Antigenic differences among Campylobacter fetus S-layer proteins

The Campylobacter fetus S layer provides resistance to photoactivated zinc oxide nanoparticles

The antimicrobial activity of metal-based compounds, including metal oxides, has resulted in numerous agricultural, industrial, and medical applications. Zinc oxide nanoparticles are toxic to Gram-positive and Gram-negative bacteria as well as to some fungi. In this study we assess the sensitivity of Campylobacter fetus, a Gram-negative bacterial pathogen of humans and animals, to ZnO nanoparticles and determine whether the S layer protects C. fetus from the antibacterial action of these nanoparticles. Broth and agar dilution assays revealed that ZnO nanoparticles at 100 μg/mL were bacteriocidal for C. fetus. Resazurin reduction assays confirmed the absence of metabolic activity, indicating that C. fetus cells had not entered into a viable but nonculturable state. Photoactivation of ZnO nanoparticles greatly enhanced their antibacterial activity, as evidenced by minimum bacteriocidal concentration (MBC) values decreasing to 16-62.5 μg/mL as a function of strain. MBC assays completed in the presence and absence of catalase revealed that H₂O₂, a product of ZnO nanoparticle photoactivation, contributed to C. fetus but not to C. jejuni cell death. S-layer-expressing C. fetus strains were more resistant to H₂O₂-mediated cell killing than were isogenic S-layer-deficient strains. These data indicate that C. fetus is sensitive to the antibacterial activity of ZnO nanoparticles and that the C. fetus S layer imparts protection against photoactivated nanoparticles.

S-layers: principles and applications

Monomolecular arrays of protein or glycoprotein subunits forming surface layers (S-layers) are one of the most commonly observed prokaryotic cell envelope components. S-layers are generally the most abundantly expressed proteins, have been observed in species of nearly every taxonomical group of walled bacteria, and represent an almost universal feature of archaeal envelopes. The isoporous lattices completely covering the cell surface provide organisms with various selection advantages including functioning as protective coats, molecular sieves and ion traps, as structures involved in surface recognition and cell adhesion, and as antifouling layers. S-layers are also identified to contribute to virulence when present as a structural component of pathogens. In Archaea, most of which possess S-layers as exclusive wall component, they are involved in determining cell shape and cell division. Studies on structure, chemistry, genetics, assembly, function, and evolutionary relationship of S-layers revealed considerable application potential in (nano)biotechnology, biomimetics, biomedicine, and synthetic biology.

Identification of antigenic epitopes of the SapA protein of Campylobacter fetus using a phage display peptide library

In this study, we immunized mice with prokaryotically expressed recombinant surface layer protein, SapA, of Campylobacter fetus, generated hybridomas secreting mouse monoclonal antibodies (mAb) targeting SapA, and purified the mAb A2D5 from mouse ascites using saturated ammonium sulfate solution. The mAb A2D5, coated onto ELISA plates, was used to screen the phage random 12-peptide library through three rounds of panning. Following panning, 15 phage clones were randomly chosen and tested for reactivity with mAb A2D5 by indirect ELISA. Single-stranded DNA from positive clones was sequenced and compared with the sequence of SapA to predict the key epitope. ELISA and/or Western blot analyses further validated that synthetic peptides and recombinant peptide mimotopes all interact with mAb A2D5. Nine of ten positive phage clones identified by screening were sequenced successfully. Seven clones shared the same sequence HYDRHNYHWWHT; one had the sequence LSKNLPLTALGN; and the final one had the sequence SGMKEPELRSYS. These three sequences shared high homology with SapA J05577 in the region GNEKDFVTKIYSIALGNTSDVDGINYW, in which the underlined amino acids may serve as key residues in the epitope. ELISA and/or Western blot analyses showed that mAb A2D5 not only interacted with the four synthetic peptide mimotopes, but also with 14 prokaryotically expressed recombinant peptide mimotopes. The mimotopes identified in this study will aid future studies into the pathological processes and immune mechanisms of the SapA protein of C. fetus.

So close and yet so far - Molecular Microbiology of Campylobacter fetus subspecies

Campylobacter fetus comprises two subspecies, C. fetus subsp. fetus and C. fetus subsp. venerealis, which are considered emerging pathogens in humans and animals. Comparisons at the genome level have revealed modest subspecies-specific variation; nevertheless, these two subspecies show distinct host and niche preferences. C. fetus subsp. fetus is a commensal and pathogen of domesticated animals that can be transmitted to humans via contaminated food. The clinical features of human infection can be severe, especially in impaired hosts. In contrast, C. fetus subsp. venerealis is a sexually transmitted pathogen essentially restricted to cattle. Infections leading to bovine venereal campylobacteriosis cause substantial economic losses due to abortion and infertility. Recent genome sequencing of the two subspecies has advanced our understanding of C. fetus adaptations through comparative genomics and the identification of subspecies-specific gene regions predicted to be involved in pathogenesis. The most striking difference between the subspecies is the highly subspecies-specific association of a pathogenicity island in the C. fetus subsp. venerealis chromosome. The inserted region encodes a Type 4 secretion system, which contributes to virulence properties of this organism in vitro. This review describes the main differences in epidemiological, phenotypic, and molecular characteristics of the two subspecies and summarizes recent advances towards understanding the molecular mechanisms of C. fetus pathogenesis.

Campylobacter fetus translocation across Caco-2 cell monolayers

Campylobacter fetus is a recognized pathogen of cattle and sheep, though human infection has also been reported. Ingestion of contaminated food or water is a proposed route of transmission for both humans and animals. The subsequent detection of the organism from extra-intestinal and systemic locations implies an ability to translocate across epithelial barriers. To determine how C. fetus disseminates from the intestine, Caco-2 cells cultured on porous membrane supports, were used as model intestinal epithelial cell monolayers. C. fetus was found to translocate equally well in both apical-to-basolateral and basolateral-to-apical directions for up to 24 h without altering Caco-2 cell monolayer permeability as assessed by transepithelial resistance and absence of paracellular diffusion of FITC-inulin. Using modified antibiotic protection assays, C. fetus was also observed to invade and subsequently egress from Caco-2 cells. Caco-2 cell invasion and translocation occurred independently of C. fetus S layer expression. Scanning and transmission electron microscopy revealed the presence of C. fetus associated with both apical and basal surfaces as well as in intracellular locations. C. fetus was, however, never observed in paracellular locations nor associated with Caco-2 cells junctions. Neither C. fetus invasion nor translocation across Caco-2 cell monolayers was impacted by latrunculin A, though translocation was enhanced in the presence of cytochalasin D which disrupted tight junctions. Tubulin cytoskeleton disrupting agents, colchicine and vinblastine, did inhibit C. fetus translocation though entry into Caco-2 cells remained unaffected. Together, translocation without disrupting monolayer integrity, invasion and egression from Caco-2 cells, electron microscopy observations and the requirement of a functional tubulin cytoskeleton for translocation, support a transcellular mechanism of C. fetus translocation across Caco-2 cell monolayers. The ability to invade and subsequently egress would contribute to establishment of an infecting C. fetus population in the host, while the demonstrated ability to translocate across model intestinal epithelial barriers accounts for the observed in vivo recovery of C. fetus from extra-intestinal locations.

