987P fimbrial gene identification and protein characterization by T7 RNA polymerase-induced transcription and TnphoA mutagenesis

Sef fimbria operon construction, expression, and function for direct rapid detection of Salmonella Enteritidis

Salmonella Enteritidis (SE) causes both horizontal and vertical transmission of diseases in poultry industry and is also one of the main causes of human food poisoning. Sequence analysis of the sef operon of poultry-derived Salmonella serotypes showed the presence of an entire sef operon in SE, whereas only sef pseudogenes were found in Salmonella Gallinarum and Salmonella Pullorum. Subsequently, the sef operon of SE was cloned into the pBR322 plasmid and expressed in a modified Escherichia coli strain SE5000. sef operon expression was demonstrated using sodium dodecyl sulfate-polyacrylamide gel electrophoresis, western blot, agglutination assay, and transmission electron microscopy. The results showed that SE5000+Sef, but not SE5000+pBR322, could specifically react with SE-positive chicken serum in an agglutination assay, which could be clearly visualized by the naked eye within less than 2 min. In contrast, SE5000+Sef could not be recognized in Salmonella Gallinarum- and Salmonella Pullorum-positive chicken sera. Next, taking advantage of the exclusive presence of an entire sef operon in SE, we set up an agglutination-based detection system to monitor the dynamics of Sef-targeted antibody from SE-infected chicks for 47 days. Using the proposed detection method, SE was readily detectable starting from 2 weeks post-infection. Finally, we compared the proposed SE5000+Sef-based detection system with commercially available agglutination antigen using the classical bacterial isolation and identification procedure as reference. The results showed that the SE5000+Sef system was more consistent with the results of bacterial isolation and identification with almost 100% accuracy. We established a simple, sensitive, and cheap agglutination method for rapid and specific detection of SE-infected chickens, which can facilitate epidemiological investigation and eradication of SE infections. KEY POINTS: • Only the Salmonella Enteritidis serotype expressed Sef fimbriae in chicken infected with SE. • A rapid, large-scale method of detection by the naked eye of detection of SE-infected chicken is presented.

Animal Enterotoxigenic Escherichia coli

Enterotoxigenic Escherichia coli (ETEC) is the most common cause of E. coli diarrhea in farm animals. ETEC are characterized by the ability to produce two types of virulence factors: adhesins that promote binding to specific enterocyte receptors for intestinal colonization and enterotoxins responsible for fluid secretion. The best-characterized adhesins are expressed in the context of fimbriae, such as the F4 (also designated K88), F5 (K99), F6 (987P), F17, and F18 fimbriae. Once established in the animal small intestine, ETEC produce enterotoxin(s) that lead to diarrhea. The enterotoxins belong to two major classes: heat-labile toxins that consist of one active and five binding subunits (LT), and heat-stable toxins that are small polypeptides (STa, STb, and EAST1). This review describes the disease and pathogenesis of animal ETEC, the corresponding virulence genes and protein products of these bacteria, their regulation and targets in animal hosts, as well as mechanisms of action. Furthermore, vaccines, inhibitors, probiotics, and the identification of potential new targets by genomics are presented in the context of animal ETEC.

Adhesins of Enterotoxigenic Escherichia coli Strains That Infect Animals

The first described adhesive antigen of Escherichia coli strains isolated from animals was the K88 antigen, expressed by strains from diarrheic pigs. The K88 antigen was visible by electron microscopy as a surface-exposed filament that was thin and flexible and had hemagglutinating properties. Many different fimbriae have been identified in animal enterotoxigenic E. coli (ETEC) and have been discussed in this article. The role of these fimbriae in the pathogenesis of ETEC has been best studied with K88, K99, 987P, and F41. Each fimbrial type carries at least one adhesive moiety that is specific for a certain host receptor, determining host species, age, and tissue specificities. ETEC are the most frequently diagnosed pathogens among neonatal and post-weaning piglets that die of diarrhea. Immune electron microscopy of animal ETEC fimbriae usually shows that the minor subunits are located at the fimbrial tips and at discrete sites along the fimbrial threads. Since fimbriae most frequently act like lectins by binding to the carbohydrate moieties of glycoproteins or glycolipids, fimbrial receptors have frequently been studied with red blood cells of various animal species. Identification and characterization of the binding moieties of ETEC fimbrial adhesins should be useful for the design of new prophylactic or therapeutic strategies. Some studies describing potential receptor or adhesin analogues that interfere with fimbria-mediated colonization have been described in the article.

