Presence and Characterization of a Mosaic Genomic Island Which Distinguishes Sorbitol-Fermenting Enterohemorrhagic Escherichia coli O157:H? from E. coli O157:H7

A mosaic genomic island comprising Shigella resistance locus (SRL) sequences flanked by segments of Escherichia coli O157:H7 strain EDL933 O islands 43, 81, and 82 was identified in sorbitol-fermenting (SF) enterohemorrhagic Escherichia coli (EHEC) O157:H⁻ strain 493/89. This mosaic island is absent from strain EDL933. PCR targeting the SRL-related sequence is a useful tool to distinguish SF EHEC O157:H⁻ from EHEC O157:H7.     

Osmotic and Desiccation Tolerance in Escherichia coli O157:H7 Requires rpoS (σ38)

The contribution of RecA, Dps, and RpoS to survival of Escherichia coli O157:H7 during desiccation and osmotic stress was determined in Luria–Bertani broth with 12?% NaCl (LB-12) at 30 and 37?°C, on filter disks at 23 and 30?°C, and in sterile bovine feces at 30?°C. RecA did not significantly contribute to survival in any condition or temperature. The contribution of Dps to survival was only significant in LB-12 at 37?°C. RpoS was necessary for survival during desiccation and osmotic stress, and survival of the RpoS mutant was significantly less than the parent in all conditions and temperatures. The RpoS mutant survived up to 21?days in bovine feces,?<4?days on filter disks, and?>8 and?<4?days in LB-12 at 30 and 37?°C, respectively. The parent, ΔrecA, dps, and dps/ΔrecA mutant strains survived?>8?days in LB-12,?>28?days on filter disks, and?>28?days in bovine feces. Increased incubation temperatures were associated with decreased survival. E. coli O157:H7 can persist in desiccating and osmotically challenging environments, especially sterile feces, for an extended period time.     

Complete DNA sequence analysis of enterohemorrhagic Escherichia coli plasmid pO157_2 in β-glucuronidase-positive E. coli O157:H7 reveals a novel evolutionary path

Strains of enterohemorragic Escherichia coli (EHEC) O157:H7 that are non-sorbitol fermenting (NSF) and β-glucuronidase negative (GUD(-)) carry a large virulence plasmid, pO157 (>90,000 bp), whereas closely related sorbitol-fermenting (SF) E. coli O157:H(-) strains carry plasmid pSFO157 (>120,000 bp). GUD(+) NSF O157:H7 strains are presumed to be precursors of GUD(-) NSF O157:H7 strains that also carry pO157. In this study, we report the complete sequence of a novel virulence plasmid, pO157-2 (89,762 bp), isolated from GUD(+) NSF O157:H7 strain G5101. PCR analysis confirmed the presence of pO157-2 in six other strains of GUD(+) NSF O157:H7. pO157-2 carries genes associated with virulence (e.g., hemolysin genes) and conjugation (tra and trb genes) but lacks katP and espP present in pO157. Comparative analysis of the three EHEC plasmids shows that pO157-2 is highly related to pO157 and pSFO157 but not ancestral to pO157. These results indicated that GUD(+) NSF O157:H7 strains might not be direct precursors to GUD(-) NSF O157:H7 as previously proposed but rather have evolved independently from a common ancestor.     

Modulation of Neutrophil Function by a Secreted Mucinase of Escherichia coli O157∶H7