Fibronectin enhances Campylobacter fetus interaction with extracellular matrix components and INT 407 cells

Campylobacter fetus is a recognized pathogen of cattle and sheep that can also infect humans. No adhesins specific for C. fetus have to date been identified; however, bacterial attachment is essential to establish an infecting population. Scanning electron microscopy revealed C. fetus attachment to the serosal surface of human colonic biopsy explants, a location consistent with the presence of the extracellular matrix (ECM). To determine whether the ECM mediated C. fetus adherence, 7 C. fetus strains were assessed in a solid-phase binding assay for their ability to bind to immobilized ECM components. Of the ECM components assayed, adherence to fibronectin was noted for all strains. Attachment to ECM components was neither correlated with S-layer expression nor with cell-surface hydrophobicity. Ligand immunoblots, however, identified the S-layer protein as a major site of fibronectin binding, and modified ECM binding assays revealed that soluble fibronectin significantly enhanced the attachment of S-layer-expressing C. fetus strains to other ECM components. Soluble fibronectin also increased C. fetus adherence to INT 407 cells. This adherence was inhibited when INT 407 cells were incubated with synthetic peptides containing an RGD sequence, indicating that integrin receptors were involved in fibronectin-mediated attachment. Together, this data suggests that C. fetus can bind to immobilized fibronectin and use soluble fibronectin to enhance attachment to other ECM components and intestinal epithelial cells. In vivo, fibronectin would promote bacterial adherence, thereby, contributing to the initial interaction of C. fetus with mucosal and submucosal surfaces.

Gram-negative bacterial ATP-binding cassette protein exporter family and diverse secretory proteins

Protein translocation to the extracellular space is essential for the invasion, colonization, and survival of pathogenic gram-negative bacteria within a host organism. In addition to the N-terminal signal sequence-dependent secretion system, which is specific for protein transport to the periplasmic space, there are five major systems (type I, II, III, IV, and V) that are known to be involved in protein secretion into the extracellular space. Of the systems, the type I pathway, which is composed of three membrane components including an ATP-binding cassette (ABC) protein, translocates proteins into the extracellular space from the cytosol by directly using the energy generated from ATP hydrolysis, and therefore, the system is a member of the ABC transporter family and is also known as the ABC exporter. To date, ABC exporters have been discovered to be involved in the secretion of a wide variety of exoproteins including RTX (repeats-in-toxin) toxins, cell surface layer proteins, proteases, lipases, bacteriocins, heme-acquisition proteins, and nodulation-related proteins such as the exoglucanases of gram-negative bacteria. A secretory protein and its associated specific ABC exporter are encoded in the same gene cluster in most cases, and ABC exporters show substrate specificity for secretion. Consequently, ABC exporters are present based primarily on the number of secretory protein genes. A secretion signal is situated in the C-terminal region of secretory proteins, however, the characteristics of the secretion signal are not fully understood. Secretory substrates and their linked ABC exporters are reviewed in the following paper.

Conservation and diversity of sap homologues and their organization among Campylobacter fetus isolates

Campylobacter fetus surface layer proteins (SLPs), encoded by sapA homologues, are important in virulence. In wild-type C. fetus strain 23D, all eight sapA homologues are located in the 54-kb sap island, and SLP expression reflects the position of a unique sapA promoter in relation to the sapA homologues. The extensive homologies in the sap island include both direct and inverted repeats, which allow DNA rearrangements, deletion, or duplication; these elements confer substantial potential for genomic plasticity. To better understand C. fetus sap island diversity and variation mechanisms, we investigated the organization and distribution of sapA homologues among 18 C. fetus strains of different subspecies, serotypes, and origins. For all type A strains, the boundaries of the sap island were relatively consistent. A 187-bp noncoding DNA insertion near the upstream boundary of the sap island was found in two of three reptile strains studied. The sapA homologue profiles were strain specific, and six new sapA homologues were recognized. Several homologues from reptile strains are remarkably conserved in relation to their corresponding mammalian homologues. In total, the observed differences suggest that the sap island has evolved differing genotypes that are plastic, perhaps enabling colonization of varied niches, in addition to antigenic variation.

Structure and genotypic plasticity of the Campylobacter fetus sap locus

The Campylobacter fetus surface layer proteins (SLPs), encoded by five to nine sapA homologues, are major virulence factors. To characterize the sapA homologues further, a 65.9 kb C. fetus genomic region encompassing the sap locus from wild-type strain 23D was completely sequenced and analysed; 44 predicted open reading frames (ORFs) were recognized. The 53.8 kb sap locus contained eight complete and one partial sapA homologues, varying from 2769 to 3879 bp, sharing conserved 553-2622 bp 5' regions, with partial sharing of 5' and 3' non-coding regions. All eight sapA homologues were expressed in Escherichia coli as antigenic proteins and reattached to the surface of SLP- strain 23B, indicating their conserved function. Analysis of the sap homologues indicated three phylogenetic groups. Promoter-specific polymerase chain reactions (PCRs) and sapA homologue-specific reverse transcription (RT)-PCRs showed that the unique sapA promoter can potentially express all eight sapA homologues. Reciprocal DNA recombination based on the 5' conserved regions can involve each of the eight sapA homologues, with frequencies from 10(-1) to 10(-3). Intragenic recombination between sapA7 and sapAp8, mediated by their conserved regions with a 10(-1)-10(-2) frequency, allows the formation of new sap homologues. As divergent SLP C-termini possess multiple antigenic sites, their reciprocal recombination behind the unique sap promoter leads to continuing antigenic variation.

