Up-regulation of both intimin and eae-independent adherence of shiga toxigenic Escherichia coli O157 by ler and phenotypic impact of a naturally occurring ler mutation

Virulence Gene Regulation in Escherichia coli

Escherichia colicauses three types of illnesses in humans: diarrhea, urinary tract infections, and meningitis in newborns. The acquisition of virulence-associated genes and the ability to properly regulate these, often horizontally transferred, loci distinguishes pathogens from the normally harmless commensal E. coli found within the human intestine. This review addresses our current understanding of virulence gene regulation in several important diarrhea-causing pathotypes, including enteropathogenic, enterohemorrhagic,enterotoxigenic, and enteroaggregativeE. coli-EPEC, EHEC, ETEC and EAEC, respectively. The intensely studied regulatory circuitry controlling virulence of uropathogenicE. coli, or UPEC, is also reviewed, as is that of MNEC, a common cause of meningitis in neonates. Specific topics covered include the regulation of initial attachment events necessary for infection, environmental cues affecting virulence gene expression, control of attaching and effacing lesionformation, and control of effector molecule expression and secretion via the type III secretion systems by EPEC and EHEC. How phage control virulence and the expression of the Stx toxins of EHEC, phase variation, quorum sensing, and posttranscriptional regulation of virulence determinants are also addressed. A number of important virulence regulators are described, including the AraC-like molecules PerA of EPEC, CfaR and Rns of ETEC, and AggR of EAEC;the Ler protein of EPEC and EHEC;RfaH of UPEC;and the H-NS molecule that acts to silence gene expression. The regulatory circuitry controlling virulence of these greatly varied E. colipathotypes is complex, but common themes offerinsight into the signals and regulators necessary forE. coli disease progression.

Escherichia coli O157:H7-Clinical aspects and novel treatment approaches

Escherichia coli O157:H7 is a notorious pathogen often contracted by intake of contaminated water or food. Infection with this agent is associated with a broad spectrum of illness ranging from mild diarrhea and hemorrhagic colitis to the potentially fatal hemolytic uremic syndrome (HUS). Treating E. coli O157:H7 infection with antimicrobial agents is associated with an increased risk of severe sequelae such as HUS. The difficulty in treating this bacterium using conventional modalities of antimicrobial agent administration has sparked an interest in investigating new therapeutic approaches to this bacterium. These approaches have included the use of probiotic agents and natural products with variable success rates. In addition, novel modalities and regimen of antimicrobial agent administration have been assessed in an attempt at decreasing their association with aggravating infection outcomes.

Association of nucleotide polymorphisms within the O-antigen gene cluster of Escherichia coli O26, O45, O103, O111, O121, and O145 with serogroups and genetic subtypes

Shiga toxin-producing Escherichia coli (STEC) strains are important food-borne pathogens capable of causing hemolytic-uremic syndrome. STEC O157:H7 strains cause the majority of severe disease in the United States; however, there is a growing concern for the amount and severity of illness attributable to non-O157 STEC. Recently, the Food Safety and Inspection Service (FSIS) published the intent to regulate the presence of STEC belonging to serogroups O26, O45, O103, O111, O121, and O145 in nonintact beef products. To ensure the effective control of these bacteria, sensitive and specific tests for their detection will be needed. In this study, we identified single nucleotide polymorphisms (SNPs) in the O-antigen gene cluster that could be used to detect STEC strains of the above-described serogroups. Using comparative DNA sequence analysis, we identified 22 potentially informative SNPs among 164 STEC and non-STEC strains of the above-described serogroups and designed matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF) assays to test the STEC allele frequencies in an independent panel of bacterial strains. We found at least one SNP that was specific to each serogroup and also differentiated between STEC and non-STEC strains. Differences in the DNA sequence of the O-antigen gene cluster corresponded well with differences in the virulence gene profiles and provided evidence of different lineages for STEC and non-STEC strains. The SNPs discovered in this study can be used to develop tests that will not only accurately identify O26, O45, O103, O111, O121, and O145 strains but also predict whether strains detected in the above-described serogroups contain Shiga toxin-encoding genes.