Porcine intestinal glycosphingolipids recognized by F6-fimbriated enterotoxigenic Escherichia coli

One important virulence factor of enterotoxigenic Escherichia coli is their ability to adhere via fimbrial adhesins to specific receptors located on the intestinal mucosa. Here, the potential glycosphingolipid receptors of enterotoxigenic F6-fimbriated E. coli were examined by binding of purified F6 fimbriae, and F6-expressing bacteria, to glycosphingolipids on thin-layer chromatograms. When intestinal mucosal non-acid glycosphingolipids from single pigs were assayed for F6 binding capacity, a selective interaction with two glycosphingolipids was observed. The binding-active glycosphingolipids were isolated and characterized as lactotriaosylceramide (GlcNAcβ3Galβ4Glcβ1Cer) and lactotetraosylceramide (Galβ3GlcNAcβ3Galβ4Glcβ1Cer). Further binding assays using a panel of reference glycosphingolipids showed a specific interaction between the F6 fimbriae and a number of neolacto core chain (Galβ4GlcNAc) glycosphingolipids. In addition, an occasional binding of the F6 fimbriae to sulfatide, galactosylceramide, lactosylceramide with phytosphingosine and/or hydroxy fatty acids, isoglobotriaosylceramide, gangliotriaosylceramide, and gangliotetraosylceramide was obtained. From the results we conclude that lactotriaosylceramide and lactotetraosylceramide are major porcine intestinal receptors for F6-fimbriated E. coli.

Identification and characterization of assembly proteins of CS5 pili from enterotoxigenic Escherichia coli

This study investigated the role of three genes comprising part of the operon which encodes CS5 pili from enterotoxigenic Escherichia coli. In-frame gene deletions were constructed, and the effects on biogenesis of the pili were examined. A deletion in csfB abolished CsfA major subunit accumulation in the periplasm, which could be restored by trans-complementation with a complete copy of the csfB gene. Localization studies using an antibody against CsfB showed that this protein was periplasmically located, and thus CsfB is likely to function as the specific chaperone for CsfA. An in-frame deletion mutation in the csfE gene resulted in pili approximately three times longer than those of the wild-type strain, thereby indicating a role for CsfE in pilus length regulation. Localization studies using an antibody generated against CsfE showed low-level CsfE accumulation in the outer membranes. Modulation of csfE expression in trans did not reduce the mean length of the pilus below that of the wild type, which indicated that CsfE is not rate-limiting for termination of pilus assembly. Interestingly, a deletion in the csfF gene also resulted in an elongated pilus morphology identical to that of the csfE deletion strain. However, unlike CsfE, CsfF was shown to be rate-limiting for termination of assembly, since overexpression of CsfF in a csfF deletion strain resulted in a significant decrease in the mean length of the pilus compared to that of the wild type. When the same construct was introduced into the wild-type strain, pilus expression was abolished. Since CsfF bears significant homology to the proposed CsfB chaperone, CsfF was predicted to act as the specific chaperone for CsfE. A double deletion in the csfB and csfF genes was shown to abolish the periplasmic accumulation of both CsfA and CsfD pilins, which could be restored individually only when the strain was trans-complemented with a wild-type copy of csfB or csfF, respectively. Therefore, CsfF may chaperone not only CsfE but also CsfD. A model for CS5 biogenesis is also proposed based on these and previous observations.

Lysine residue 117 of the FasG adhesin of enterotoxigenic Escherichia coli is essential for binding of 987P fimbriae to sulfatide

The FasG subunit of the 987P fimbriae of enterotoxigenic strains of Escherichia coli was previously shown to mediate fimbrial binding to a glycoprotein and a sulfatide receptor on intestinal brush borders of piglets. Moreover, the 987P adhesin FasG is required for fimbrial expression, since fasG null mutants are nonfimbriated. In this study, fasG was modified by site-directed mutagenesis to study its sulfatide binding properties. Twenty single mutants were generated by replacing positively charged lysine (K) or arginine (R) residues with small, nonpolar alanine (A) residues. Reduced levels of binding to sulfatide-containing liposomes correlated with reduced fimbriation and FasG surface display in four fasG mutants (R27A, R286A, R226A, and R368). Among the 16 remaining normally fimbriated mutants with wild-type levels of surface-exposed FasG, only one mutant (K117A) did not interact at all with sulfatide-containing liposomes. Four mutants (K117A, R116A, K118A, and R200A) demonstrated reduced binding to such liposomes. Since complete phenotypic dissociation between the structure and specific function of 987P was observed only with mutant K117A, this residue is proposed to play an essential role in the FasG-sulfatide interaction, possibly communicating with the sulfate group of sulfatide by hydrogen bonding and/or salt bridge formation. Residues K17, R116, K118, and R200 may stabilize this interaction.