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.

Campylobacter fetus adheres to and enters INT 407 cells

Campylobacter fetus is a Gram-negative bacterial pathogen of humans and ungulates and is normally transmitted via ingestion of contaminated food or water with infection resulting in mild to severe enteritis. However, despite clinical evidence that C. fetus infection often involves transient bacteremic states from which systemic infection may develop and the frequent isolation of C. fetus from extra-intestinal sites, this organism displays very poor invasiveness in in vitro models of infection. In this study, immunofluorescence microscopy and gentamicin protection assays were used to investigate the ability of six clinical isolates and one reference strain of C. fetus to adhere to and invade the human intestinal epithelial cell line, INT 407. During an initial 4-h infection period, all C. fetus strains were detected intracellularly using both techniques, though adherence and internalization levels were very low when determined from gentamicin protection assays. Microscopy results indicated that during a 4-h infection period, four of the five clinical strains tested were adherent to 41.3-87.3% of INT 407 cells observed and that 25.2-34.6% of INT 407 cells contained intracellular C. fetus. The C. fetus reference strain displayed the lowest levels of adherence and internalization. A modified infection assay revealed that C. fetus adherence did not necessarily culminate in internalization. Despite the large percentage of INT 407 cells with adherent bacteria, the percentage of INT 407 cells with intracellular bacteria remained unchanged when incubation was extended from 4 h to 20 h. However, microscopy of INT 407 cells 24 h postinfection (p.i.) revealed that infected host cells contained clusters of densely packed C. fetus cells. Gentamicin protection assays revealed that intracellular C. fetus cells were not only viable 24 h p.i. but also that C. fetus had increased in number approximately three- to fourfold between 4 and 24 h p.i., indicative of intracellular replication. Investigation of the role of the host cell cytoskeleton revealed that pretreatment of host cells with cytochalasin D, colchicine, vinblastine, taxol, or dimethyl sulfoxide (DMSO) did not impact upon C. fetus adherence or internalization of INT 407 cells. Microscopy indicated neither rearrangement nor colocalization of either microtubules or microfilaments in INT 407 cells in response to C. fetus adherence or internalization. Together, these data indicate that clinical isolates of C. fetus are capable of adhering, entering, and surviving within the nonphagocytic epithelial cell line, INT 407.

Campylobacter fetus uses multiple loci for DNA inversion within the 5' conserved regions of sap homologs

Campylobacter fetus cells possess multiple promoterless sap homologs, each capable of expressing a surface layer protein (SLP) by utilizing a unique promoter present on a 6.2-kb invertible element. Each sap homolog includes a 626-bp 5' conserved region (FCR) with 74 bp upstream and 552 bp within the open reading frame. After DNA inversion, the splice is seamless because the FCRs are identical. In mutant strain 23D:ACA2K101, in which sapA and sapA2 flanking the invertible element in opposite orientations were disrupted by promoterless chloramphenicol resistance (Cm(r)) and kanamycin resistance (Km(r)) cassettes, respectively, the frequency of DNA inversion is 100-fold lower than that of wild-type strain 23D. To define the roles of a 15-bp inverted repeat (IR) and a Chi-like site (CLS) in the FCR, we mutagenized each upstream of sapA2 in 23D:ACA2K101 by introducing NotI and KpnI sites to create strains 23D:ACA2K101N and 23D:ACA2K101K, respectively. Alternatively selecting colonies for Cm(r) or Km(r) showed that mutagenizing the IR or CLS had no apparent effect on the frequency of the DNA inversion. However, mapping the unique NotI or KpnI site in relation to the Cm(r) or Km(r) cassette in the cells that changed phenotype showed that splices occurred both upstream and downstream of the mutated sites. PCR and sequence analyses also showed that the splice could occur in the 425-bp portion of the FCR downstream of the cassettes. In total, these data indicate that C. fetus can use multiple sites within the FCR for its sap-related DNA inversion.

Evidence that the Campylobacter fetus sap locus is an ancient genomic constituent with origins before mammals and reptiles diverged

Campylobacter fetus bacteria, isolated from both mammals and reptiles, may be either subsp. fetus or subsp. venerealis and either serotype A or serotype B. Surface layer proteins, expressed and secreted by genes in the sap locus, play an important role in C. fetus virulence. To assess whether the sap locus represents a pathogenicity island and to gain further insights into C. fetus evolution, we examined several C. fetus genes in 18 isolates. All of the isolates had 5 to 9 sapA or sapB homologs. One strain (85-387) possessed both sapA and sapB homologs, suggesting a recombinational event in the sap locus between sapA and sapB strains. When we amplified and analyzed nucleotide sequences from portions of housekeeping gene recA (501 bp) and sapD (450 bp), a part of the 6-kb sap invertible element, the phylogenies of the genes were highly parallel. Among the 15 isolates from mammals, serotype A and serotype B strains generally had consistent positions. The fact that the serotype A C. fetus subsp. fetus and subsp. venerealis strains were on the same branch suggests that their differentiation occurred after the type A-type B split. Isolates from mammals and reptiles formed two distinct tight phylogenetic clusters that were well separated. Sequence analysis of 16S rRNA showed that the reptile strains form a distinct phylotype between mammalian C. fetus and Campylobacter hyointestinalis. The phylogenies and sequence results showing that sapD and recA have similar G + C contents and substitution rates suggest that the sap locus is not a pathogenicity island but rather is an ancient constituent of the C. fetus genome, integral to its biology.

S-layer variation in Bacillus stearothermophilus PV72 is based on DNA rearrangements between the chromosome and the naturally occurring megaplasmids

Bacillus stearothermophilus PV72 expresses different S-layer genes (sbsA and sbsB) under different growth conditions. No stretches of significant sequence identity between sbsA and sbsB were detected. In order to investigate S-layer gene regulation in B. stearothermophilus PV72, we characterized the upstream regulatory region of sbsA and sbsB by sequencing and primer extension analysis. Both genes are transcribed from unique but different promoters, independently of the growth phase. Localization of sbsB in the sbsA-expressing strain PV72/p6 revealed that the coding region of the second S-layer gene sbsB is located not on the chromosome but on a natural megaplasmid of the strain, whereas the upstream regulatory region of sbsB was exclusively detected on the chromosome of PV72/p6. For sbsB expression, the coding region has to be integrated into the chromosomally located expression site. After the switch to sbsB expression, the sbsA coding region was removed from the chromosome but could still be detected on the plasmid of the sbsB-expressing strain PV72/p2. The sbsA upstream regulatory region, however, remained on the chromosome. This is the first report of S-layer variation not caused by intrachromosomal DNA rearrangements, but where variant formation depends on recombinational events between the plasmid and the chromosome.