Escherichia coli O157∶H7 is a human enteric pathogen that causes hemorrhagic colitis which can progress to hemolytic uremic syndrome, a severe kidney disease with immune involvement. During infection, E. coli O157∶H7 secretes StcE, a metalloprotease that promotes the formation of attaching and effacing lesions and inhibits the complement cascade via cleavage of mucin-type glycoproteins. We found that StcE cleaved the mucin-like, immune cell-restricted glycoproteins CD43 and CD45 on the neutrophil surface and altered neutrophil function. Treatment of human neutrophils with StcE led to increased respiratory burst production and increased cell adhesion. StcE-treated neutrophils exhibited an elongated morphology with defective rear detachment and impaired migration, suggesting that removal of the anti-adhesive capability of CD43 by StcE impairs rear release. Use of zebrafish embryos to model neutrophil migration revealed that StcE induced neutrophil retention in the fin after tissue wounding, suggesting that StcE modulates neutrophil-mediated inflammation in vivo. Neutrophils are crucial innate effectors of the antibacterial immune response and can contribute to severe complications caused by infection with E. coli O157∶H7. Our data suggest that the StcE mucinase can play an immunomodulatory role by directly altering neutrophil function during infection. StcE may contribute to inflammation and tissue destruction by mediating inappropriate neutrophil adhesion and activation.     

Biochemical characterization of WbdN, a β1,3-glucosyltransferase involved in O-antigen synthesis in enterohemorrhagic Escherichia coli O157

The enterohemorrhagic O157 strain of Escherichia coli, which is one of the most well-known bacterial pathogens, has an O-antigen repeating unit structure with the sequence [-2-d-Rha4NAcα1-3-l-Fucα1-4-d-Glcβ1-3-d-GalNAcα1-]. The O-antigen gene cluster of E. coli O157 contains the genes responsible for the assembly of this repeating unit and includes wbdN. In spite of cloning many O-antigen genes, biochemical characterization has been done on very few enzymes involved in O-antigen synthesis. In this work, we expressed the wbdN gene in E. coli BL21, and the His-tagged protein was purified. WbdN activity was characterized using the donor substrate UDP-[(14)C]Glc and the synthetic acceptor substrate GalNAcα-O-PO(3)-PO(3)-(CH(2))(11)-O-Ph. The enzyme product was isolated by high pressure liquid chromatography, and mass spectrometry showed that one Glc residue was transferred to the acceptor by WbdN. Nuclear magnetic resonance analysis of the product structure indicated that Glc was β1-3 linked to GalNAc. WbdN contains a conserved DxD motif and requires divalent metal ions for full activity. WbdN activity has an optimal pH between 7 and 8 and is highly specific for UDP-Glc as the donor substrate. GalNAcα derivatives lacking the diphosphate group were inactive as substrates, and the enzyme did not transfer Glc to GlcNAcα-O-PO(3)-PO(3)-(CH(2))(11)-O-Ph. Our results illustrate that WbdN is a specific UDP-Glc:GalNAcα-diphosphate-lipid β1,3-Glc-transferase. The enzyme is a target for the development of inhibitors to block O157-antigen synthesis.     

Coexpression of β-mannanase and xylanase genes in Escherichia coli

[目的]β-甘露聚糖酶和木聚糖酶都属于半纤维素酶,它们已经同时运用于工农业生产的许多领域.构建β-甘露聚糖酶和木聚糖酶共表达菌株并进行相关评价.[方法]通过设计一个共同的酶切位点,将菌株Bacillus subtilis BE-91中的β-甘露聚糖酶和木聚糖酶基因串联到表达载体pET28a(+)上,转化大肠杆菌构建了一株能够共表达β-甘露聚糖酶和木聚糖酶的菌株B.pET28a-man-xyl.[结果]菌株诱导21h后,发酵液中β-甘露聚糖酶和木聚糖酶的酶活分别为713.34 U/mL和1455.83 U/mL,是胞内酶活的11.8倍和2.53倍.[结论]SDS-PAGE分析、水解圈活性检测和胞外酶与胞内酶酶活检测表明:两个酶均以功能蛋白独立分泌到胞外.此外,与β-甘露聚糖酶和木聚糖酶单独酶解半纤维素相比,复合酶的酶解效果更好.菌株的成功构建为复合酶制剂(半纤维素酶制剂)的研究和生产奠定基础.     

