology and disease pathogenesis.

Cloning and characterization of two bistructural S-layer-RTX proteins from Campylobacter rectus

Campylobacter rectus is an important periodontal pathogen in humans. A surface-layer (S-layer) protein and a cytotoxic activity have been characterized and are thought to be its major virulence factors. The cytotoxic activity was suggested to be due to a pore-forming protein toxin belonging to the RTX (repeats in the structural toxins) family. In the present work, two closely related genes, csxA and csxB (for C. rectus S-layer and RTX protein) were cloned from C. rectus and characterized. The Csx proteins appear to be bifunctional and possess two structurally different domains. The N-terminal part shows similarity with S-layer protein, especially SapA and SapB of C. fetus and Crs of C. rectus. The C-terminal part comprising most of CsxA and CsxB is a domain with 48 and 59 glycine-rich canonical nonapeptide repeats, respectively, arranged in three blocks. Purified recombinant Csx peptides bind Ca2+. These are characteristic traits of RTX toxin proteins. The S-layer and RTX domains of Csx are separated by a proline-rich stretch of 48 amino acids. All C. rectus isolates studied contained copies of either the csxA or csxB gene or both; csx genes were absent from all other Campylobacter and Helicobacter species examined. Serum of a patient with acute gingivitis showed a strong reaction to recombinant Csx protein on immunoblots.

Surface structure, hydrophobicity, phagocytosis, and adherence to matrix proteins of Bacillus cereus cells with and without the crystalline surface protein layer

Nonopsonic phagocytosis of Bacillus cereus by human polymorphonuclear leukocytes (PMNs) with particular attention to bacterial surface properties and structure was studied. Two reference strains (ATCC 14579(T) and ATCC 4342) and two clinical isolates (OH599 and OH600) from periodontal and endodontic infections were assessed for adherence to matrix proteins, such as type I collagen, fibronectin, laminin, and fibrinogen. One-day-old cultures of strains OH599 and OH600 were readily ingested by PMNs in the absence of opsonins, while cells from 6-day-old cultures were resistant. Both young and old cultures of the reference strains of B. cereus were resistant to PMN ingestion. Preincubation of PMNs with the phagocytosis-resistant strains of B. cereus did not affect the phagocytosis of the sensitive strain. Negatively stained cells of OH599 and OH600 studied by electron microscopy had a crystalline protein layer on the cell surface. In thin-sectioned cells of older cultures (3 to 6 days old), the S-layer was observed to peel off from the cells. No S-layer was detected on the reference strains. Extraction of cells with detergent followed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed a major 97-kDa protein from the strains OH599 and OH600 but only a weak 97-kDa band from the reference strain ATCC 4342. One-day-old cultures of the clinical strains (hydrophobicity, 5.9 to 6.0%) showed strong binding to type I collagen, laminin, and fibronectin. In contrast, reference strains (hydrophobicity, -1.0 to 4.2%) as well as 6-day-old cultures of clinical strains (hydrophobicity, 19.0 to 53.0%) bound in only low numbers to the proteins. Gold-labelled biotinylated fibronectin was localized on the S-layer on the cell surface as well as on fragments of S-layer peeling off the cells of a 6-day-old culture of B. cereus OH599. Lactose, fibronectin, laminin, and antibodies against the S-protein reduced binding to laminin but not to fibronectin. Heating the cells at 84 degreesC totally abolished binding to both proteins. Benzamidine, a noncompetitive serine protease inhibitor, strongly inhibited binding to fibronectin whereas binding to laminin was increased. Overall, the results indicate that changes in the surface structure, evidently involving the S-layer, during growth of the clinical strains of B. cereus cause a shift from susceptibility to PMN ingestion and strong binding to matrix and basement membrane proteins. Furthermore, it seems that binding to laminin is mediated by the S-protein while binding to fibronectin is dependent on active protease evidently attached to the S-layer.

The S-layer protein from Campylobacter rectus: sequence determination and function of the recombinant protein

The gene encoding the crystalline surface layer (S-layer) protein from Campylobacter rectus, designated slp, was sequenced and the recombinant gene product was expressed in Escherichia coli. The gene consisted of 4086 nucleotides encoding a protein with 1361 amino acids. The N-terminal amino acid sequence revealed that Slp did not contain a signal sequence, but that the initial methionine residue was processed. The deduced amino acid sequence displayed some common characteristic features of S-layer proteins previously reported. A homology search showed a high similarity to the Campylobacter fetus S-layer proteins, especially in their N-terminus. The C-terminal third of Slp exhibited homology with the RTX toxins from Gram-negative bacteria via the region including the glycine-rich repeats. The Slp protein had the same N-terminal sequence as a 104-kDa cytotoxin isolated from the culture supernatants of C. rectus. However, neither native nor recombinant Slp showed cytotoxicity against HL-60 cells or human peripheral white blood cells. These data support the idea that the N-terminus acts as an anchor to the cell surface components and that the C-terminus is involved in the assembly and/or transport of the protein.

A new member of the S-layer protein family: characterization of the crs gene from Campylobacter rectus

Strains of the periodontal pathogen Campylobacter rectus express a 150- to 166-kDa protein on their cell surface. This protein forms a paracrystalline lattice, called the surface layer (S-layer), on the outer membrane of this gram-negative bacterium. To initiate a genetic analysis of the function of the S-layer in the pathogenesis of C. rectus, we have cloned and characterized its gene. The S-layer gene (crs) from C. rectus 314 encodes a cell surface protein which does not have a cleaved signal peptide at its amino terminus. Although the amino acid sequence deduced from the crs gene has 50% identity with the amino-terminal 30 amino acids of the four S-layer proteins from Campylobacter fetus, the similarity decreases to less than 16% over the rest of the protein. Thus, the crs gene from C. rectus encodes a novel S-layer protein whose precise role in pathogenesis may differ from that of S-layer proteins from other organisms. Southern and Northern blot analyses with probes from different segments of the crs gene indicate that the S-layer gene is a single-copy, monocistronic gene in C. rectus. RNA end mapping and sequence analyses were used to define the crs promoter; there is an exact match to the Escherichia coli -10 promoter consensus sequence but only a weak match to the -35 consensus element. Southern blots of DNA from another strain of C. rectus, ATCC 33238, demonstrated that the crs gene is also present in that strain but that there are numerous restriction fragment length polymorphisms in the second half of the gene. This finding suggests that the carboxy halves of the S-layer proteins from strains 314 and 33238 differ. It remains to be determined whether the diversities in sequence are reflected in functional or antigenic differences important for the pathogenesis of different C. rectus isolates.