Cloning of the genes for AF/R1 pili from rabbit enteroadherent Escherichia coli RDEC-1 and DNA sequence of the major structural subunit

Evolution of the chaperone/usher assembly pathway: fimbrial classification goes Greek

Many Proteobacteria use the chaperone/usher pathway to assemble proteinaceous filaments on the bacterial surface. These filaments can curl into fimbrial or nonfimbrial surface structures (e.g., a capsule or spore coat). This article reviews the phylogeny of operons belonging to the chaperone/usher assembly class to explore the utility of establishing a scheme for subdividing them into clades of phylogenetically related gene clusters. Based on usher amino acid sequence comparisons, our analysis shows that the chaperone/usher assembly class is subdivided into six major phylogenetic clades, which we have termed alpha-, beta-, gamma-, kappa-, pi-, and sigma-fimbriae. Members of each clade share related operon structures and encode fimbrial subunits with similar protein domains. The proposed classification system offers a simple and convenient method for assigning newly discovered chaperone/usher systems to one of the six major phylogenetic groups.

Contribution of a novel gene, rpeA, encoding a putative autotransporter adhesin to intestinal colonization by rabbit-specific enteropathogenic Escherichia coli

Rabbit-specific enteropathogenic Escherichia coli (REPEC) is an attaching and effacing pathogen of young rabbits. Using signature-tagged mutagenesis, we identified several known colonization factors of REPEC as well as a gene predicted to encode a novel autotransporter protein. This novel gene was termed rpeA for REPEC plasmid-encoded autotransporter.

Towards a vaccine for attaching/effacing Escherichia coli: a LEE encoded regulator (ler) mutant of rabbit enteropathogenic Escherichia coli is attenuated, immunogenic, and protects rabbits from lethal challenge with the wild-type virulent strain

The ler (LEE encoded regulator) gene product is a central regulator for the genes encoded on the locus of enterocyte effacement (LEE) pathogenicity island of attaching/effacing (A/E) pathogens, including human enteropathogenic E. coli (EPEC) and enterohemorrhagic E. coli (EHEC) as well as animal isolates. Although an in vivo role for Ler in bacterial virulence has not been documented, we hypothesized that a Ler deletion mutant should be attenuated for virulence but might retain immunogenicity. The goals of this study were to genetically characterize ler of a rabbit EPEC (rEPEC) strain (O103:H2), to examine the effect of ler on in vivo virulence, and to determine if intragastric inoculation of an attenuated rEPEC ler mutant was immunogenic and could protect rabbits against subsequent challenge with the wild-type virulent parent strain. The predicted ler gene product of rEPEC strain O103:H2 shares high homology (over 95% amino acid identity) with the Lers of another rEPEC strain RDEC-1 (O15:H-) and human EPEC and EHEC. A defined internal ler deletion mutant of rEPEC O103:H2 showed reduced production of secreted proteins. Although orogastric inoculation of rabbits with the virulent parent O103:H2 strain induced severe diarrhea, significant weight loss and early mortality with adherent mucosal bacteria found at sacrifice, the isogeneic ler mutant strain was well tolerated. Animals gained weight and showed no clinical signs of disease. Examination of histological sections of intestinal segments revealed the absence of mucosal bacterial adherence. This result demonstrates an essential role for Ler in in vivo pathogenicity of A/E E. coli. Single dose orogastric immunization with the rEPEC ler mutant induced serum IgG antibody to whole bacteria (but not to intimin). Immunized animals were protected against enteric infection with the WT virulent parent strain exhibiting normal weight gain, absence of diarrhea and absence of mucosally adherent bacteria at sacrifice. Such attenuated ler mutant strains may have potential for use as oral vaccines, or as vaccine vectors for delivery of foreign antigens. It remains to be determined whether such regulatory mutants can protect against infection with A/E bacteria of differing serotypes affecting different hosts.

Predominance of afr2 and ral fimbrial genes related to those encoding the K88 and CS31A fimbrial adhesins in enteropathogenic Escherichia coli isolates from rabbits with postweaning diarrhea in Central Europe

PCR tests designed in these studies identified three rabbit adhesive factor genes among 43 enteropathogenic E. coli (EPEC) strains: afr1 (2 strains), the F4(K88)/CS31A-related afr2 (10 strains), and ral (15 strains). Several EPEC strains (i.e., O153:H7 and O157:H2) lacked these genes but did adhere to HeLa cells and produced attaching and effacing lesions in rabbits.