Bovine β-defensins: Identification and characterization of novel bovine β-defensin genes and their expression in mammarygland tissue

-Defensin genes code for multifunctional peptides with a broad-range antimicrobial activity. In this project we hypothesized that -defensin genes may be candidate genes for resistance to mastitis. In this article we describe the identification and genomic characterization of eight bovine -defensin genes, including six novel defensin genes and two pseudogenes. Expression in the bovine mammary gland of one of the novel genes, DEFB401, has been demonstrated, as well as the expression of LAP, TAP, DEFB1, BNBD3, BNBD9, and BNBD12. For genomic characterization, 20 BACs from two different bovine BAC libraries (RZPD numbers 750 and 754) were isolated by PCR screening with -defensin consensus primers derived from published sequences. PCR products from BACs generated with consensus primers have been subcloned and sequenced, revealing a total of 16 genes and two pseudogenes. Six novel -defensin genes share the typical exon–intron structure and are highly homologous to published bovine -defensin genes. They are named DEFB401–DEFB405 and LAP-like, and two novel pseudogenes are named EBD-P and EBD-P2. Analysis of mammary gland tissue-derived cDNA from nine cows with different clinical findings demonstrated the expression of several -defensin genes mentioned above. First results indicate that the lactational status of the cow presumably has no influence on gene expression. Competent knowledge of antimicrobial activity of -defensins from literature, the abundance of -defensin mRNA in the bovine mammary gland, and the inducibility of some genes give first evidence that -defensins may play a role in local host defense during udder infections.The nucleotide sequence data reported in this article have been submitted to EMBL and have been assigned the accession numbers AJ563279–AJ563283, AJ567353–AJ567365, AJ567990–AJ567993, and AJ620296.     

Expression and characterization of Kluyveromyces lactis β-galactosidase in Escherichia coli

Kluyveromyces lactis -galactosidase gene, LAC4, was expressed in Escherichia coli as a soluble His-tagged recombinant enzyme under the optimized culture conditions. The expressed protein was multimeric with a subunit molecular mass of 118 kDa. The dimeric form of the -galactosidase was the major fraction but had a lower activity than those of the multimeric forms. The purified enzyme required Mn²⁺ for activity and was inactivated irreversibly by imidazole above 50 mM. The activity was optimal at 37 and 40 °C for o-nitrophenyl--d-galactopyranoside (oNPG) and lactose, respectively. The optimum pH value is 7. The K _m and V _max values of the purified enzyme for oNPG were 1.5 mM and 560 mol min^–1 mg^–1, and for lactose 20 mM and 570 mol min^–1 mg^–1, respectively.     

Expression and characterization of Pichia etchellsii β-glucosidase in Escherichia coli

The β-glucosidase enzyme is important as the terminal enzyme involved in hydrolysis of cellobiose and short-chain cellodextrins generated during enzymatic cellulose degradation. Under controlled reaction conditions the enzyme also displays cello-oligosaccharide synthesizing ability (based on either the thermodynamic or kinetic approach). We present here the purification of the enzyme β-glucosidase (BGL) of Pichia etchellsii from recombinant pBG55 Escherichia coli clone. The kinetic parameters, substrate specificity and oligosaccharide synthesizing ability of the purified enzyme are also reported. The purified 200-kDa protein (tetramer of 50 kDa) was identified as a broad-substrate-specificity enzyme exhibiting increased temperature and glucose tolerance compared to the native yeast enzyme. Temperature directed substrate specificity for aryl β,1–4 linkage, and β(1–2), β(1–4), β(1–6) and β(2-1) linkages in various natural disaccharides was observed. Glycosylation of the enzyme was found to be unimportant for enzyme activity. With both cellobiose and glucose, oligosaccharide synthesis was detected. The implications of this information with regard to cellulose hydrolysis and oligosaccharide synthesis are discussed.     

Does Enterohemorrhagic Escherichia coli O157:H7 Enter the Viable but Nonculturable State in Salted Salmon Roe?