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.

Campylobacter fetus surface layer proteins are transported by a type I secretion system

The virulence of Campylobacter fetus, a bacterial pathogen of ungulates and humans, is mediated in part by the presence of a paracrystalline surface layer (S-layer) that confers serum resistance. The subunits of the S-layer are S-layer proteins (SLPs) that are secreted in the absence of an N-terminal signal sequence and attach to either type A or B C. fetus lipopolysaccharide in a serospecific manner. Antigenic variation of multiple SLPs (encoded by sapA homologs) of type A strain 23D occurs by inversion of a promoter-containing DNA element flanked by two sapA homologs. Cloning and sequencing of the entire 6.2-kb invertible region from C. fetus 23D revealed a probable 5.6-kb operon of four overlapping genes (sapCDEF, with sizes of 1,035, 1,752, 1,284, and 1,302 bp, respectively) transcribed in the opposite direction from sapA. The four genes also were present in the invertible region of type B strain 84-107 and were virtually identical to their counterparts in the type A strain. Although SapC had no database homologies, SapD, SapE, and SapF had predicted amino acid homologies with type I protein secretion systems (typified by Escherichia coli HlyBD/TolC or Erwinia chrysanthemi PrtDEF) that utilize C-terminal secretion signals to mediate the secretion of hemolysins, leukotoxins, or proteases from other bacterial species. Analysis of the C termini of four C. fetus SLPs revealed conserved structures that are potential secretion signals. A C. fetus sapD mutant neither produced nor secreted SLPs. E. coli expressing C. fetus sapA and sapCDEF secreted SapA, indicating that the sapCDEF genes are sufficient for SLP secretion. C. fetus SLPs therefore are transported to the cell surface by a type I secretion system.

Serratia marcescens S-layer protein is secreted extracellularly via an ATP-binding cassette exporter, the Lip system

The Serratia marcescens Lip exporter belonging to the ATP-binding cassette (ABC) exporter is known to be involved in signal peptide-independent extracellular secretion of a lipase and a metalloprotease. Although the genes of secretory proteins and their ABC exporters are usually all reported to be linked in several gram-negative bacteria, neither the lipase nor the protease gene is located close to the Lip exporter genes, lipBCD. A gene (slaA) located upstream of the lipBCD genes was cloned, revealing that it encodes a polypeptide of 100 kDa and is partially similar to the Caulobacter crescentus paracrystalline cell surface layer (S-layer) protein. The Lip exporter-deficient mutants of S. marcescens failed to secrete the SlaA protein. Electron micrography demonstrated the cell surface layer of S. marcescens. The S-layer protein was secreted to the cultured media in Escherichia coli cells carrying the Lip exporter. Three ABC exporters, Prt, Has and Hly systems, could not allow the S-layer secretion, indicating that the S. marcescens S-layer protein is strictly recognized by the Lip system. This is the first report concerning secretion of an S-layer protein via its own secretion system.

TheCampylobacter fetusS layer is not essential for initial interaction with HEp-2 cells

In vitro adherence assays were used to determine whether the S layer mediated interactions between Campylobacter fetus subsp. venerealis strains and HEp-2 cells. At multiplicity of infection ratios ranging from 0.1:1 through 100:1, quantitation of bacterial adherence by light microscopy revealed that S layer deficient isogenic C. fetus 809K and C. fetus 810K were not less efficient in their attachment to HEp-2 cells; either S layer deficient C. fetus strains interacted with HEp-2 cells in greater numbers than the corresponding wild-type parent strains 809 and 810 or there was no significant difference in adherence levels between wild-type and mutant strains. Adherence of C. fetus strains to HEp-2 cells increased most during the first 2 h of a 22-h incubation period with only a slight increase in C. fetus cell numbers occuring subsequent to 2 h. At each assay point throughout this 22-h time period, equivalent numbers of wild-type and S layer deficient C. fetus strains were observed associated with HEp-2 cells. Prior to 2 h, adherence levels of all C. fetus strains exceeded those of Escherichia coli AB264 and Salmonella typhimurium SL1344. And, unlike S. typhimurium, C. fetus did not undergo significant replication following initial adherence to HEp-2 cells. Campylobacter fetus did not adhere to HEp-2 cells in a localized or aggregative pattern but were randomly distributed over individual HEp-2 cells and at no time during the assay with C. fetus were changes in HEp-2 cell morphology apparent. These data suggest that the S layer is not essential for mediating initial interactions between C. fetus and HEp-2 cells.Key words: Campylobacter fetus, S layer, HEp-2.

Functions of S-layers

Although S-layers are being increasingly identified on Bacteria and Archaea, it is enigmatic that in most cases S-layer function continues to elude us. In a few instances, S-layers have been shown to be virulence factors on pathogens (e.g. Campylobacter fetus ssp. fetus and Aeromonas salmonicida), protective against Bdellovibrio, a depository for surface-exposed enzymes (e.g. Bacillus stearothermophilus), shape-determining agents (e.g. Thermoproteus tenax) and nucleation factors for fine-grain mineral development (e.g. Synechococcus GL 24). Yet, for the vast majority of S-layered bacteria, the natural function of these crystalline arrays continues to be evasive. The following review up-dates the functional basis of S-layers and describes such diverse topics as the effect of S-layers on the Gram stain, bacteriophage adsorption in lactobacilli, phagocytosis by human polymorphonuclear leukocytes, the adhesion of a high-molecular-mass amylase, outer membrane porosity, and the secretion of extracellular enzymes of Thermoanaerobacterium. In addition, the functional aspect of calcium on the Caulobacter S-layer is explained.