Contribution of long polar fimbriae to the virulence of rabbit-specific enteropathogenic Escherichia coli

Enteropathogenic Escherichia coli (EPEC) is a major of cause of diarrhea among children in developing countries. Although EPEC is a human specific pathogen, some related strains are natural pathogens of animals, including laboratory-bred rabbits. We have identified two chromosomal loci in rabbit-specific EPEC (REPEC) O15:H- strain 83/39, which are predicted to encode long polar fimbriae (LPF). lpf(R154) was identical to a fimbrial gene cluster, lpf(O113), identified previously in enterohemorrhagic E. coli (EHEC) O113:H21. The second locus, lpf(R141), comprised a novel sequence with five predicted open reading frames, lpfA to lpfE, that encoded long fine fimbriae in nonfimbriated E. coli ORN103. The predicted products of lpf(R141) shared identity with components of the lpfABCC'DE gene cluster from EHEC O157:H7, and the fimbriae were similar in morphology and length to LPF from EHEC O157:H7. Interruption of lpf(R141) resulted in significant attenuation of REPEC 83/39 for rabbits with respect to the early stages of colonization and severity of diarrhea. However, there was no significant difference in the number of bacteria shed at later time points or in overall body weight and mortality rate of rabbits infected with lpf(R141) mutant strains or wild-type REPEC 83/39. Although rabbits infected with the lpf(R141) mutants did not develop severe diarrhea, there was evidence of attaching and effacing histopathology, which was indistinguishable in morphology, location, and extent compared to rabbits infected with wild-type REPEC 83/39. The results suggested that lpf(R141) contributes to the early stages of REPEC-mediated disease and that this is important for the development of severe diarrhea in susceptible animals.

Characterization and evidence of mobilization of the LEE pathogenicity island of rabbit-specific strains of enteropathogenic Escherichia coli

We have characterized the LEE pathogenicity islands (PAIs) of two rabbit-specific strains of enteropathogenic E. coli (REPEC), 83/39 (serotype O15:H-) and 84/110-1 (O103:H2), and have compared them to homologous loci from the human enteropathogenic and enterohaemorrhagic E. coli strains, E2348/69 and EDL933, and another REPEC strain, RDEC-1. All five PAIs contain a 34 kb core region that is highly conserved in gene order and nucleotide sequence. However, the LEE of 83/39 is significantly larger (59 540 basepairs) than those of the human strains, which are less than 44 kb, and has inserted into pheU tRNA. The regions flanking the 34 kb core of 83/39 contain homologues of two putative virulence determinants, efa1/lifA and senA. The LEE of 84/110-1 is approximately 85 kb and is located at pheV tRNA. Its core is almost identical to those of 83/39 and RDEC-1, apart from a larger espF gene, but its flanking regions contain trcA, a putative virulence determinant of EPEC. All three REPEC LEE PAIs contain a gene for an integrase, Int-phe. The LEE PAI of 84/110-1 is also flanked by short direct repeats (representing the 3'-end of pheV tRNA), suggesting that it may be unstable. To investigate this possibility, we constructed a LEE::sacB derivative of 84/110-1 and showed that the PAI was capable of spontaneous deletion. We also showed that Int-phe can mediate site-specific integration of foreign DNA at the pheU tRNA locus of E. coli DH1. Together these results indicate possible mechanisms of mobilization and integration of the LEE PAI.

The transfer region of IncI1 plasmid R64: similarities between R64 tra and legionella icm/dot genes

The entire nucleotide sequence of the transfer region of IncI1 plasmid R64 was determined together with previously reported sequences. Twenty-two transfer genes, traE-Y and nuc, were newly identified in the present study. The protein products of 17 genes were detected by maxicell experiments or by the T7 RNA polymerase expression system. Mutagenesis experiments indicated that 16 genes were indispensable for R64 transfer both in liquid and on surfaces. In summary, the R64 transfer region located within an approximately 54 kb DNA segment was shown to encode the most complex transfer system so far studied. It contains at least 49 genes and may produce 58 different proteins as a result of shufflon DNA rearrangement and overlapping genes. Among the 49 genes, 23 tra, trb and nik genes have been shown to be indispensable for R64 conjugal transfer in liquid and on surfaces. Twelve additional pil genes are required only for liquid matings. The amino acid sequences of 10 R64 tra/trb products share similarity with those of the icm/dot products of Legionella pneumophila that are responsible for its virulence, suggesting that the R64 transfer and L. pneumophila icm/dot systems have evolved from a common ancestral genetic system.

