Shiga toxin 2 overexpression in Escherichia coli O157:H7 strains associated with severe human disease

Variation in disease severity among Escherichia coli O157:H7 infections may result from differential expression of Shiga toxin 2 (Stx2). Eleven strains belonging to four prominent phylogenetic clades, including clade 8 strains representative of the 2006 U.S. spinach outbreak, were examined for stx2 expression by real-time PCR and western blot analysis. Clade 8 strains were shown to overexpress stx2 basally, and following induction with ciprofloxacin when compared to strains from clades 1–3. Differences in stx2 expression generally correlated with Stx2 protein levels. Single-nucleotide polymorphisms identified in regions upstream of stx2AB in clade 8 strains were largely absent in non-clade 8 strains. This study concludes that stx2 overexpression is common to strains from clade 8 associated with hemolytic uremic syndrome, and describes SNPs which may affect stx2 expression and which could be useful in the genetic differentiation of highly-virulent strains.     

Structural and functional differences between disease-associated genes of enterohaemorrhagic Escherichia coli O111

We analysed 72 clinical isolates of enterohaemorrhagic Escherichia coli (EHEC) O111 from patients with diarrhoea or haemolytic uraemic syndrome (HUS) isolated during the period from 1955 to 2005 and identified six motile strains (flagellar antigens 8, 10 and 11); the remaining 66 (92%) were nonmotile (NM) and could not be typed by conventional H serotyping. To improve subtyping methodologies, we determined genotypes of the flagellin-encoding fliC. Three fliC genotypes were found which were identical to those of motile EHEC O111 with H antigens 8, 10 and 11 and designated fliC(H8), fliC(H10) and fliC(H11). The IS629 insertion element was present, identically located, in six epidemiologically unrelated isolates with fliC(H8). The prevalence of the fliC genotypes in the 72 EHEC O111 strains were fliC(H8) (89%), fliC(H10) (7%) and fliC(H11) (4%). Within these fliC genotypes, a high degree of homogeneity for the presence of disease-associated genes was found. The adhesins-encoding genes eae and efa-1 were present in all strains with fliC(H8) and fliC(H11), but absent from strains with fliC(H10). The latter strains have not been reported previously. Strains with fliC(H10) and fliC(H11), but not those with fliC(H8), retained intact cadA and cadC loci and decarboxylated lysine. Three different stx genotypes including stx(1), stx(2) and stx(1)/stx(2) were determined among the 72 EHEC O111. We observed a significant increase over time in the frequency of strains harbouring both stx(1) and stx(2). The presence of stx(2) both alone and in combination with stx(1) was significantly (chi(2)=23.16, P<0.00001, CI(95) [2.29; 9.76]) associated with HUS. Therefore, the emergence of EHEC O111 should be monitored carefully. We conclude that EHEC O111 strains can be differentiated using specific loci required for motility, adherence, Stx production, and lysine decarboxylation. The divergence within EHEC O111 makes it possible to subtype these emerging pathogens in the laboratory thereby providing a basis for further investigations into their ecological niches and survival capabilities.     

Depletion of liver and splenic macrophages reduces the lethality of Shiga toxin-2 in a mouse model.

The haemolytic uraemic syndrome (HUS) is a clinical syndrome consisting of haemolytic anaemia, thrombocytopenia, and acute renal insufficiency. HUS is the most frequent cause of acute renal failure in childhood. It has been previously suggested that the presence of Shiga toxin (Stx) is necessary but not sufficient for HUS development, and cytokines such as tumour necrosis factor-alpha (TNF-alpha) and IL-1beta appear to be necessary to develop the syndrome. Since the mononuclear phagocytic system (MPS) is the major source of these cytokines, macrophages might be one of the relevant targets for Stx action in the pathophysiology of HUS. In this study our objective was to examine the role of the hepatic and splenic macrophages in a mouse model of HUS induced by injection of Shiga toxin type-2 (Stx2) or Stx2 plus lipopolysaccharide (LPS). For this purpose, depletion of mice macrophages by liposome-encapsulated clodronate (lip-clod), followed by injection of STx2 or Stx2 plus LPS, was assayed. In this study we show that depletion of hepatic and splenic macrophages by clodronate treatment induces a survival of 50% in animals treated with Stx2 alone or in presence of LPS. This maximal effect was observed when lip-clod was injected 48-72 h before Stx2 injection. Biochemical and histological parameters show characteristics of the lesion produced by Stx2, discarding non-specific damage due to LPS or lip-clod. In addition, we determined that the toxic action of Stx2 is similar in BALB/c and N:NIH nude mice, indicating the T cell compartment is not involved in the Stx2 toxicity. Briefly, we demonstrate that macrophages play a central role in the pathophysiology of HUS, and that the systemic production of cytokines by liver and/or spleen is for Stx2 to manifest its full cytotoxic effect. In addition, the toxicity of Stx2 alone, or in presence of LPS, is independent of the T cell compartment.     