An outbreak caused by salted salmon roe contaminated with enterohemorrhagic Escherichia coli O157 occurred in Japan in 1998. Since about 0.75 to 1.5 viable cells were estimated to cause infection, we presumed that O157 might enter the viable but nonculturable (VNC) state in salted salmon roe and consequently that viable cell numbers might be underestimated. Although patient-originating O157 cells could not grow on agar plates after 72 h of incubation in 13% NaCl, they were resuscitated in yeast extract broth, and more than 90% of the cells were shown to be viable by fluorescent staining, suggesting that almost all of them could enter the VNC state in NaCl water. Roe-originating O157 was resistant to NaCl because it could grow on agar after 72 h of incubation in NaCl water, but about 20% of cells appeared to enter the VNC state. Therefore, germfree mice were infected with O157 to examine the resuscitation of cells in the VNC state and the retention of pathogenicity. O157 that originated in roe, but not patients, killed mice and was isolated from the intestine. However, these isolates had become sensitive to NaCl. O157 cells of roe origin incubated in normal media also killed mice and were isolated from the intestine, but they also became transiently NaCl sensitive. We therefore propose that bacterial cells might enter the VNC state under conditions of stress, such as those encountered in vivo or in high salt concentrations, and then revive when those conditions have eased. If so, the VNC state in food is potentially dangerous from a public health viewpoint and may have to be considered at the time of food inspection. Finally, the establishment of a simple recovery system for VNC cells should be established.     

Does enterohemorrhagic Escherichia coli O157:H7 enter the viable but nonculturable state in salted salmon roe?

An outbreak caused by salted salmon roe contaminated with enterohemorrhagic Escherichia coli O157 occurred in Japan in 1998. Since about 0.75 to 1.5 viable cells were estimated to cause infection, we presumed that O157 might enter the viable but nonculturable (VNC) state in salted salmon roe and consequently that viable cell numbers might be underestimated. Although patient-originating O157 cells could not grow on agar plates after 72 h of incubation in 13% NaCl, they were resuscitated in yeast extract broth, and more than 90% of the cells were shown to be viable by fluorescent staining, suggesting that almost all of them could enter the VNC state in NaCl water. Roe-originating O157 was resistant to NaCl because it could grow on agar after 72 h of incubation in NaCl water, but about 20% of cells appeared to enter the VNC state. Therefore, germfree mice were infected with O157 to examine the resuscitation of cells in the VNC state and the retention of pathogenicity. O157 that originated in roe, but not patients, killed mice and was isolated from the intestine. However, these isolates had become sensitive to NaCl. O157 cells of roe origin incubated in normal media also killed mice and were isolated from the intestine, but they also became transiently NaCl sensitive. We therefore propose that bacterial cells might enter the VNC state under conditions of stress, such as those encountered in vivo or in high salt concentrations, and then revive when those conditions have eased. If so, the VNC state in food is potentially dangerous from a public health viewpoint and may have to be considered at the time of food inspection. Finally, the establishment of a simple recovery system for VNC cells should be established.     

Identifying mechanisms by which Escherichia coli O157:H7 subverts interferon-γ mediated signal transducer and activator of transcription-1 activation

Enterohemorrhagic Escherichia coli serotype O157:H7 is a food borne enteric bacterial pathogen that causes significant morbidity and mortality in both developing and industrialized nations. E. coli O157:H7 infection of host epithelial cells inhibits the interferon gamma pro-inflammatory signaling pathway, which is important for host defense against microbial pathogens, through the inhibition of Stat-1 tyrosine phosphorylation. The aim of this study was to determine which bacterial factors are involved in the inhibition of Stat-1 tyrosine phosphorylation. Human epithelial cells were challenged with either live bacteria or bacterial-derived culture supernatants, stimulated with interferon-gamma, and epithelial cell protein extracts were then analyzed by immunoblotting. The results show that Stat-1 tyrosine phosphorylation was inhibited by E. coli O157:H7 secreted proteins. Using sequential anion exchange and size exclusion chromatography, YodA was identified, but not confirmed to mediate subversion of the Stat-1 signaling pathway using isogenic mutants. We conclude that E. coli O157:H7 subverts Stat-1 tyrosine phosphorylation in response to interferon-gamma through a still as yet unidentified secreted bacterial protein.     