Polymeric display of immunogenic epitopes from herpes simplex virus and transmissible gastroenteritis virus surface proteins on an enteroadherent fimbria

The strong immunogenicity of bacterial fimbriae results from their polymeric and proteinaceous nature, and the protective role of these immunogens in experimental or commercial vaccines is associated with their capacity to induce antiadhesive antibodies. Fimbria-mediated intestinal colonization by enteropathogens typically leads to similar antibody responses. The possibility of taking advantage of these properties was investigated by determining whether enteroadhesive fimbriae, like the 987P fimbriae of enterotoxigenic Escherichia coli, can serve as carriers for foreign antigens without losing their adhesive characteristics. Random linker insertion mutagenesis of the fasA gene encoding the major 987P subunit identified five different mutants expressing wild-type levels of fimbriation. The linker insertion sites of these mutants were used to introduce three continuous segments of viral surface glycoproteins known to be accessible to antibodies. These segments encode residues 11 to 19 or 272 to 279 of herpes simplex virus type 1 (HSV-1) glycoprotein D [gD(11-19) and gD(272-279), respectively] or residues 379 to 388 of the transmissible gastroenteritis virus (TGEV) spike protein [S(379-388)]. Studies of bacteria expressing fimbriae incorporating mutated FasA subunits alone or together with wild-type FasA subunits (hybrid fimbriae) indicated that foreign epitopes were best exported and displayed on assembled fimbriae when they were inserted near the amino terminus of FasA. Fimbriated bacteria expressing FasA subunits carrying the HSV gD(11-19) or the TGEV S(379-388) epitope inserted between the second and third residues of mature FasA elicited high levels of foreign epitope antibodies in all rabbits immunized parenterally. Antibodies against the HSV epitope were also shown to recognize the epitope in the context of the whole gD protein. Because the 987P adhesive subunit FasG was shown to be present on mutated fimbriae and to mediate bacterial attachment to porcine intestinal receptors, polymeric display of foreign epitopes on 987P offers new opportunities to test the potential beneficial effect of enteroadhesion for mucosal immunization and protection against various enteric pathogens.

Domains within the Vibrio cholerae toxin coregulated pilin subunit that mediate bacterial colonization

Several experimental approaches have provided evidence suggesting that a domain within the C-terminal region of the TcpA pilin, delineated by the single disulfide loop, is directly responsible for the colonization function mediated by the toxin coregulated pilus (TCP) of Vibrio cholerae. This evidence includes the mapping of domains recognized by protective monoclonal antibodies to this region, the ability of peptides from within this region to elicit cholera protective antibody, the construction of tcpA missense mutations that abolish TCP function, and the requirement of a periplasmic disulfide isomerase to produce functional TCP.

Porcine 987P glycolipid receptors on intestinal brush borders and their cognate bacterial ligands

Certain strains of enterotoxigenic Escherichia coli adhere to piglet intestinal epithelial cells by means of the 987P fimbriae. The 987P fimbrial structure consists of a helical arrangement of three fimbrial proteins, namely, the major subunit FasA and two minor subunits, FasF and FasG. FasG, which is located at the fimbrial tip and at various positions along the fimbriae, mediates 987P binding to glycoprotein receptors. In this study, we isolated and analyzed the structure of piglet glycolipid brush border receptors and characterized their cognate ligands on the 987P fimbriae. Two major glycolipid bands recognized by 987P fimbrial probes in thin-layer chromatography overlay assays were further purified by high-performance thin-layer chromatography and shown to comigrate with control galactosylceramide containing hydroxylated fatty acids and with sulfatide. Their structures were confirmed by fast atom bombardment mass spectrometry, which detected homologous series of ceramide monohexoside and sulfatide with hydroxylated fatty acyl chains ranging from h16:0 to h24:0. Assembled 987P fimbriae, pre- and postassembly dissociated fimbrial subunits, and Fab fragments of specific anti-FasG, -FasF, and -FasA were used to inhibit 987P-mediated bacterial binding to the two identified piglet glycolipids and corresponding isoreceptor controls. Only assembled fimbriae and anti-FasG Fab fragments were significantly able to inhibit bacterial binding to sulfatide, indicating that in addition to glycoproteins, FasG recognizes a specific glycolipid of piglet brush borders. In contrast, only anti-FasA Fab fragments were significantly able to inhibit bacterial binding to galactosylceramide with hydroxylated fatty acids and piglet hydroxylated ceramide monohexoside, indicating that FasA may determine a third type of ligand-receptor interaction in the piglet intestines. Since these bacterial adhesins recognize their respective glycolipid receptors only after being assembled in their final fimbrial quaternary structure, adhesin binding may involve cooperative interactions and the subunits by themselves may have very low binding affinities. Alternatively, conformation-sensitive domains of these subunits present in the assembled fimbriae may be required for glycolipid binding.