Enterohemorrhagic Escherichia coli O157:H7 produces Tir, which is translocated to the host cell membrane but is not tyrosine phosphorylated

Intimate attachment to the host cell leading to the formation of attaching and effacing (A/E) lesions is an essential feature of enterohemorrhagic Escherichia coli (EHEC) O157:H7 pathogenesis. In a related pathogen, enteropathogenic E. coli (EPEC), this activity is dependent upon translocation of the intimin receptor, Tir, which becomes tyrosine phosphorylated within the host cell membrane. In contrast, the accumulation of tyrosine-phosphorylated proteins beneath adherent EHEC bacteria does not occur, leading to questions about whether EHEC uses a Tir-based mechanism for adherence and A/E lesion formation. In this report, we demonstrate that EHEC produces a functional Tir that is inserted into host cell membranes, where it serves as an intimin receptor. However, unlike in EPEC, in EHEC Tir is not tyrosine phosphorylated yet plays a key role in both bacterial adherence to epithelial cells and pedestal formation. EHEC, but not EPEC, was unable to synthesize Tir in Luria-Bertani medium but was able to secrete Tir into M9 medium, suggesting that Tir synthesis and secretion may be regulated differently in these two pathogens. EHEC Tir and EPEC Tir both bind intimin and focus cytoskeletal rearrangements, indicating that tyrosine phosphorylation is not needed for pedestal formation. EHEC and EPEC intimins are functionally interchangeable, but EHEC Tir shows a much greater affinity for EHEC intimin than for EPEC intimin. These findings highlight some of the differences and similarities between EHEC and EPEC virulence mechanisms, which can be exploited to further define the molecular basis of pedestal formation.

Characterization of the Escherichia coli AF/R1 pilus operon: novel genes necessary for transcriptional regulation and for pilus-mediated adherence

We isolated the genetic determinant of AF/R1 pilus production in attaching/effacing Escherichia coli RDEC-1 and identified seven genes required for pilus expression and function. DNA sequence analysis of the structural subunit gene afrA corrected an error in the published sequence and extended homology with the F18 pilus subunit of pig edema E. coli strains. AfrB and AfrC, encoded downstream from AfrA, were required for pilus expression. AfrB was related to the usher protein PefC of Salmonella typhimurium plasmid-encoded fimbriae, and AfrC was related to PefD, a chaperone protein. AfrD and AfrE, encoded downstream from AfrC, were not necessary for the expression of AF/R1 pili but were required for ileal adherence as assayed by ileal brush border aggregation. Thus, the adhesive subunit of the AF/R1 pilus is distinct from the structural subunit, as is the case for Pap pili and type 1 pili. AfrD was related to FedE of the F18 fimbrial operon of the E. coli strain that causes edema disease in pigs. AfrE was a novel protein. AfrR and AfrS are encoded upstream from AfrA, in the opposite orientation. AfrR is related to the AraC family of transcriptional regulators, and AfrR and AfrS interact to function in a novel mode of transcriptional activation of afrA. AF/R1 pili mediate the adherence to Peyer's patch M cells, ileal mucosa, and colonic mucosa in a rabbit model of diarrhea caused by enteropathogenic E. coli. Our observations will facilitate the further study of the phenomena of M-cell adherence.

Complete DNA sequence and detailed analysis of the Yersinia pestis KIM5 plasmid encoding murine toxin and capsular antigen