Spread and change in stress resistance of Shiga toxin-producing Escherichia coli?O157 on fungal colonies

To elucidate the effect of fungal hyphae on the behaviour of Shiga toxin-producing Escherichia coli (STEC) O157, the spread and change in stress resistance of the bacterium were evaluated after coculture with 11 species of food-related fungi including fermentation starters. Spread distances of STEC O157 varied depending on the co-cultured fungal species, and the motile bacterial strain spread for longer distances than the non-motile strain. The population of STEC O157 increased when co-cultured on colonies of nine fungal species but decreased on colonies of Emericella nidulans and Aspergillus ochraceus. Confocal scanning microscopy visualization of green fluorescent protein-tagged STEC O157 on fungal hyphae revealed that the bacterium colonized in the water film that existed on and between hyphae. To investigate the physiological changes in STEC O157 caused by co-culturing with fungi, the bacterium was harvested after 7 days of co-culturing and tested for acid resistance. After co-culture with eight fungal species, STEC O157 showed greater acid resistance compared to those cultured without fungi. Our results indicate that fungal hyphae can spread the contamination of STEC O157 and can also enhance the stress resistance of the bacteria.     

Characterization of Escherichia coli O157:H7 Strains from Contaminated Raw Beef Trim during “High Event Periods”

The development and implementation of effective antimicrobial interventions by the beef processing industry in the United States have dramatically reduced the incidence of beef trim contamination by Escherichia coli O157:H7. However, individual processing plants still experience sporadic peaks in contamination rates where multiple E. coli O157:H7-positive lots are clustered in a short time frame. These peaks have been referred to as “high event periods” (HEP) of contamination. The results reported here detail the characterization of E. coli O157:H7 isolates from 21 HEP across multiple companies and processing plants to gain insight regarding the mechanisms causing these incidents. Strain genotypes were determined by pulsed-field gel electrophoresis, and isolates were investigated for characteristics linking them to human illness. Through these analyses, it was determined that individual HEP show little to no diversity in strain genotypes. Hence, each HEP has one strain type that makes up most, if not all, of the contamination. This is shown to differ from the genotypic diversity of E. coli O157:H7 found on the hides of cattle entering processing plants. In addition, it was found that a large proportion (81%) of HEP are caused by strain types associated with human illness. These results pose a potential challenge to the current model for finished product contamination during beef processing.     

A comparison of the retention of pathogenic Escherichia coli?O157 by sprouts,leaves and fruits

The retention (binding to or association with the plant) of Escherichia coli by cut leaves and fruits after vigorous water washing was compared with that by sprouts. Retention by fruits and leaves was similar but differed from retention by sprouts in rate, effect of wounding and requirement for poly-β,1-6-N-acetyl-D-glucosamine. Escherichia coli was retained by cut ends of lettuce leaves within 5 min while more than 1 h was required for retention by the intact epidermis of leaves and fruits, and more than 1 day for sprouts. Retention after 5 min at the cut leaf edge was specific for E. coli and was not shown by the plant-associated bacteria Agrobacterium tumefaciens and Sinorhizobium meliloti. Escherichia coli was retained by lettuce, spinach, alfalfa, bean, tomato, Arabidopsis thaliana, cucumber, and pepper leaves and fruits faster than by sprouts. Wounding of leaves and fruits but not sprouts increased bacterial retention. Mutations in the exopolysaccharide synthesis genes yhjN and wcaD reduced the numbers of bacteria retained. PgaC mutants were retained by cut leaves and fruits but not by sprouts. There was no significant difference in the retention of an O157 and a K12 strain by fruits or leaves. However, retention by sprouts of O157 strains was significantly greater than K12 strains. These findings suggest that there are differences in the mechanisms of E coli retention among sprouts, and leaves and fruits.     