Oral immunization of mice with Lactococcus lactis expressing Shiga toxin truncate confers enhanced protection against Shiga toxins of Escherichia coli O157:H7 and Shigella dysenteriae

Regardless of the communal impact of Shiga toxins, till today neither a specific treatment nor licensed vaccine is available. Lactococcus lactis (L. lactis), generally regarded as safe organism, is well known to provide a valuable approach regarding the oral delivery of vaccines. This study was undertaken to evaluate the protective efficacy of Stx2a1 expressed in nisin‐inducible L. lactis, against Shiga toxins (Stx1, Stx2) in mouse model. Oral immunization of BALB/c mice with LL‐Stx2a1 elicited significant serum antibody titer with elevated fecal and serum IgA, along with minimized intestinal and kidney damage resulting in survival of immunized animals at 84% and 100% when challenged with 10 × LD₅₀ of Escherichia coli O157 and Shigella dysenteriae toxins, respectively. HeLa cells incubated with immune sera and toxin mixture revealed high neutralizing capacity with 90% cell survivability against both the toxins. Mice immunized passively with both toxins and antibody mixture survived the observation period of 15 days, and the controls administered with sham sera and toxins were succumbed to death within 3 days. Our results revealed protective efficacy and toxin neutralization ability of LL‐Stx2a1, proposing it as an oral vaccine candidate against Shiga toxicity mediated by E. coli O157 and S. dysenteriae.     

Intraperitoneal administration of Shiga toxin 2 induced neuronal alterations and reduced the expression levels of aquaporin 1 and aquaporin 4 in rat brain

Shiga toxin-producing Escherichia coli produces watery and hemorrhagic diarrhea, and hemolytic uremic syndrome (HUS) characterized by thrombocytopenia, microangiopathic hemolytic anemia, and acute renal failure. Central nervous system (CNS) complications are observed in around 30% of infant population with HUS. Common signs of severe CNS involvement leading to death include seizures, alteration of consciousness, hemiparesis, visual disturbances, and brain stem symptoms. The purpose of the present work was to study the effects of Shiga toxin 2 (Stx2) in the brain of rats intraperitoneally (i.p.) injected with a supernatant from recombinant E. coli expressing Stx2 (sStx2). Neurological alterations such as postural and motor abnormalities including lethargy, abnormal walking, and paralysis of hind legs, were observed in this experimental model of HUS in rats. Neuronal damage, as well as significant decrease in aquaporin 1 (AQP1) and aquaporin 4 (AQP4) expression levels were observed in the brain of rats, 2 days after sStx2 injection, compared to controls. Downregulation of aquaporin protein levels, and neuronal alterations, observed in brain of rats injected with sStx2, may be involved in edema formation and in neurological manifestations characteristic of HUS.     

Antagonistic effects of probiotic Escherichia coli Nissle 1917 on EHEC strains of serotype O104:H4 and O157:H7