Yersinia pestis, the causative agent of plague, harbors at least three plasmids necessary for full virulence of the organism, two of which are species specific. One of the Y. pestis-specific plasmids, pMT1, is thought to promote deep tissue invasion, resulting in more acute onset of symptoms and death. We determined the entire nucleotide sequence of Y. pestis KIM5 pMT1 and identified potential open reading frames (ORFs) encoded by the 100,990-bp molecule. Based on codon usage for known yersinial genes, homology with known proteins in the databases, and potential ribosome binding sites, we determined that 115 of the potential ORFs which we considered could encode polypeptides in Y. pestis. Five of these ORFs were genes previously identified as being necessary for production of the classic virulence factors, murine toxin (MT), and the fraction 1 (F1) capsule antigen. The regions of pMT1 encoding MT and F1 were surrounded by remnants of multiple transposition events and bacteriophage, respectively, suggesting horizontal gene transfer of these virulence factors. We identified seven new potential virulence factors that might interact with the mammalian host or flea vector. Forty-three of the remaining 115 putative ORFs did not display any significant homology with proteins in the current databases. Furthermore, DNA sequence analysis allowed the determination of the putative replication and partitioning regions of pMT1. We identified a single 2,450-bp region within pMT1 that could function as the origin of replication, including a RepA-like protein similar to RepFIB, RepHI1B, and P1 and P7 replicons. Plasmid partitioning function was located ca. 36 kb from the putative origin of replication and was most similar to the parABS bacteriophage P1 and P7 system. Y. pestis pMT1 encoded potential genes with a high degree of similarity to a wide variety of organisms, plasmids, and bacteriophage. Accordingly, our analysis of the pMT1 DNA sequence emphasized the mosaic nature of this large bacterial virulence plasmid and provided implications as to its evolution.

Initial binding of Shiga toxin-producing Escherichia coli to host cells and subsequent induction of actin rearrangements depend on filamentous EspA-containing surface appendages

Shiga toxin-producing Escherichia coli (STEC) induce so-called attaching and effacing lesions that enable the tight adherence of these pathogens to the gut epithelium. All of the genes necessary for this process are present in the locus of enterocyte effacement, which encodes a type III secretion system, the secreted Esp proteins and the surface protein intimin. In this study we sequenced the espA gene of STEC, generated and characterized a corresponding deletion mutant and raised EspA-specific monoclonal antibodies to analyse the functional role of this protein during infection. EspA was detected in often filament-like structures decorating all bacteria that had attached to HeLa cells. These appendages were especially prominent on bacteria that had not yet induced the formation of actin pedestals, indicating that they mediate the initial contact of STEC to their target cells. Consistently, a deletion of the espA gene completely abolished the capacity of such STEC mutants to bind to HeLa cells and to induce actin rearrangements. Surface appendages similar to those described in this study are also formed by Pseudomonas syringae and may represent a structural element common to many bacterial pathogens that deliver proteins into their target cells via a type III secretion system.

Proliferative enterocolitis associated with dual infection with enteropathogenic Escherichia coli and Lawsonia intracellularis in rabbits

Both enteropathogenic Escherichia coli (EPEC) and an obligate intracellular bacterium, previously referred to as an intracellular Campylobacter-like organism and now designated Lawsonia intracellularis, have been reported as causes of enterocolitis in rabbits. An outbreak of enterocolitis in a group of rabbits, characterized by an unusually high rate of mortality, was found to be associated with dual infection with EPEC and L. intracellularis. The EPEC strain was found to have eaeA gene homology but was negative for afrA homology. The absence of the afrA gene, which encodes the structural subunit for the AF/R1 pilus, indicates that this rabbit EPEC strain is distinct from the prototypic RDEC-1 strain. This finding suggests that rabbit EPEC strains widely reported in Western Europe, which lack AF/R1 pili, are also present in rabbits in the United States. Dual infection with these two pathogens in rabbits has not been previously reported and may have contributed to the unusually high mortality observed in this outbreak.

Arac/XylS family of transcriptional regulators

The ArC/XylS family of prokaryotic positive transcriptional regulators includes more than 100 proteins and polypeptides derived from open reading frames translated from DNA sequences. Members of this family are widely distributed and have been found in the gamma subgroup of the proteobacteria, low- and high-G + C-content gram-positive bacteria, and cyanobacteria. These proteins are defined by a profile that can be accessed from PROSITE PS01124. Members of the family are about 300 amino acids long and have three main regulatory functions in common: carbon metabolism, stress response, and pathogenesis. Multiple alignments of the proteins of the family define a conserved stretch of 99 amino acids usually located at the C-terminal region of the regulator and connected to a nonconserved region via a linker. The conserved stretch contains all the elements required to bind DNA target sequences and to activate transcription from cognate promoters. Secondary analysis of the conserved region suggests that it contains two potential alpha-helix-turn-alpha-helix DNA binding motifs. The first, and better-fitting motif is supported by biochemical data, whereas existing biochemical data neither support nor refute the proposal that the second region possesses this structure. The phylogenetic relationship suggests that members of the family have recruited the nonconserved domain(s) into a series of existing domains involved in DNA recognition and transcription stimulation and that this recruited domain governs the role that the regulator carries out. For some regulators, it has been demonstrated that the nonconserved region contains the dimerization domain. For the regulators involved in carbon metabolism, the effector binding determinants are also in this region. Most regulators belonging to the AraC/XylS family recognize multiple binding sites in the regulated promoters. One of the motifs usually overlaps or is adjacent to the -35 region of the cognate promoters. Footprinting assays have suggested that these regulators protect a stretch of up to 20 bp in the target promoters, and multiple alignments of binding sites for a number of regulators have shown that the proteins recognize short motifs within the protected region.