Purification and characterization of Campylobacter rectus surface layer proteins

Campylobacter rectus is a putative periodontopathogen which expresses a proteinaceous surface layer (S-layer) external to the outer membrane. S-layers are considered to play a protective role for the microorganism in hostile environments. The S-layer proteins from six different C. rectus strains (five human isolates and a nonhuman primate [NHP] isolate) were isolated, purified, and characterized. The S-layer proteins of these strains varied in molecular mass (ca. 150 to 166 kDa) as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. They all reacted with monospecific rabbit antiserum to the purified S-layer of C. rectus 314, but a quantitative enzyme-linked immunosorbent assay demonstrated a strong antigenic relationship among the five human strains, while the NHP strain, 6250, showed weaker reactivity. Amino acid composition analysis showed that the S-layers of four C. rectus strains contained large proportions of acidic amino acids (13 to 27%) and that >34% of the amino acid residues were hydrophobic. Amino acid sequence analysis of six S-layer proteins revealed that the first 15 amino-terminal amino acids were identical and showed seven residues of identity with the amino-terminal sequence of the Campylobacter fetus S-layer protein SapA1. CNBr peptide profiles of the S-layer proteins from C. rectus 314, ATCC 33238, and 6250 confirmed that the S-layer proteins from the human strains were similar to each other and somewhat different from that of the NHP isolate (strain 6250). However, the S-layer proteins from the two human isolates do show some structural heterogeneity. For example, there was a 17-kDa fragment unique to the C. rectus 314 S-layer. The amino-terminal sequence of this peptide had homology with the C. rectus 51-kDa porin and was composed of nearly 50% hydrophobic residues. Thus, the S-layer protein from C. rectus has structural heterogeneity among different human strains and immunoheterogeneity with the NHP strain.

Dynamics in oxygen-induced changes in S-layer protein synthesis from Bacillus stearothermophilus PV72 and the S-layer-deficient variant T5 in continuous culture and studies of the cell wall composition

Stable synthesis of the hexagonally ordered (p6) S-layer protein from the wild-type strain of Bacillus stearothermophilus PV72 could be achieved in continuous culture on complex medium only under oxygen-limited conditions when glucose was used as the sole carbon source. Depending on the adaptation of the wild-type strain to low oxygen supply, the dynamics in oxygen-induced changes in S-layer protein synthesis was different when the rate of aeration was increased to a level that allowed dissimilation of amino acids. If oxygen supply was increased at the beginning of continuous culture, synthesis of the p6 S-layer protein from the wild-type strain (encoded by the sbsA gene) was immediately stopped and replaced by that of a new type of S-layer protein (encoded by the sbsB gene) which assembled into an oblique (p2) lattice. In cells adapted to a prolonged low oxygen supply, first, low-level p2 S-layer protein synthesis and second, synchronous synthesis of comparable amounts of both types of S-layer proteins could be induced by stepwise increasing the rate of aeration. The time course of changes in S-layer protein synthesis was followed up by immunogold labelling of whole cells. Synthesis of the p2 S-layer protein could also be induced in the p6-deficient variant T5. Hybridization data obtained by applying the radiolabelled N-terminal and C-terminal sbsA fragments and the N-terminal sbsB fragment to the genomic DNA of all the three organisms indicated that changes in S-layer protein synthesis were accompanied by chromosomal rearrangement. Chemical analysis of peptidoglycan-containing sacculi and extraction and recrystallization experiments revealed that at least for the wild-type strain, a cell wall polymer consisting of N-acetylglucosamine and glucose is responsible for binding of the p6 S-layer protein to the rigid cell wall layer.

Southern blotting analyses of strains of Campylobacter fetus using the conserved region of sapA

Chromosomal DNA of 27 strains of Campylobacter fetus was analyzed by Southern blotting with a probe of the conserved region of sapA. The probe hybridized with 23 strains that produced type A lipopolysaccharide. These strains had more than six sapA homologs. In Southern blots of SalI-digested chromosomal DNA separated by pulsed-field gel electrophoresis, one fragment from 19 strains and two fragments from 4 strains hybridized. These data indicate that multiple sapA homologs are localized to a limited region on the chromosomal DNA of C. fetus and thus suggest the possibility of developing a typing system using this method.

Segmental conservation of sapA sequences in type B Campylobacter fetus cells

Campylobacter fetus cells may exist as either of two defined serogroups (type A or B) based on their lipopolysaccharide (LPS) composition. Wild-type strains contain surface array proteins (S-layer proteins) that have partial antigenic cross-reactivity but bind exclusively to LPS from homologous (type A or B) cells. Type A cells possess 8 homologs of sapA, which encodes a 97-kDa S-layer protein; the gene products of these homologs have a conserved N terminus of 184 amino acids. To further explore the structural relationships between the C. fetus S-layer proteins and their encoding genes, we sought to clone and express an S-layer protein from type B strain 84-91. The cloned type B gene (sapB) was similar in structure to the previously cloned type A gene (sapA) and encoded a full-length 936-amino acid (97-kDa) S-layer protein. Sequence analysis of sapB indicated that the conserved N-terminal encoding region in sapA was absent but that the remainder of the ORF (encoding 751 amino acids) was identical to that of sapA in spite of the nonconserved nature of this region among sapA homologs. Noncoding sequences both 300 base pairs 5' and 1000 base pairs 3' to the sapB and sapA ORFs, including the sapA promoter and transcriptional terminator sequences, were essentially identical. Southern analyses revealed that the sapB N-terminal encoding region was conserved in multiple copies in type B strains but was absent in type A strains. Recombinant sapA and sapB products bound to a substantially greater degree to cells of the homologous LPS type compared with the heterologous LPS type, indicating that the conserved sapA- and sapB-encoded N termini are critical for LPS binding specificity. The parallel genetic organization and identity at the nucleotide level in both coding and noncoding regions for sap homologs in types A and B cells indicates the necessity of both homolog conservation and high fidelity DNA replication in the biology of sap diversity.

Analysis of strains of Campylobacter fetus by pulsed-field gel electrophoresis

Campylobacter fetus chromosomal DNA from 21 strains was analyzed by pulsed-field gel electrophoresis. The fingerprint patterns generated with SmaI and SalI were distinctive. Using the profiles obtained by pulsed-field gel electrophoresis, we established the phylogenetic dendrogram of C. fetus to identify the genetic relationship of the strains.