检测大肠杆菌(Escherichia coli) O157:H7免疫磁珠的制备、保存与应用

【背景】大肠杆菌(Escherichia coli) O157:H7是导致肠出血性大肠杆菌食源性疾病暴发的主要血清型,免疫磁珠(Immunomagnetic beads,IMBS)在E. coli O157的检测中发挥着重要作用,而免疫磁珠的稳定性、特异性、广谱性等性能指标关系着在实际应用中的使用效果。【目的】制备高效、稳定且具有广谱性的免疫磁珠,联合分子检测技术如环介导恒温扩增 (Loop-mediated isothermal amplification,LAMP)技术、PCR等,提高目标菌的检出率。【方法】采用新型的磁珠活化剂MIX&GO制备E. coli O157免疫磁珠,并进行广谱性以及特异性检测;针对6种试剂牛血清白蛋白(Bovine serum albumin,BSA)、酪蛋白(Casein)、海藻糖(Trehalose)、聚乙烯吡咯烷酮(Polyvinyl pyrrolidone,PVP)、抗坏血酸(Vitamin C)和防腐剂ProClin 300,利用正交试验L18(37)优化免疫磁珠保存液组分;采用IMBS-LAMP、IMBS-PCR、IMBS-生化、菌液-LAMP、菌液-PCR、显色平板-生化鉴定6种方式对20份生猪肉样品进行检测。【结果】利用MIX&GO活化剂制备的免疫磁珠捕获率最高达到81.5%±1.3%;免疫磁珠保存液最优配方为：牛血清白蛋白15.0 g/L,酪蛋白10.0 g/L,海藻糖10.0 g/L,PVP 2.0 g/L,抗坏血酸5.0 g/L,ProClin 300 2.5 g/L,保存6个月后免疫磁珠捕获率为75.5%;在20份生猪肉样品的检测中,自制磁珠和商品化磁珠与LAMP联用均检出9例阳性样品;IMBS-LAMP在6种检测方式中具有最高的检测灵敏度,但检出的样品会因磁珠抗体的差异而有所不同。【结论】与商品化磁珠相比,实验制备的免疫磁珠具有良好的特异性和广谱性,免疫磁珠-LAMP联用提高了目标菌的检出率,是一种高灵敏度、具有应用前景的检测方法。     

Expression of human CYP27B1 in Escherichia coli and characterization in phospholipid vesicles

CYP27B1 is a mitochondrial cytochrome P450 that catalyses the hydroxylation of 25-hydroxyvitamin D3 at the C1α-position to give the hormonally active form of vitamin D3, 1α,25-dihydroxyvitamin D3. We successfully expressed human CYP27B1 in Escherichia?coli and partially purified this labile enzyme and carried out a detailed characterization of its kinetic properties in a reconstituted membrane environment. The phospholipid concentration did not affect the enzyme activity in the vesicle-reconstituted system, although it was influenced by the phospholipid composition, with the addition of cardiolipin lowering the K(m) for 25-hydroxyvitamin D3. These data are consistent with the enzyme accessing substrate from the hydrophobic domain of the vesicle membrane. Cardiolipin also caused the appearance of inhibition of activity at high substrate concentrations. This substrate inhibition fitted a model for one catalytic and two inhibitory sites on the enzyme for the binding of substrate. The K(m) for human adrenodoxin was observed to decrease with decreasing substrate concentration, with the catalytic efficiency (k(cat) /K(m) ) being largely independent of adrenodoxin concentration. Human CYP27B1 was also active on 25-hydroxyvitamin D(2) and on intermediates of the CYP24A1-mediated inactivation pathway, 24R,25-dihydroxyvitamin D3, 24-oxo-25-hydroxyvitamin D3 and 24-oxo-23,25-dihydroxyvitamin D3, with all these substrates showing comparable k(cat) values of 50-71?min(-1) , similar to 25-hydroxyvitamin D3. The latter two substrates gave higher K(m) values than that for 25-hydroxy-vitamin D3. The present study shows that human CYP27B1 can be partially purified in an active form with the enzyme displaying high activity towards a range of substrates in a phospholipid vesicle-reconstituted system that mimics the inner-mitochondrial membrane.