Escherichia coli O115 forms fewer attaching and effacing lesions in the ovine colon in the presence of E. coli O157:H7

Escherichia coli O115 has been isolated from healthy sheep and was shown to be associated with attaching-effacing (AE) lesions in the large intestine. Following previous observations of interactions between E. coli O157 and O26, the aim of the present study was to assess what influence an O115 AE E. coli (AEEC) would have on E. coli O157 colonisation in vitro and in vivo. We report that E. coli O115- and O157-associated AE lesions were observed on HEp-2 cells and on the mucosa of ligated ovine spiral colon. In single strain inoculum, E. coli O115 associated intimately with HEp-2 cells and the spiral colon in greater numbers than E. coli O157:H7. However, in mixed inoculum studies, the number of E. coli O115 AE lesions was significantly reduced suggesting negative interference by E. coli O157. Use of the ligated colon model in the present work has allowed in vitro observations to be extended and confirmed whilst using a minimum of experimental animals. The findings support a hypothesis that some AEEC can inhibit adhesion of other AEEC in vivo. The mechanisms involved may prove to be of utility in the control of AE pathovars.

Novel fusion protein protects against adherence and toxicity of enterohemorrhagic Escherichia coli O157:H7 in mice

Infection with Escherichia coli (E. coli) O157:H7 may develop into bloody diarrhea, or hemorrhagic uremic syndrome (HUS), which usually causes kidney failure or even death. Considered as the pathogenesis mechanism of E. coli O157:H7 infection, attachment or adhesion that is directly mediated by intimin is the first step of E. coli O157:H7 interaction with its host, and all these serious sequelae are mainly due to Shiga toxins (Stxs) released by E. coli O157:H7. In this study, a novel SSI fusion protein that contains the critical toxin-antigens Stx2B and Stx1B, and the critical adhesion-antigen fragment Int281 was constructed. The protein induced complete immune protection, with both anti-toxin and anti-adhesion effects. The dominant increase in IgG1 and the high level of Th2-typical cytokine (IL-4 and IL-10) expression showed that SSI significantly induced Th2-mediated humoral immune response. In the mouse model, the SSI fusion protein not only elicited neutralizing antibodies against both Stx1 and Stx2 toxins, but also induced a high level of anti-adhesion antibodies. The SSI-immunized mice did not show any pathologic changes. SSI provides evident protection with two-time immunization against a highly lethal dose of E. coli O157:H7.

Development of a multiplex real-time polymerase chain reaction for simultaneous detection of enterohemorrhagic Escherichia coli and enteropathogenic Escherichia coli strains

A multiplex real-time polymerase chain reaction (PCR) was developed for the simultaneous detection of genes encoding intimin (eae) and all variants of Shiga toxins 1 and 2 (stx1 and stx2) in diagnostic samples. The uidA gene encoding a beta-glucuronidase specific for Escherichia coli and Shigella spp. was included in the multiplex PCR assay as an internal amplification control. The multiplex PCR was tested on 30 E. coli reference strains and 174 diagnostic samples already characterized as harboring stx1, stx2, and eae genes. The multiplex PCR correctly detected the genes in all strains examined. No cross reaction was observed with 68 strains representing other gastrointestinal pathogens, normal gastrointestinal flora, or closely related bacteria, reflecting 100% specificity of the assay. The detection limits of the multiplex PCR were 5 genome equivalents for stx2 and 50 genome equivalents for eae and stx1.

Contribution of the Ler- and H-NS-regulated long polar fimbriae of Escherichia coli O157:H7 during binding to tissue-cultured cells

The expression of the long polar fimbriae (LPF) of enterohemorrhagic Escherichia coli (EHEC) O157:H7 is controlled by a tightly regulated process, and, therefore, the role of these fimbriae during binding to epithelial cells has been difficult to establish. We recently found that histone-like nucleoid-structuring protein (H-NS) binds to the regulatory sequence of the E. coli O157:H7 lpf1 operon and "silences" its transcription, while Ler inhibits the action of the H-NS protein and allows lpf1 to be expressed. In the present study, we determined how the deregulated expression of LPF affects binding of EHEC O157:H7 to tissue-cultured cells, correlating the adherence phenotype with lpf1 expression. We tested the adherence properties of EHEC hns mutant and found that this strain adhered 2.8-fold better than the wild type. In contrast, the EHEC ler mutant adhered 2.1-fold less than the wild type. The EHEC hns ler mutant constitutively expressed the lpf genes, and, therefore, we observed that the double mutant adhered 5.6-fold times better than the wild type. Disruption of lpfA in the EHEC hns and hns ler mutants or the addition of anti-LpfA serum caused a reduction in adhesion, demonstrating that the increased adherence was due to the expression of LPF. Immunogold-labeling electron microscopy showed that LPF is present on the surface of EHEC lpfA(+) strains. Furthermore, we showed that EHEC expressing LPF agglutinates red blood cells from different species and that the agglutination was blocked by the addition of anti-LpfA serum. Overall, our data confirmed that expression of LPF is a tightly regulated process and, for the first time, demonstrated that these fimbriae are associated with adherence and hemagglutination phenotypes in EHEC O157:H7.