Purification and characterization of the major surface array protein from the avirulent Bacillus anthracis Delta Sterne-1

Many prokaryotic organisms possess surface layer (S-layer) proteins that are components of the outermost cell envelope. With immunogold labeling, it was demonstrated that the protein extractable antigen 1 (EA1) was localized on the outer surface and specifically to cell wall fragments from Bacillus anthracis which retained the S layer. When grown in rich medium under aerobic conditions, the avirulent strain Delta Sterne-1 released large amounts of EA1 into the medium. This EA1 had no higher-order structure initially but formed two-dimensional crystals under defined conditions. The released EA1 was purified in aqueous buffers with a three-step procedure and found to have a mass of 95 kDa when subjected to denaturing sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). N-terminal sequence data revealed exact identity to the first eight residues of the S-layer protein from B. thuringiensis 4045. Gel permeation chromatography of the purified EA1 under nondenaturing conditions revealed a single peak corresponding to a mass of approximately 400 kDa, suggesting that a tetramer or dimer of dimers was the primary species in solution. SDS-PAGE of EA1 purified in the absence of protease inhibitors revealed specific proteolytic processing to an 80-kDa form, which immunoreacted with polyclonal anti-EA1 antibodies. This proteolytic cleavage of EA1 to 80 kDa was duplicated with purified EA1 and the protease trypsin or pronase.

A lipopolysaccharide-binding domain of the Campylobacter fetus S-layer protein resides within the conserved N terminus of a family of silent and divergent homologs

Campylobacter fetus cells can produce multiple S-layer proteins ranging from 97 to 149 kDa, with a single form predominating in cultured cells. We have cloned, sequenced, and expressed in Escherichia coli a sapA homolog, sapA2, which encodes a full-length 1,109-amino-acid (112-kDa) S-layer protein. Comparison with the two previously cloned sapA homologs has demonstrated two regions of identity, approximately 70 bp before the open reading frame (ORF) and proceeding 550 bp into the ORF and immediately downstream of the ORF. The entire genome contains eight copies of each of these conserved regions. Southern analyses has demonstrated that sapA2 existed as a complete copy within the genome in all strains examined, although Northern (RNA) analysis has demonstrated that sapA2 was not expressed in the C. fetus strain from which it was cloned. Further Southern analyses revealed increasing sapA diversity as probes increasingly 3' within the ORF were used. Pulsed-field gel electrophoresis and then Southern blotting with the conserved N-terminal region of the sapA homologs as a probe showed that these genes were tightly clustered on the chromosome. Deletion mutagenesis revealed that the S-layer protein bound serospecifically to the C. fetus lipopolysaccharide via its conserved N-terminal region. These data indicated that the S-layer proteins shared functional activity in the conserved N terminus but diverged in a semiconservative manner for the remainder of the molecule. Variation in S-layer protein expression may involve rearrangement of complete gene copies from a single large locus containing multiple sapA homologs.

Protein shift and antigenic variation in the S-layer of Campylobacter fetus subsp. venerealis during bovine infection accompanied by genomic rearrangement of sapA homologs

Campylobacter fetus subsp. venerealis isolated from a case of human vaginosis was inoculated into the uterus of a C. fetus-negative heifer. Isolates obtained weekly from the vaginal mucus exhibited variations in high-molecular-mass-protein profiles from that of the original inoculum, which had a dominant 110-kDa S-layer protein. Immunoblots of the weekly isolates with monoclonal antibody probes against the 110-kDa S-layer protein and other C. fetus S-layer proteins demonstrated antigenic shifts. Genomic digests of the isolates probed with a 75-mer oligonucleotide of the conserved sapA region also indicated that antigenic variation of the S-layer is accompanied by DNA rearrangement.

Localization of the sapA gene on a physical map of Campylobacter fetus chromosomal DNA

We constructed a physical map of Campylobacter fetus TK(+) chromosomal DNA digested by either SmaI, SalI, or NotI using pulsed-field gel electrophoresis and Southern hybridization data. The genome size of C. fetus TK(+) is 2016kb, larger than that reported by the others. To locate the sapA gene, which encodes the surface array protein (SAP), on the physical map, we performed Southern hybridizations with probes based on the conserved region of the sapA gene. The results showed that more than seven copies of the conserved region were present on C. fetus chromosomal DNA and that the sapA gene was located on a limited number of fragments forming a cluster of genes. By comparing fingerprint patterns of strain TK(+) and strain TK(-), which lost the ability to produce SAP during culture on agar medium, an approximately 10kb deletion was observed in the fragments of strain TK(-). The results of Southern hybridization with two probes, one from the upstream region and the other from the variable region of sapA, suggest that the loss of SAP expression might not be the result of the loss of the sapA gene itself, but only a loss of its control systems.

High-frequency S-layer protein variation in Campylobacter fetus revealed by sapA mutagenesis

Campylobacter fetus utilizes paracrystalline surface (S-) layer proteins that confer complement resistance and that undergo antigenic variation to facilitate persistent mucosal colonization in ungulates. C. fetus possesses multiple homologues of sapA, each of which encode full-length S-layer proteins. Disruption of sapA by a gene targeting method (insertion of kanamycin (km) resistance) caused the loss of C. fetus cells bearing full-length S-layer proteins and their replacement by cells bearing a 50 kDa truncated protein that was not exported to the cell surface. After incubation of the mutants with serum, the survival rate was approximately 2 x 10(-2). Immunoblots of survivors showed that phenotypic reversion involving high-level production of full-length (98, 127 or 149 kDa) S-layer proteins had occurred. Revertants were serum resistant but caused approximately 10-fold less bacteraemia in orally challenged mice than did the wild-type strain. Southern hybridizations of the revertants showed rearrangement of sapA homologues and retention of the km marker. These results indicate that there exists high-frequency generation of C. fetus sapA antigenic variants, and that intracellular mechanisms acting at the level of DNA reciprocal recombination play key roles in this phenomenon.

Isolation and immunogenicity of Campylobacter fetus subsp. fetus from an abdominal aortic aneurysm

A Campylobacter fetus strain was isolated from an abdominal aortic aneurysm in a 56-year-old man who had no significant chronic illness except arteriosclerosis. The strain was identified on the basis of the usual taxonomic markers and fatty acid profiles. The patient's immunological status regarding Campylobacter fetus was studied by Western blot. The strong immune response observed against the 99 kDa protein of the strain confirms that the S-layer protein is the immunodominant antigen of Campylobacter fetus.