Characterization of two virulence proteins secreted by rabbit enteropathogenic Escherichia coli, EspA and EspB, whose maximal expression is sensitive to host body temperature

Enteropathogenic Escherichia coli (EPEC) and rabbit EPEC (RDEC-1) cause unique histopathological features on intestinal mucosa, including attaching/effacing (A/E) lesions. Due to the human specificity of EPEC, RDEC-1 has been used as an animal model to study EPEC pathogenesis. At least two of the previously identified EPEC-secreted proteins, EspA and EspB, are required for triggering host epithelial signal transduction pathways, intimate adherence, and A/E lesions. However, the functions of these secreted proteins and their roles in pathogenesis have not been characterized. To investigate the function of EspA and EspB in RDEC-1, the espA and espB genes were cloned and their sequences were compared to that of EPEC O127. The EspA proteins showed high similarity (88.5% identity), while EspB was heterogeneous in internal regions (69.8% identity). However, RDEC-1 EspB was identical to that of enterohemorrhagic E. coli serotype O26. Mutations in RDEC-1 espA and espB revealed that the corresponding RDEC-1 gene products are essential for triggering of host signal transduction pathways and invasion into HeLa cells. Complementation with plasmids containing EPEC espA or/and espB genes into RDEC-1 mutant strains demonstrated that they were functionally interchangeable, although the EPEC proteins mediated higher levels of invasion. Furthermore, maximal expression of RDEC-1 and EPEC-secreted proteins occurred at their respective host body temperatures, which may contribute to the lack of EPEC infectivity in rabbits.

Hrp pilus: an hrp-dependent bacterial surface appendage produced by Pseudomonas syringae pv. tomato DC3000

Hypersensitive response and pathogenicity (hrp) genes control the ability of major groups of plant pathogenic bacteria to elicit the hypersensitive response (HR) in resistant plants and to cause disease in susceptible plants. A number of Hrp proteins share significant similarities with components of the type III secretion apparatus and flagellar assembly apparatus in animal pathogenic bacteria. Here we report that Pseudomonas syringae pv. tomato strain DC3000 (race 0) produces a filamentous surface appendage (Hrp pilus) of 6-8 nm in diameter in a solid minimal medium that induces hrp genes. Formation of the Hrp pilus is dependent on at least two hrp genes, hrpS and hrpH (recently renamed hrcC), which are involved in gene regulation and protein secretion, respectively. Our finding of the Hrp pilus, together with recent reports of Salmonella typhimurium surface appendages that are involved in bacterial invasion into the animal cell and of the Agrobacterium tumefaciens virB-dependent pilus that is involved in the transfer of T-DNA into plant cells, suggests that surface appendage formation is a common feature of animal and plant pathogenic bacteria in the infection of eukaryotic cells. Furthermore, we have identified HrpA as a major structural protein of the Hrp pilus. Finally, we show that a nonpolar hrpA mutant of P. syringae pv. tomato DC3000 is unable to form the Hrp pilus or to cause either an HR or disease in plants.

Adhesive factor/rabbit 2, a new fimbrial adhesin and a virulence factor from Escherichia coli O103, a serogroup enteropathogenic for rabbits

Enteropathogenic Escherichia coli-like E. coli strains belonging to serovar O103:K-:H2 and rhamnose-negative biotypes are highly pathogenic diarrhea-inducing strains for weaned European rabbits. We describe here the cloning and sequencing of the major subunit gene of a new fimbrial adhesin, adhesive factor/rabbit 2 (AF/R2), which confers on these strains the ability to attach to rabbit enterocytes and to HeLa cells in a diffuse manner and which is associated with in vivo virulence. The chromosomal operon that encodes functional AF/R2 has been cloned from strain B10. The major subunit gene afr2G, as well as an adjacent open reading frame, afr2H, has been sequenced. The Afr2G protein shows homologies with FaeG and ClpG, which are the respective major subunits of fimbrial adhesin K88 (F4) and afimbrial adhesin CS31A. Plasmid carrying the operon transcomplements an AF/R2-negative TnphoA mutant for its ability to express AF/R2. As a whole, AF/R2 is a new member of the E. coli K88 adhesin family which is associated with virulence and which may serve in the design of vaccines.