The largest EHEC outbreak up to now in Germany occurred in 2011. It was caused by the non-O157:H7 Shiga-toxinogenic enterohemorrhagic E. coli strain O104:H4. This strain encodes in addition to the Shiga toxin 2 (Stx2), responsible for the hemolytic uremic syndrome (HUS), several adhesins such as aggregative adherence fimbriae. Currently, there is no effective prophylaxis and treatment available for EHEC infections in humans. Especially antibiotics are not indicated for treatment as they may induce Stx production, thus worsening the symptoms. Alternative therapeutics are therefore desperately needed. We tested the probiotic Escherichia coli strain Nissle 1917 (EcN) for antagonistic effects on two O104:H4 EHEC isolates from the 2011 outbreak and on the classical O157:H7 EHEC strain EDL933. These tests included effects on adherence, growth, and Stx production in monoculture and co-culture together with EcN. The inoculum of each co-culture contained EcN and the respective EHEC strain either at a ratio of 1:1 or 10:1 (EcN:EHEC). Adhesion of EHEC strains to Caco-2 cells and to the mucin-producing LS-174T cells was reduced significantly in co-culture with EcN at the 1:1 ratio and very dramatically at the 10:1 ratio. This inhibitory effect of EcN on EHEC adherence was most likely not due to occupation of adhesion sites on the epithelial cells, because in monocultures EcN adhered with much lower bacterial numbers than the EHEC strains. Both EHEC strains of serotype O104:H4 showed reduced growth in the presence of EcN (10:1 ratio). EHEC strain EDL933 grew in co-culture with EcN only during the first 2 h of incubation. Thereafter, EHEC counts declined. At 24 h, the numbers of viable EDL933 was at or slightly below the numbers at the time of inoculation. The amount of Stx2 after 24 h co-incubation with EcN (EcN:EHEC ratio 10:1) was for all 3 EHEC strains tested significantly reduced in comparison to EHEC monocultures.     

Role of tir and intimin in the virulence of rabbit enteropathogenic Escherichia coli serotype O103:H2

Attaching and effacing (A/E) rabbit enteropathogenic Escherichia coli (REPEC) strains belonging to serogroup O103 are an important cause of diarrhea in weaned rabbits. Like human EPEC strains, they possess the locus of enterocyte effacement clustering the genes involved in the formation of the A/E lesions. In addition, pathogenic REPEC O103 strains produce an Esp-dependent but Eae (intimin)-independent alteration of the host cell cytoskeleton characterized by the formation of focal adhesion complexes and the reorganization of the actin cytoskeleton into bundles of stress fibers. To investigate the role of intimin and its translocated coreceptor (Tir) in the pathogenicity of REPEC, we have used a newly constructed isogenic tir null mutant together with a previously described eae null mutant. When human HeLa epithelial cells were infected, the tir mutant was still able to induce the formation of stress fibers as previously reported for the eae null mutant. When the rabbit epithelial cell line RK13 was used, REPEC O103 produced a classical fluorescent actin staining (FAS) effect, whereas both the eae and tir mutants were FAS negative. In a rabbit ligated ileal loop model, neither mutant was able to induce A/E lesions. In contrast to the parental strain, which intimately adhered to the enterocytes and destroyed the brush border microvilli, bacteria of both mutants were clustered in the mucus without reaching and damaging the microvilli. The role of intimin and Tir was then analyzed in vivo by oral inoculation of weaned rabbits. Although both mutants were still present in the intestinal flora of the rabbits 3 weeks after oral inoculation, neither mutant strain induced any clinical signs or significant weight loss in the inoculated rabbits whereas the parental strain caused the death of 90% of the inoculated rabbits. Nevertheless, an inflammatory infiltrate was present in the lamina propria of the rabbits infected with both mutants, with an inflammatory response greater for the eae null mutant. In conclusion, we have confirmed the role of intimin in virulence, and we have shown, for the first time, that Tir is also a key factor in vivo for pathogenicity.     

EHEC O157:H7志贺样毒素Ⅱ(Stx2)的克隆表达与活性研究

目的克隆表达肠出血性大肠杆菌(EHEC)O157：H7志贺样毒素Ⅱ(Shiga like toxinⅡ,Stx2),并鉴定其生物学活性。方法采用PCR法自EHEC O157：H7基因组扩增志贺样毒素Ⅱ的编码基因stx2,T-A克隆后构建原核表达质粒pET-11c(+)-six2,经测序鉴定后转化E．coli BL21(DE3),IPTG诱导表达,以SDS-PAGE、Western blot分析目的蛋白的表达。以EHEC O157：H7野生菌株作对照,通过对HeLa细胞的细胞毒作用及对小鼠攻毒实验,鉴定重组蛋白的生物学活性。结果扩增的stx2基因约1230bp；原核表达质粒pET-11c(+)-stx2经酶切和测序鉴定与预期序列一致；并在E．coli BL21 (DE3)中得到表达,蛋白表达率约25％；PAGE初步测定目的蛋白的相对分子质量(M_r)约72×10~3；重组蛋白Stx2对HeLa细胞具有细胞毒作用,CD_(50)为35pg/ml；Stx2对小鼠致死剂量LD_(100)为10μg。结论成功克隆并表达了Stx2重组蛋白,为探讨O157：H7菌的感染机制及其防治研究奠定了一定的基础。     