Ler and H-NS, regulators controlling expression of the long polar fimbriae of Escherichia coli O157:H7

Enterohemorrhagic Escherichia coli (EHEC) serotype O157:H7 colonizes the human intestine and is responsible for diarrheal outbreaks worldwide. Previously we showed that EHEC produces long polar fimbriae (LPF) and that maximum expression is observed during the exponential phase of growth at 37 degrees C and pH 6.5. In this study, we analyzed the roles of several regulators in the expression of LPF using the beta-galactosidase reporter system, and we found that H-NS functions as a transcriptional silencer while Ler functions as an antisilencer of LPF expression. Interestingly, deletion of the hns and ler genes in EHEC caused constitutive expression of the fusion reporter protein. Semiquantitative reverse transcription (RT)-PCR was also used to analyze LPF expression in the EHEC ler or hns mutant strain. The hns mutant exhibited an increase in lpf mRNA expression, while expression in the ler mutant was decreased, compared to that in the wild-type strain. Using primer extension analysis, we identified two potential transcriptional start sites within the regulatory region of lpf and located consensus hexamers of -10 (CAAGAT) and -35 (TTCAAA), which are commonly found in sigma(70)-dependent promoters. Further, we determined whether H-NS and Ler interact directly with the lpf promoter region by using purified His-tagged proteins and electrophoretic mobility shift assays. Our data are the first to show direct binding interactions between the H-NS and Ler proteins within the regulatory sequence of the lpf operon. Based on the electrophoretic mobility shift assay, RT-PCR, primer extension, and beta-galactosidase assay results, we concluded that the E. coli O157:H7 lpf operon possesses a promoter dependent on sigma(70), that H-NS binds to the regulatory sequence of lpfA and "silences" the transcription of lpf, and that Ler binds to the regulatory sequence and inhibits the action of the H-NS protein.

The global regulator Ler is necessary for enteropathogenic Escherichia coli colonization of Caenorhabditis elegans

Enteropathogenic Escherichia coli (EPEC) is an important cause of infant diarrhea in developing countries and is useful for general investigations of the bacterial infection process. However, the study of the molecular pathogenesis of EPEC has been hampered by the lack of genetically tractable, convenient animal models. We have therefore developed the use of the nematode Caenorhabditis elegans as a small animal model of infection for this diarrheal pathogen. We found that nematodes died faster on nematode growth medium in the presence of EPEC pathogens than in the presence of the laboratory control strain MG1655. Increased numbers of pathogens in the gut, determined by standard plate count assays and fluorescence microscopy using green fluorescent protein-expressing bacteria, correlated with killing. Deletion of the gene encoding the global regulator Ler severely reduced the ability of EPEC to colonize the nematode gut and could be complemented by providing the ler gene on a multicopy plasmid in trans. Neither the type III secretion system nor the type IV bundle-forming pilus was required for colonization. Combined, the similarities and distinct differences between EPEC infection of nematodes and that of humans offer a unique opportunity to study several stages of the infection process, namely, attachment, colonization, and persistence, in a genetically tractable, inexpensive, and convenient in vivo system.

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.

Enhanced CXC chemokine responses of human colonic epithelial cells to locus of enterocyte effacement-negative shiga-toxigenic Escherichia coli