Genotypic and phenotypic characterization of Escherichia coli isolates from dogs manifesting attaching and effacing lesions

Thirteen Escherichia coli isolates from dogs manifesting attaching and effacing lesions were characterized genetically with respect to the presence of the following virulence determinants associated with human enteropathogenic E. coli (EPEC): eaeA, encoding the outer membrane protein intimin; eaeB, which is necessary for inducing signal transduction; bfpA, encoding the bundle-forming pilus; and the EAF (stands for EPEC adherence factor) plasmid. These isolates were also analyzed phenotypically with respect to adherence to mammalian cells in vivo and in vitro. Nine of these 13 isolates were found to be eaeA positive by PCR: four of these nine were eaeB positive. The 5' end, but not the 3' end, of the eaeA gene was amplified by PCR when primers derived from the eaeA gene of EPEC were used. Six and eight of these 13 isolates were found to be bfpA positive and EAF positive, respectively. The bfpA gene and EAF locus were found on high-molecular-weight plasmids, whereas the eaeA and eaeB genes were chromosomally located when present. Only one canine E. coli isolate, 4221, which was positive for eaeA, eaeB, bfpA, and EAF, adhered to HEp-2 cells in a localized manner and was positive in the fluorescence actin staining test. The nine eaeA-positive isolates adhered to the mucosal surface of piglet ileal explants and induced some microvillus effacement. However, when tested in experimentally inoculated gnotobiotic piglets, isolate 4221 did not induce attaching and effacing lesions at any level of the intestinal tract. Our results indicate that canine E. coli isolates associated with attaching and effacing lesions share some properties with human EPEC but form a heterogeneous group.

Colonization factors of diarrheagenic E. coli and their intestinal receptors

While Escherichia coli is common as a commensal organism in the distal ileum and colon, the presence of colonization factors (CF) on pathogenic strains of E. coli facilitates attachment of the organism to intestinal receptor molecules in a species- and tissue-specific fashion. After the initial adherence, colonization occurs, and the involvement of additional virulence determinants leads to illness. Enterotoxigenic E. coli (ETEC) is the most extensively studied of the five categories of E. coli that cause diarrheal disease, and has the greatest impact on health worldwide. ETEC can be isolated from domestic animals and humans. The biochemistry, genetics, epidemiology, antigenic characteristics, and cell and receptor binding properties of ETEC have been extensively described. Another major category, enteropathogenic E. coli (EPEC), has virulence mechanisms, primarily effacement and cytoskeletal rearrangement of intestinal brush borders, that are distinct from ETEC. An EPEC CF receptor has been purified and characterized as a sialidated transmembrane glycoprotein complex directly attached to actin, thereby associating CF-binding with host-cell response. Three additional categories of E. coli diarrheal disease, their colonization factors and their host cell receptors, are discussed. It appears that biofilms exist in the intestine in a manner similar to oral bacterial biofilms, and that E. coli is part of these biofilms as both commensals and pathogens.

Activation of multiple antibiotic resistance and binding of stress-inducible promoters by Escherichia coli Rob protein

Multiple antibiotic resistance in Escherichia coli can be mediated by induction of the SoxS or MarA protein, triggered by oxygen radicals (in the soxRS regulon) or certain antibiotics (in the marRAB regulon), respectively. These small proteins (SoxS, 107 residues; MarA, 127 residues) are homologous to the C terminus of the XylS-AraC family of proteins and are more closely related to a approximately 100-residue segment in the N terminus of Rob protein, which binds the right arm of the replication origin, oriC. We investigated whether the SoxS-MarA homology in Rob might extend to the regulation of some of the same inducible genes. Overexpression of Rob indeed conferred multiple antibiotic resistance similar to that known for SoxS and MarA (against chloramphenicol, tetracycline, nalidixic acid, and puromycin), as well as resistance to the superoxide-generating compound phenazine methosulfate. The Rob-induced antibiotic resistance depended only partially on the micF antisense RNA that down-regulates the OmpF outer membrane porin to limit antibiotic uptake. Similar antibiotic resistance was conferred by expression of a Rob fragment containing only the N-terminal 123 residues that constitute the SoxS-MarA homology. Both intact Rob and the N-terminal fragment activated expression of stress genes (inaA, fumC, sodA) but with a pattern distinct from that found for SoxS and MarA. Purified Rob protein bound a DNA fragment containing the micF promoter (50% bound at approximately 10(-9) M Rob) as strongly as it did oriC, and it bound more weakly to DNA containing the sodA, nfo, or zwf promoter (50% bound at 10(-8) to 10(-7) M). Rob formed multiple DNA-protein complexes with these fragments, as seen previously for SoxS. These data point to a DNA-binding gene activator module used in different protein contexts.