Regulatory elements of stx2 gene and the expression level of Shiga-like toxin 2 in Escherichia coli O157:H7

Background/Purpose

Shiga-like toxin (Stx) is an important factor in the pathogenesis of Escherichia coli O157:H7 infection and is responsible for some severe complications. Stx2 is usually associated with hemolytic uremic syndrome in humans. Its expression is regulated by elements located upstream of the stx2 gene, including stx2-promoter sequence, ribosome binding site, and the antiterminator q gene. The present study aimed to find the correlation between regulatory elements and the expression level of Stx2 in two local isolates of E. coli O157:H7.

肠出血性大肠埃希菌O157:H7 TCCP蛋白与宿主细胞IRTKS蛋白的相互作用

目的通过体外结合实验验证EHEC O157:H7内膜素受体偶联细胞骨架蛋白重复片段与人肠上皮细胞IRTKS蛋白SH3结构域之间的相互作用,并制备二者蛋白复合体,为利用晶体学方法研究其相互作用奠定基础。方法将IRTKS蛋白的SH3结构域cDNA序列克隆至pET30a表达质粒,转化BL21(DE3),诱导表达纯化SH3(IRTKS)蛋白;制备TCCP重复片段TC-CP(3R)蛋白;将SH3(IRTKS)与TCCP(3R)按照不同比例混合后,非变性丙烯酰胺凝胶电泳鉴定两者相互结合后产物;利用分子筛层析收集标准蛋白、TCCP(3R)、SH3(IRTKS)和TCCP(3R)-SH3(IRTKS)复合体的出峰体积,回归计算复合物的分子量,分析蛋白相互结合的比例。结果成功克隆表达了SH3(IRTKS)重组质粒;并制备了高纯度的SH3(IRTKS)和TCCP(3R)蛋白;非变性丙烯酰胺凝胶电泳结果证实TCCP(3R)与SH3(IRTKS)发生相互作用形成了蛋白质复合体;复合体的分子量约为33 000,二者相互结合的比例约为1:1。结论本研究为TCCP重复片段与SH3(IRTKS)相互作用提供了直接证据,同时复合体的制备,为利用晶体学研究两者的相互作用奠定了基础。     

Antimicrobial resistance of Shiga toxin (verotoxin)-producing Escherichia coli O157:H7 and non-O157 strains isolated from humans, cattle, sheep and food in Spain

A total of 722 Shiga toxin-producing Escherichia coli (STEC) isolates recovered from humans, cattle, ovines and food during the period from 1992 to 1999 in Spain were examined to determine antimicrobial resistance profiles and their association with serotypes, phage types and virulence genes. Fifty-eight (41%) out of 141 STEC O157:H7 strains and 240 (41%) out of 581 non-O157 STEC strains showed resistance to at least one of the 26 antimicrobial agents tested. STEC O157:H7 showed a higher percentage of resistant strains recovered from bovine (53%) and beef meat (57%) than from human (23%) and ovine (20%) sources, whereas the highest prevalence of antimicrobial resistance in non-O157 STEC was found among isolates recovered from beef meat (55%) and human patients (47%). Sulfisoxazole (36%) had the most common antimicrobial resistance, followed by tetracycline (32%), streptomycin (29%), ampicillin (10%), trimethoprim (8%), cotrimoxazole (8%), chloramphenicol (7%), kanamycin (7%), piperacillin (6%), and neomycin (5%). The multiple resistance pattern most often observed was that of streptomycin, sulfisoxazole, and tetracycline. Ten (7%) STEC O157:H7 and 71 (12%) non-O157 strains were resistant to five or more antimicrobial agents. Most strains showing resistance to five or more antimicrobial agents belonged to serotypes O4:H4 (4 strains), O8:H21 (3 strains), O20:H19 (6 strains), O26:H11 (8 strains eae-beta1), O111:H- (3 strains eae-gamma2), O118:H- (2 strains eae-beta1), O118:H16 (5 strains eae-beta1), O128:H- (2 strains), O145:H8 or O145:H- (2 strains eae-gamma1), O157:H7 (10 strains eae-gamma1), O171:H25 (3 strains), O177:H11 (5 strains eae-beta1), ONT:H- (3 strains/1 eae-beta1) and ONT:H21 (2 strains). Interestingly, most of these serotypes, i.e., those indicated in bold) were found among human STEC strains isolated from patients with hemolytic uremic-syndrome (HUS) reported in previous studies. We also detected, among non-O157 strains, an association between a higher level of multiple resistance to antibiotics and the presence of the virulence genes eae and stx(1). Moreover, STEC O157:H7, showed an association between certain phage types, PT21/28 (90%), PT23 (75%), PT34 (75%), and PT2 (54%), with a higher number of resistant strains. We conclude that the high prevalence of antimicrobial resistance detected in our study is a source of concern, and cautious use of antibiotics in animals is highly recommended.     