There is increasing evidence that by facilitating translocation of Shiga toxin (Stx) across the intestinal epithelium and by transporting bound toxin to remote sites such as the renal endothelium, polymorphonuclear leukocytes (PMNs) play a key role in the pathogenesis of Shiga-toxigenic Escherichia coli (STEC) disease. Plasma levels of PMN-attracting CXC chemokines such as interleukin-8 (IL-8) also appear to correlate in humans with the severity of disease. Thus, the capacity of STEC strains to elicit CXC chemokine responses in intestinal epithelial cells may be a crucial step in pathogenesis. Accordingly, we attempted to determine which STEC factors are responsible for CXC chemokine induction in human colonic epithelial cells. Infection of Hct-8 cells with locus for enterocyte effacement (LEE)-negative STEC strains isolated from patients with severe STEC disease resulted in up-regulation of IL-8, macrophage inflammatory protein 2alpha (MIP-2alpha), MIP-2beta, and ENA-78 mRNA significantly higher and earlier than that elicited by several LEE-positive STEC strains, including the O157:H7 strain EDL933. Similarly, levels of IL-8 protein in LEE-negative STEC-infected Hct-8 culture supernatants were significantly higher than in LEE-positive STEC-infected culture supernatants. The difference in responses could not be attributed to the expression or nonexpression of LEE genes, the presence or absence of an STEC megaplasmid, or differences in O serogroups or in the type or amount of Stx produced. Interestingly, however, several of the LEE-negative STEC strains eliciting the strongest chemokine responses belonged to flagellar serotype H21. Incubation of Hct-8 cells with isolated H21 flagellin elicited IL-8 and MIP-2alpha responses similar to those seen in the presence of the most potent LEE-negative STEC strains. Deletion of the fliC gene, but not the stx(2) gene, largely abolished the capacity of O113:H21 LEE-negative STEC strain 98NK2 to elicit IL-8 and MIP-2alpha responses in Hct-8 cells. Taken together, these data suggest that although Stx is capable of inducing CXC chemokine responses, the elevated responses seen in cells infected with certain STEC strains are largely attributable to the production of flagellin.

StcE, a metalloprotease secreted by Escherichia coli O157:H7, specifically cleaves C1 esterase inhibitor

Escherichia coli O157:H7 causes diarrhoea, haemorrhagic colitis, and the haemolytic uraemic syndrome. We have identified a protein of previously unknown function encoded on the pO157 virulence plasmid of E. coli O157:H7, which is the first described protease that specifically cleaves C1 esterase inhibitor (C1-INH), a member of the serine protease inhibitor family. The protein, named StcE for secreted protease of C1 esterase inhibitor from EHEC (formerly Tagn), cleaves C1-INH to produce (unique) approximately 60-65 kDa fragments. StcE does not digest other serine protease inhibitors, extracellular matrix proteins or universal protease targets. We also observed that StcE causes the aggregation of cultured human T cells but not macrophage-like cells or B cells. Substitution of aspartic acid for glutamic acid at StcE position 435 within the consensus metalloprotease active site ablates its abilities to digest C1-INH and to aggregate T cells. StcE is secreted by the etp type II secretion pathway encoded on pO157, and extracellular StcE levels are positively regulated by the LEE-encoded regulator, Ler. StcE antigen and activity were detected in the faeces of a child with an E. coli O157:H7 infection, demonstrating the expression of StcE during human disease. Cleavage of C1-INH by StcE could plausibly cause localized pro-inflammatory and coagulation responses resulting in tissue damage, intestinal oedema and thrombotic abnormalities.

Immunological characterization of Escherichia coli O157:H7 intimin gamma1

Portions of the intimin genes of Escherichia coli O157:H7 strain E319 and of the enteropathogenic E. coli O127:H6 strain E2348/69 were amplified by PCR and cloned into pET-28a+ expression vectors. The entire 934 amino acids (aa) of E. coli O157:H7 intimin, the C-terminal 306 aa of E. coli O157:H7 intimin, and the C-terminal 311 aa of E. coli O127:H6 intimin were expressed as proteins fused with a six-histidine residue tag (six-His tag) in pET-28a+. Rabbit antisera raised against the six-His tag-full-length E. coli O157:H7 intimin protein fusion cross-reacted in slot and Western blots with outer membrane protein preparations from the majority of enterohemorrhagic and enteropathogenic E. coli serotypes which have the intimin gene. The E. coli strains tested included isolates from humans and animals which produce intimin types alpha (O serogroups 86, 127, and 142), beta1 (O serogroups 5, 26, 46, 69, 111, 126, and 128), gamma 1 (O serogroups 55, 145, and 157), gamma 2 (O serogroups 111 and 103), and epsilon (O serogroup 103) and a nontypeable intimin (O serogroup 80), results based on intimin type-specific PCR assays. Rabbit antisera raised against the E. coli O157:H7 C-terminal fusion protein were much more intimin type-specific than those raised against the full-length intimin fusion protein, but some cross-reaction with other intimin types was also observed for these antisera. In contrast, the monoclonal antibody Intgamma1.C11, raised against the C-terminal E. coli O157 intimin, reacted only with preparations from intimin gamma 1-producing E. coli strains such as E. coli O157:H7.