Virulence properties and attaching-effacing activity of Escherichia coli O45 from swine postweaning diarrhea

Escherichia coli O45 isolates associated with swine postweaning diarrhea in Québec were characterized with respect to virulence determinants genetically and investigated for their attaching and effacing (A/E) activities by experimental inoculation of gnotobiotic piglets and by the HEp-2 cell adherence assay. All of 32 isolates tested were negative for enterotoxigenic and verotoxigenic E. coli virulence determinants, heat-labile enterotoxin (LT), heat-stable enterotoxins (STap, STb), verotoxins (VT1, VT2), and F4 (K88), F5 (K99), F6 (987P), and F41, except one STb-positive and two F4-positive isolates. A total of 25 isolates hybridized with an EaeA probe, and 11 hybridized with an enteropathogenic E. coli adherence factor (EAF) probe. None of 32 isolates hybridized with a bundle-forming pilus (BFP) probe. The EAF, EaeA, and BFP factors have been associated with human enteropathogenic E. coli strains. A total of 10 of 12 eaeA-positive porcine O45 isolates induced A/E lesions characterized by intimate adherence of bacteria to the intestinal epithelial cell membrane with effacement of the microvilli, similar to those of human attaching-effacing E. coli. However, A/E lesions were not observed in the piglets inoculated with any one of three eaeA-negative O45 isolates. All E. coli O45 isolates were non-adherent to HEp-2 cells. Thus, we have demonstrated the production of typical A/E lesions by nonenterotoxigenic E. coli O45 isolates from swine postweaning diarrhea. The results indicate the significance of the eaeA gene in A/E activities of these isolates and suggest that EAF and BFP are not involved in O45 E. coli infection of weaning piglets.

Adherence characteristics of attaching and effacing strains of Escherichia coli from rabbits

Twelve strains of Escherichia coli previously reported to cause diarrhea in rabbits were examined for properties associated with virulence. Ten strains met the criteria for classification as enteropathogenic E. coli in that they were diarrheagenic strains that evoked attaching-effacing lesions in the small intestine and did not produce detectable enterotoxins or cytotoxins. These bacteria exhibited a variety of patterns when investigated for adherence to HEp-2 epithelial cells. Although several strains displayed localized and/or diffuse adherence to epithelial cells, they did not hybridize with DNA probes that recognize the genes responsible for these phenotypes in diarrheagenic E. coli from humans. The bacteria also varied in their ability to bind to erythrocytes and intestinal brush borders from various animal species. Six strains adhered to rabbit brush borders; two of these also adhered to brush borders from other animals. Two strains that did not adhere to rabbit brush borders adhered to those from guinea pigs or sheep. Only one of the strains investigated carried AF/R1 fimbriae, which are believed to govern the host specificity of this category of diarrheagenic E. coli. This strain was E. coli RDEC-1, which remains the only E. coli strain to date that is known to carry fimbriae of this type. The results indicate that although diarrheagenic E. coli strains from rabbits may have common properties associated with the ability to produce attaching-effacing lesions, they differ from each other and from enteropathogenic E. coli of humans in terms of some of the adhesins that mediate binding to eukaryotic cells.

Identification of eae sequences in enteropathogenic Escherichia coli strains from rabbits

DNA sequences coding for attachment and for verotoxin production were investigated in a collection of enteropathogenic Escherichia coli strains from rabbits. All of the strains produced diarrhea after experimental infection, attached to the brush borders of the intestinal lining, and possessed homology to the eae probe, whereas strains isolated from healthy rabbits did not. Sequences homologous to the AF/R1 fimbriae of strain RDEC-1 were not found. One strain reacted with the probe for the Shiga-like toxin type I gene.