Selection of an Escherichia coli O157:H7 bacteriophage for persistence in the circulatory system of mice infected experimentally

A bacteriophage lytic for Escherichia coli O157:H7 was isolated from bovine manure. Following in-vivo selection, the phage acquired the capacity to persist in the circulatory system of mice for at least 38 days. When mice were infected experimentally with E. coli O157:H7 (10(7) CFU/mouse), simultaneous injection of the mice with phage (10(8) PFU/mouse) cleared E. coli O157:H7 from the mice within 48 h.     

Construction of genetically defined aroA mutant of a native E. coli O78:K80 isolated from avian colibacillosis, in Iran

A precisely defined, single gene deletion of aroA was made systematically by Datsenko and Wanner based approach in native Escherichia coli O78: K80, and its nature was examined. The aroA gene encodes 5-enolpyrovylshikimate3-phosphate synthase and participates in the aromatic amino acids and folic acid, which are universal metabolic pathways of bacteria. For construction of mutant strain, the gene was replaced by recombination with a chloramphenicol cassette, flanked by FLP recognition target site. Primers designed to create in-frame deletion upon excision of the resistance cassette. The aroA deletion was confirmed by both polymerase chain reaction and failure of the mutant to replicate and grow on minimum medium lacking aromatic amino acids. On the other hand, the wild-type parent strain grows well in this medium. The sequence of the aroA gene in native strain was different, and knowing this subject was important to produce a mutant strain with deletion-based methods of mutation. Furthermore, use of antibiotic cassette replacement facilitates mutant construction in a more cost-effective manner in comparison with other techniques such as use of suicide vectors.     

A dietary probiotic (Bifidobacterium lactis HN019) reduces the severity of Escherichia coli O157:H7 infection in mice

The protective effects of the probiotic Bifidobacterium lactis HN019 against Escherichia coli O157:H7 were investigated in murine challenge infection models. BALB/c or C57BL/6 mice were fed milk-based diets supplemented with B. lactis HN019 (3 × 10⁸ cfu/g) for 7 days prior to and following oral challenge with E. coli O157:H7. Behavioral parameters (morbidity, feed intake) were measured for 7 days following challenge; immunological responses (phagocytosis, antibody) and pathogen translocation were measured in a sub-sample of ostensibly healthy animals 1 week post-challenge. Results showed that HN019-fed mice maintained significantly higher post-challenge feed intake and exhibited a lower cumulative morbidity rate, compared to control mice which did not receive the probiotic. Significantly higher proportions of phagocytically active cells in the blood and peritoneum, and higher intestinal tract IgA anti-E. coli antibody responses, were recorded among HN019-fed mice compared to controls. Among HN019-fed mice, pathogen translocation was identified in one of five BALB/c and one of five C57 mice; the comparative figures in control mice were two of five and three of five, respectively, and the mean bacterial burdens in these mice were over 100-fold higher than in HN019-fed mice. These results demonstrate that HN019 can reduce the severity of infection due to the enterohemolytic pathogen E. coli O157:H7, and suggest that this reduction may be associated with enhanced immune protection conferred by the probiotic. Received: 24 October 2000     

Clonal structure of Shiga toxin (Stx)-producing and beta-D-glucuronidase-positive Escherichia coli O157:H7 strains isolated from outbreaks and sporadic cases in Hokkaido, Japan