Shift in S-layer protein expression responsible for antigenic variation in Campylobacter fetus

Campylobacter fetus strains possess regular paracrystalline surface layers (S-layers) composed of high-molecular-weight proteins and can change the size and crystalline structure of the predominant protein expressed. Polyclonal antisera demonstrate antigenic cross-reactivity among these proteins but suggest differences in epitopes. Monoclonal antibodies to the 97-kDa S-layer protein of Campylobacter fetus subsp. fetus strain 82-40LP showed three different reactivities. Monoclonal antibody 1D1 recognized 97-kDa S-layer proteins from all C. fetus strains studied; reactivity of monoclonal antibody 6E4 was similar except for epitopes in S-layer proteins from reptile strains and strains with type B lipopolysaccharide. Monoclonal antibody 2E11 only recognized epitopes on S-layer proteins from strains with type A lipopolysaccharide regardless of size. In vitro shift from a 97-kDa S-layer protein to a 127-kDa S-layer protein resulted in different reactivity, indicating that size change was accompanied by antigenic variation. To examine in vivo variation, heifers were genetically challenged with Campylobacter fetus subsp. venerealis strains and the S-layer proteins from sequential isolates were characterized. Analysis with monoclonal antibodies showed that antigenic reactivities of the S-layer proteins were varied, indicating that these proteins represent a system for antigenic variation.

Rearrangement of sapA homologs with conserved and variable regions in Campylobacter fetus

The Campylobacter fetus surface-layer (S-layer) proteins mediate both complement resistance and antigenic variation in mammalian hosts. Wild-type strain 23D possesses the sapA gene, which encodes a 97-kDa S-layer protein, and several sapA homologs are present in both wild-type and mutant strains. Here we report that a cloned silent gene (sapA1) in C. fetus can express a functional full-length S-layer protein in Escherichia coli. Analysis of sapA and sapA1 and partial analysis of sapA2 indicate that a block of approximately 600 bp beginning upstream and continuing into the open reading frames is completely conserved, and then the sequences diverge completely, but immediately downstream of each gene is another conserved 50-bp sequence. Conservation of sapA1 among strains, the presence of a putative Chi (RecBCD recognition) site upstream of sapA, sapA1, and sapA2, and the sequence identities of the sapA genes suggest a system for homologous recombination. Comparison of the wild-type strain (23D) with a phenotypic variant (23D-11) indicates that variation is associated with removal of the divergent region of sapA from the expression locus and exchange with a corresponding region from a sapA homolog. We propose that site-specific reciprocal recombination between sapA homologs leads to expression of divergent S-layer proteins as one of the mechanisms that C. fetus uses for antigenic variation.

Characterization of the Campylobacter fetus sapA promoter: evidence that the sapA promoter is deleted in spontaneous mutant strains

Wild-type Campylobacter fetus cells possess S-layer proteins (S+ phenotype), whereas after laboratory passage, spontaneous stable mutants that do not express these proteins (S- phenotype) arise. To determine the molecular mechanisms by which C. fetus changes to the S- phenotype, we studied wild-type strain 23D, from which the sapA gene encoding the 97-kDa S-layer protein has been cloned, and strain 23B, a spontaneous S- mutant. We compared these strains with another pair of strains, LP (S+) and HP (S-). Southern analysis with the cloned sapA gene as a probe indicated that both pairs of strains have multiple sapA homologs. Using gene disruption and replacement techniques, we constructed an isogenic strain of 23D that differed only in sapA expression (strain 23D:401:1). A 6.0-kb HindIII fragment from 23D:401:1 containing 3.4 kb of sapA upstream region then was cloned into pBluescript to produce pBG101. Nucleotide sequence analysis of sapA upstream region revealed a consensus promoter at -121 bp from the translational start site. Primer extension analysis placed a single in vivo transcription initiation site at the -114-bp position of sapA. A DNA probe derived from the sapA promoter region hybridized to a 5.5-kb HindIII fragment of chromosomal DNA from strain 23D but not to DNA from strain 23B. Northern RNA blot analysis showed no sapA mRNA in strain 23B. These data indicate that the lack of S-layer protein expression in spontaneous mutant strains is caused by the deletion of promoter sequences.

Isolation and comparison of the paracrystalline surface layer proteins of freshwater caulobacters

Several methods for isolation of the paracrystalline surface (S) layer protein (RsaA) of Caulobacter crescentus CB15A were evaluated. Treatment of cells with HEPES (N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid) buffer at pH 2 was the most effective means of selectively removing RsaA from cells, and after neutralization, the protein was capable of reassembling into a paracrystalline structure. Ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid treatment could also be used to extract RsaA and yielded protein capable of reassembly. The success of the methods was likely related to disruption of calcium-mediated bonding; calcium was required for recrystallization, while magnesium and strontium ions were ineffective. Antibody was raised against purified RsaA and, along with the S-layer extraction techniques, was used to evaluate 42 strains of caulobacters isolated from a variety of aquatic and wastewater treatment locations. A single characteristic protein could be isolated from the 35 strains that produced an S layer; with one exception, no proteins were extracted from strains that had no S layer. The presumed S-layer proteins ranged in size from 100 to 193 kDa. All of these proteins specifically reacted with anti-RsaA serum by Western immunoblot analysis. In strain CB15A, a specific S-layer-associated oligosaccharide has been proposed to be involved in a calcium-mediated attachment of the S layer to the cell surface. This molecule was detected by Western immunoblotting with a specific antiserum and on polyacrylamide gels stained for polysaccharides. A comparable band was found in all S-layer-producing strains and for most, S-layer-associated oligosaccharide-specific antibody reacted with them in Western analysis. Overall, in freshwater caulobacters at least portions of their S-layer structures appear to be strongly conserved entities, as well as the means of attachment to the cell surface.