Identification of major and minor chaperone proteins involved in the export of 987P fimbriae

The 987P fimbriae of Escherichia coli consist mainly of the major subunit, FasA, and two minor subunits, FasF and FasG. In addition to the previously characterized outer membrane or usher protein FasD, the FasB, FasC, and FasE proteins are required for fimbriation. To better understand the roles of these minor proteins, their genes were sequenced and the predicted polypeptides were shown to be most similar to periplasmic chaperone proteins of fimbrial systems. Western blot (immunoblot) analysis and immunoprecipitation of various fas mutants with specific antibody probes identified both the subcellular localizations and associations of these minor components. FasB was shown to be a periplasmic chaperone for the major fimbrial subunit, FasA. A novel periplasmic chaperone, FasC, which stabilizes and specifically interacts with the adhesin, FasG, was identified. FasE, a chaperone-like protein, is also located in the periplasm and is required for optimal export of FasG and possibly other subunits. The use of different chaperone proteins for various 987P subunits is a novel observation for fimbrial biogenesis in bacteria. Whether other fimbrial systems use a similar tactic remains to be discovered.

Ordered translocation of 987P fimbrial subunits through the outer membrane of Escherichia coli

The 987P fimbria of enterotoxigenic Escherichia coli is a heteropolymeric structure which consists essentially of a major FasA subunit and a minor subunit, the FasG adhesin. The latter harbors the binding moiety for receptor molecules on piglet intestinal epithelial cells. In this study, anti-FasF antibody probes were developed and used to demonstrate that the FasF protein represents a new minor fimbrial component. FasF was identified in highly purified fimbriae, and its sequence demonstrated significant levels of similarity with that of FasA. Immune electron microscopy localized both the FasG and FasF proteins at the fimbrial tip as well as at broken ends and at various intervals along the fimbrial length. The presence of these minor proteins in purified 987P fimbriae was corroborated by enzyme-linked immunosorbent assay inhibitions. Finally, the use of nonfimbriated fasG, fasF, and fasA mutants indicated that subunit translocation through the outer membrane follows a specific order, FasG being the first, FasF being the second, and FasA being the third type of exported subunit. Since fimbriae are thought to grow from the base, FasG is proposed to be a tip adhesin and FasF is proposed to be a linker molecule between the adhesin and the fimbrial shaft. Moreover, export of FasG (or FasF) in the absence of FasF (or FasA) indicates that during the process of fimbrial biogenesis in the outer membrane, translocating events precede the initiation of subunit heteropolymerization.

A minor 987P protein different from the structural fimbrial subunit is the adhesin

The 987P fimbriae produced by enterotoxigenic strains of Escherichia coli isolated from piglets mediate bacterial attachment to intestinal epithelial cells. These fimbriae consist essentially of a tight helical arrangement of one structural protein subunit encoded by fasA. Fimbriation and specific adhesion requires the expression of seven additional genes (fasB to fasH). In this study, we investigated whether FasA or another Fas protein, e.g., a potential minor fimbrial component, harbors the binding moiety for the pig 987P receptor glycoproteins. Fas proteins, specifically radiolabeled with an in vivo T7 expression system, were isolated from the periplasm and incubated with receptor-containing brush borders isolated from piglet intestinal epithelial cells. FasG bound best to brush borders, whereas no FasA adhered to them. Additional evidence that FasG, and not FasA, is the 987P adhesin was provided by ligand blotting inhibition assays indicating that FasG alone inhibited fimbrial binding to 987P receptors and that in the absence of FasG, other Fas proteins were not inhibitory. FasG was identified in purified fimbrial preparations with a specific anti-FasG antibody probe. Moreover, FasG was shown to be tightly associated with the fimbrial structure, since it was released only after disassembling fimbriae by heat and sodium dodecyl sulfate treatments. The primary structure of FasG, deduced from the DNA sequence, exhibited 19.1 to 24.4% similarity to FasA and large minor components and/or adhesins of other fimbriae. FasG is the first-described minor fimbrial subunit shown to be essential for both fimbrial biogenesis and specific adhesion.