Mucosal immune response to RDEC-1 infection: study of lamina propria antibody-producing cells and biliary antibody

Infection of rabbits with Escherichia coli RDEC-1 is a useful model for diarrheal disease caused by mucosally attaching E. coli. Understanding of the protective immunity induced by RDEC-1 infection in rabbits should provide information useful in the design of vaccines for protection against this infection and other mucosally attaching organisms as well. Thus, to define the time course and location of specific immunoglobulin A secretion in relation to bacterial colonization during primary RDEC-1 infection, we infected rabbits with RDEC-1, which express AF/R1 adherence pili, and compared sites of anti-AF/R1 antibody-containing cells in the intestinal mucosa with the sites of luminal colonization and mucosal attachment of RDEC-1. Also, anti-AF/R1 antibodies in intestinal fluids and bile were measured by enzyme-linked immunosorbent assay, and attachment sites of RDEC-1 to the intestinal epithelium were determined by immunohistochemical examination. Anti-AF/R1 pilus antibody-containing cells were most numerous in the proximal intestine (duodenum and jejunum). In contrast, both luminal colonization and attachment of RDEC-1 to epithelial cells were densest in the distal intestine (cecum and colon). Anti-AF/R1 antibodies were present in approximately equal amounts in fluids collected from all levels of the gut after week 1 postinfection. Anti-AF/R1 antibody levels in undiluted bile exceeded those in gut flushes by at least 2 orders of magnitude. Loss of RDEC-1 attachment to epithelial cells preceded resolution of diarrheal illness despite the presence of large numbers of organisms in the intestinal lumen. Our studies indicate that during RDEC-1 infection (i) sites of greatest mucosal anti-AF/R1 antibody secretion are proximal to sites of maximal RDEC-1 luminal colonization and attachment, (ii) bile is a major source of specific antibodies in the intestinal lumen, and (iii) interference with RDEC-1 attachment to epithelial cells may permit resolution of disease.

A sialoglycoprotein complex linked to the microvillus cytoskeleton acts as a receptor for pilus (AF/R1) mediated adhesion of enteropathogenic Escherichia coli (RDEC-1) in rabbit small intestine

Escherichia coli strain RDEC-1 is an enteroadherent, diarrheagenic pathogen in rabbits that utilizes AF/R1 pili for initial (stage 1) adherence, but the host receptors for this adhesion are unknown. Here we demonstrate that RDEC-1 binds, via AF/R1 pili, to a specific rabbit ileal microvillus membrane glycoprotein receptor complex of subunits 130 and 140 kD. The binding involves sialic acid present on oligosaccharide moieties of the glycoprotein receptor. Furthermore, the microvillus membrane glycoprotein receptor complex appears to be associated with cytoskeletal components via brush border myosin 1. This newly described link between AF/R1 receptor and cytoskeletal components suggests that, in addition to this function in mucosal adherence, the pili may facilitate subsequent (second stage) close effacing attachment of RDEC-1 to the host epithelium by influencing cytoskeletal function.

R plasmid in Escherichia coli O103 coding for colonization of the rabbit intestinal tract

One rabbit pathogenic Escherichia coli strain, belonging to serogroup O103, harbors a self-transferable 117-kb plasmid (pREC-1) encoding resistance to several antibiotics. The role of this R plasmid in the colonization of the digestive tract in specific-pathogen-free (E. coli O103-free) rabbits was studied. Five-week-old rabbits were inoculated with the wild-type strain, with its variant cured of the plasmid, with an E. coli K-12 strain, or with an untypeable E. coli strain from a healthy rabbit. No symptoms and no mortality were observed in animals inoculated with strains without the plasmid pREC-1, but 87.5% of the rabbits infected by the wild strain died, generally with bloody diarrhea, between days 5 and 15 postinfection. The weight gain of animals was strongly reduced. Transfer of the plasmid to the cured strain or to nonvirulent strains led these strains to induce the same pathology but with a lower mortality. Colonization of the gut by the O103 strain and symptoms of bloody diarrhea are thus related to the presence of the pREC-1 plasmid. The GV strain, which does not produce classical heat-labile enterotoxin or heat-stable enterotoxin and is not invasive, could be considered an enteropathogenic E. coli-like strain. The presence of a conjugative plasmid such as pREC-1 encoding both antibiotic resistance and virulence determinants in O103 E. coli from rabbits could represent a prominent epidemiological hazard under selective pressure by antibiotic therapy.