A total of 22 clonal phenotypic variants of Shiga toxin (Stx)-producing Escherichia coli (STEC) O157:H7 was isolated from six different locations in Hokkaido, Japan. These isolates were negative for sorbitol fermentation but positive for beta-D-glucuronidase (GUD+). They carried eaeA, EHEC-hlyA, pas and etpD genes like typical E. coli O157:H7 and, in addition, st1 and stx2 genes. However, they were shown to lack katP and espP genes that are present in typical STEC O157:H7. All these atypical GUD+ STEC O157:H7 isolates had very similar antimicrobial susceptibilities. Pulsed-field gel electrophoresis analysis with XbaI, SfiI, SwaI, SpeI and NotI indicated that they were identical or closely related to one another. From their phenotypic and genotypic features, these GUD+ STEC O157:H7 isolates may represent a distinct clone among STEC O157.     

Detection in Escherichia coli of the genes encoding the major virulence factors,the genes defining the O157:H7 serotype,and components of the type 2 Shiga toxin family by multiplex PCR

Strains of Shiga toxin-producing Escherichia coli (STEC) have been associated with outbreaks of diarrhea, hemorrhagic colitis, and hemolytic-uremic syndrome in humans. Most clinical signs of disease arise as a consequence of the production of Shiga toxin 1 (Stx1), Stx2 or combinations of these toxins. Other major virulence factors include enterohemorrhagic E. coli hemolysin (EHEC hlyA), and intimin, the product of the eaeA gene that is involved in the attaching and effacing adherence phenotype. In this study, a series of multiplex-PCR assays were developed to detect the eight most-important E. coli genes associated with virulence, two that define the serotype and therefore the identity of the organism, and a built-in gene detection control. Those genes detected were stx(1), stx(2), stx(2c), stx(2d), stx(2e), stx(2f), EHEC hlyA, and eaeA, as well as rfbE, which encodes the E. coli O157 serotype; fliC, which encodes the E. coli flagellum H7 serotype; and the E. coli 16S rRNA, which was included as an internal control. A total of 129 E. coli strains, including 81 that were O157:H7, 10 that were O157:non-H7, and 38 that were non-O157 isolates, were investigated. Among the 129 samples, 101 (78.3%) were stx positive, while 28 (21.7%) were lacked stx. Of these 129 isolates, 92 (71.3%) were EHEC hlyA positive and 96 (74.4%) were eaeA positive. All STEC strains were identified by this procedure. In addition, all Stx2 subtypes, which had been initially identified by PCR-restriction fragment length polymorphism, were identified by this method. A particular strength of the assay was the identification of these 11 genes without the need to use restriction enzyme digestion. The proposed method is a simple, reliable, and rapid procedure that can detect the major virulence factors of E. coli while differentiating O157:H7 from non-O157 isolates.     

Cloning and nucleotide sequencing of Vero toxin 2 variant genes from Escherichia coli O91:H21 isolated from a patient with the hemolytic uremic syndrome

Cellular DNA extracted from Escherichia coli strain B2F1 (O91:H21) was found to contain two separate DNA sequences that hybridized with a Vero toxin 2 (VT2)-specific gene probe under stringent conditions. These two sequences were cloned and both were shown to encode a variant of Vero toxin 2 (VT2vh). The nucleotide sequences of the operons encoding VT2vh, designated as vtx2ha and vtx2hb, were determined. The two operons were nearly identical (99% overall DNA homology) and both encoded A subunits of 319 amino acid residues and B subunits of 89 amino acid residues, the A and B subunit genes being separated by a stretch of 14 bp. The A and B subunit genes of the vtx2ha operon exhibited 98.6% and 95.5% DNA homology, respectively, with those of the slt-II operon encoding Shiga-like toxin II (or VT2) cloned from a strain from a patient with hemorrhagic colitis, while the A and B subunit genes of the vtx2ha operon showed 94.5% and 82.8% DNA homology, respectively, with those of the slt-IIv operon encoding a SLT-II variant cloned from a strain isolated from a pig with edema disease. The nucleotide sequences of the presumed promoters and presumptive ribosome binding sites in the vtx2ha, vtx2hb, and slt-II, and slt-IIv operons were identical. These results indicate that nucleotide sequences encoding a family of VT2-related toxins are present in various strains of E. coli and that the sequences of the genes for A subunits are better conserved than those of the B subunit genes.