Permissive linker insertion sites in the outer membrane protein of 987P fimbriae of Escherichia coli

The FasD protein is essential for the biogenesis of 987P fimbriae of Escherichia coli. In this study, subcellular fractionation was used to demonstrate that FasD is an outer membrane protein. In addition, the accessibility of FasD to proteases established the presence of surface-exposed FasD domains on both sides of the outer membrane. The fasD gene was sequenced, and the deduced amino acid sequence was shown to share homologous domains with a family of outer membrane proteins from various fimbrial systems. Similar to porins, fimbrial outer membrane proteins are relatively polar, lack typical hydrophobic membrane-spanning domains, and posses secondary structures predicted to be rich in turns and amphipathic beta-sheets. On the basis of the experimental data and structural predictions, FasD is postulated to consist essentially of surface-exposed turns and loops and membrane-spanning interacting amphipathic beta-strands. In an attempt to test this prediction, the fasD gene was submitted to random in-frame linker insertion mutagenesis. Preliminary experiments demonstrated that it was possible to produce fasD mutants, whose products remain functional for fimbrial export and assembly. Subsequently, 11 fasD alleles, containing linker inserts encoding beta-turn-inducing residues, were shown to express functional proteins. The insertion sites were designated permissive sites. The inserts used are expected to be least detrimental to the function of FasD when they are inserted into surface-exposed domains not directly involved in fimbrial export. In contrast, FasD is not expected to accommodate such residues in its amphipathic beta-strands without being destabilized in the membrane and losing function. All permissive sites were sequenced and shown to be located in or one residue away from predicted turns. In contrast, 5 of 10 sequenced nonpermissive sites were mapped to predicted amphipathic beta-strands. These results are consistent with the structural predictions for FasD.

Identification of bacterial cell-surface virulence determinants with TnphoA

Insertion mutagenesis using TnphoA has proved to be a potent device for the creation of easily screened knockout mutations in genes encoding virulence determinants in a variety of pathogenic bacteria. Initial identification of genes with TnphoA directly initiates more sophisticated genetic and biochemical studies on these factors essential to our understanding of bacterial pathogenesis.

Identification of plasmid and chromosomal copies of 987P pilus genes in enterotoxigenic Escherichia coli 987

A DNA probe derived from the subunit gene of a cloned 987P determinant was used to characterize the locations of the 987P genes in several Escherichia coli strains. We examined E. coli 987, a nonpiliated mutant of strain 987 (I36) that does not express 987P in vitro, and a variant of I36 that expressed 987P following growth in vivo. We found that plasmid and chromosomal copies of the 987P subunit gene could be differentiated in strain 987 by restriction endonuclease analysis and Southern blot hybridization. The nonpiliated mutant I36 had lost the plasmid copy but retained the chromosomal copy of the 987P gene. A 987P-piliated variant of I36, which did not contain the 987P plasmid, colonized and caused diarrhea in neonatal pigs similarly to wild-type 987. The plasmid that hybridized with the 987P probe was transferred from strain 987 to an E. coli K-12 strain by conjugation. We were unable to demonstrate expression of 987P by these transconjugants. The data suggest that the chromosomal and plasmid copies of the 987P genes may interact to influence 987P expression.

Processing of TCP pilin by TcpJ typifies a common step intrinsic to a newly recognized pathway of extracellular protein secretion by gram-negative bacteria

Biogenesis of the Vibrio cholerae toxin-coregulated pilus (TCP) requires the activities of at least seven accessory proteins. We demonstrate that a portion of this pathway involves a novel processing step in which a hydrophilic leader peptide is proteolytically removed from TcpA by the gene product characterized in this report, TcpJ, to yield the mature, export-competent form of the pilin. Cleavage of the pilin leader peptide is independent of known signal peptidases as demonstrated by pilin-processing profiles in Escherichia coli strains conditionally defective for production of leader peptidase or grown in the presence of the antibiotic globomycin. Additionally, pilin cleavage did not rely on the SecA protein, as evidenced by TcpA processing in azide-treated cells. These results suggest that TcpJ is representative of a new class of proteins involved in SecA-independent proteolytic cleavage of a set of atypical leader peptides during extracellular export.