Antigenic diversity of the S-layer proteins from pathogenic strains of Aeromonas hydrophila and Aeromonas veronii biotype sobria

The antigenic relatedness of paracrystalline surface array proteins with subunit molecular weights of approximately 52,000 from isolates of Aeromonas hydrophila and Aeromonas veronii biotype sobria belonging to a single heat-stable serogroup was examined. Enzyme-linked immunosorbent assay and immunoblotting with two different polyclonal antisera against surface exposed and non-surface-exposed epitopes of the S-layer protein from A. hydrophila TF7 showed that the S-layer proteins of the mesophilic aeromonads were antigenically diverse. NH2-terminal amino acid sequence analysis of four antigenically different proteins showed that while the proteins were structurally related, they differed in primary sequence. Absorption experiments with heterologous live cells showed that cross-reactive epitopes were in non-surface-exposed regions of the S-layer proteins, while absorption with homologous live cells showed that the immunodominant epitopes of the S-layer protein of strain TF7 were strain specific and exposed on the surface of the native, tetragonal array produced by this strain. Proteolytic digestion of the TF7 S-layer protein with trypsin, chymotrypsin, or endoproteinase Glu-C produced an amino-terminal peptide of approximate Mr 38,000 which was refractile to further proteolytic cleavage under nondenaturing conditions. This peptide carried the immunodominant surface-exposed region of the protein, and chemical cleavage with cyanogen bromide further mapped the portion of these surface-exposed epitopes to a peptide of approximate Mr 26,000, part of which maps within the Mr 38,000 protease-resistant NH2-terminal peptide.

Reattachment of surface array proteins to Campylobacter fetus cells

Campylobacter fetus strains may be of serotype A or B, a property associated with lipopolysaccharide (LPS) structure. Wild-type C. fetus strains contain surface array proteins (S-layer proteins) that may be extracted in water and that are critical for virulence. To explore the relationship of S-layer proteins to other surface components, we reattached S-layer proteins onto S- template cells generated by spontaneous mutation or by serial extractions of S+ cells with water. Reattachment occurred in the presence of divalent (Ba2+, Ca2+, Co2+, and Mg2+) but not monovalent (H+, NH4+, Na+, K+) or trivalent (Fe3+) cations. The 98-, 125-, 127-, and 149-kDa S-layer proteins isolated from strains containing type A LPS (type A S-layer protein) all reattached to S- template cells containing type A LPS (type A cells) but not to type B cells. The 98-kDa type B S-layer protein reattached to SAP- type B cells but not to type A cells. Recombinant 98-kDa type A S-layer protein and its truncated amino-terminal 65- and 50-kDa segments expressed in Escherichia coli retained the full and specific determinants for attachment. S-layer protein and purified homologous but not heterologous LPS in the presence of calcium produced insoluble complexes. By quantitative enzyme-linked immunosorbent assay, the S-layer protein copy number per C. fetus cell was determined to be approximately 10(5). In conclusion, C. fetus cells are encapsulated by a large number of S-layer protein molecules which may be specifically attached through the N-terminal half of the molecule to LPS in the presence of divalent cations.

Freeze-substitution studies of bacteria

Typically, models of bacterial structure combine biochemical data obtained from bulk analyses of cell populations with electron microscopic observation of individual cells. Recent development of a battery of cryotechniques specific for biological electron microscopy have begun to supercede routine procedures such as conventional thin sectioning. One of these cryotechniques, freeze-substitution, combines the advantages of ultrarapid freezing with standard microtomy methods. This technique is particularly well suited to the examination of bacterial structure and has yielded additional ultrastructural information consistent with biochemical data but often challenging models of cell structure obtained from conventional microscopical methods. In addition to retaining more accurately the spatial distribution of cell components, freeze-substitution has been successfully combined with immunochemical labelling techniques and has enabled identification and localization of specific molecules both within the cell and on the cell surface. In this review, I describe current ideas on bacterial ultrastructure, modified in accordance with new data obtained from recent freeze-substitution studies.

Surface layers of bacteria

Since bacteria are so small, microscopy has traditionally been used to study them as individual cells. To this end, electron microscopy has been a most powerful tool for studying bacterial surfaces; the viewing of macromolecular arrangements of some surfaces is now possible. This review compares older conventional electron-microscopic methods with new cryotechniques currently available and the results each has produced. Emphasis is not placed on the methodology but, rather, on the importance of the results in terms of our perception of the makeup and function of bacterial surfaces and their interaction with the surrounding environment.

Correlation between molecular size of the surface array protein and morphology and antigenicity of the Campylobacter fetus S layer

The correlation between the molecular size of the surface layer protein (S protein) and both structure and antigenicity of the Campylobacter fetus surface layer (S layer) was investigated in several clinical strains and their spontaneous variants which produce S proteins of molecular weights (MW) different from those of the parents. Only three molecular sizes of the S proteins were observed (98, 127, and 149 kDa) in the parental and variant strains. Immunologically, the 98-kDa protein and the 149-kDa protein but not the 127-kDa protein were cross-reactive. Freeze-etching analysis showed that the 98-kDa S protein formed a hexagonal arrangement with a 24-nm center-to-center space and that the S proteins with larger MW (127 or 149 kDa) formed tetragonal ones with an 8-nm center-to-center space. Thus, the MW changes of the S proteins seen in the variant strains were associated with both morphological and antigenic changes in S layer. These observations support the hypothesis that the pattern and antigenicity of the C. fetus S layer is determined by the particular type of S protein. Furthermore, the presence of the two different S layer patterns on a single bacterial cell indicates that multiple S proteins can be produced and expressed in a single cell.

Production and purification of heat-stable enterotoxin b from a porcine Escherichia coli strain

Production of heat-stable enterotoxin b (STb) by porcine Escherichia coli strains belonging to serogroup O115 was evaluated in ligated intestinal segments of adult rats. The conditions for optimal production and detection of STb were studied by using the STb-producing strain 4247. As STb production was similar in complex Trypticase soy broth and minimal Davis medium, the latter was used for the fermentation of strain 4247 and the production of STb in large quantities. STb was then purified to apparent homogeneity by sequential ultrafiltration, ultracentrifugation, and preparative gel electrophoresis. The enterotoxin was purified more than 500-fold and exhibited a molecular weight of approximately 5,000 as determined by urea-sodium dodecyl sulfate gel electrophoresis. Purified STb retained such chemical characteristics as resistance to heating (60 degrees C/30 min) and sensitivity to trypsin. A rabbit polyclonal antiserum was produced against the purified toxin. Numerous booster doses were required to obtain a significant enzyme-linked immunosorbent assay titer, suggesting that STb is a poor immunogen. Nevertheless, the antiserum was used successfully to discriminate between culture supernatants of STb-positive and STb-negative O115 E. coli strains, thus demonstrating the immunogenicity of purified STb.