Shiga toxin: biochemistry, genetics, mode of action, and role in pathogenesis

Function-based classification of hazardous biological sequences: Demonstration of a new paradigm for biohazard assessments

Bioengineering applies analytical and engineering principles to identify functional biological building blocks for biotechnology applications. While these building blocks are leveraged to improve the human condition, the lack of simplistic, machine-readable definition of biohazards at the function level is creating a gap for biosafety practices. More specifically, traditional safety practices focus on the biohazards of known pathogens at the organism-level and may not accurately consider novel biodesigns with engineered functionalities at the genetic component-level. This gap is motivating the need for a paradigm shift from organism-centric procedures to function-centric biohazard identification and classification practices. To address this challenge, we present a novel methodology for classifying biohazards at the individual sequence level, which we then compiled to distinguish the biohazardous property of pathogenicity at the whole genome level. Our methodology is rooted in compilation of hazardous functions, defined as a set of sequences and associated metadata that describe coarse-level functions associated with pathogens (e.g., adherence, immune subversion). We demonstrate that the resulting database can be used to develop hazardous “fingerprints” based on the functional metadata categories. We verified that these hazardous functions are found at higher levels in pathogens compared to non-pathogens, and hierarchical clustering of the fingerprints can distinguish between these two groups. The methodology presented here defines the hazardous functions associated with bioengineering functional building blocks at the sequence level, which provide a foundational framework for classifying biological hazards at the organism level, thus leading to the improvement and standardization of current biosecurity and biosafety practices.

High-Relaxivity Molecular MRI Contrast Agent to Target Gb3-Expressing Cancer Cells

Targeted contrast agents (CAs) can improve magnetic resonance imaging (MRI) for accurate cancer diagnosis. In this work, we used the Shiga toxin B-subunit (STxB) as a targeting agent, which binds to Gb3, a glycosphingolipid highly overexpressed on the surface of tumor cells. We developed STxB-targeted MRI probes from cyclic peptide scaffolds functionalized with six to nine monoamide DO3A[Gd(III)] chelates. The influence of structural constraints on the longitudinal relaxivity (r1) of the CAs has been studied. The cyclic peptide carrying nine monoamide DO3A[Gd(III)] exhibited a r1 per compound of 32 and 93 mM-1s-1 at 9.4 and 1.5 T, respectively. Its conjugation to the pentameric STxB protein led to a 70 kDa compound with a higher r1 of 150 and 475 mM-1 s-1 at 9.4 and 1.5 T, respectively. Specific accumulation and cellular distribution of this conjugate in Gb3-expressing cancer cells were demonstrated using immunofluorescence microscopy and quantified by an inductively coupled plasma-mass spectrometry dosage of Gd(III). Such an agent should enable the in vivo detection by MRI of tumors expressing Gb3 receptors.

Genomic Characterization of hlyF-positive Shiga Toxin-Producing Escherichia coli, Italy and the Netherlands, 2000-2019

Shiga toxin–producing Escherichia coli (STEC) O80:H2 has emerged in Europe as a cause of hemolytic uremic syndrome associated with bacteremia. STEC O80:H2 harbors the mosaic plasmid pR444_A, which combines several virulence genes, including hlyF and antimicrobial resistance genes. pR444_A is found in some extraintestinal pathogenic E. coli (ExPEC) strains. We identified and characterized 53 STEC strains with ExPEC-associated virulence genes isolated in Italy and the Netherlands during 2000–2019. The isolates belong to 2 major populations: 1 belongs to sequence type 301 and harbors diverse stx₂ subtypes, the intimin variant eae-ξ, and pO157-like and pR444_A plasmids; 1 consists of strains belonging to various sequence types, some of which lack the pO157 plasmid, the locus of enterocyte effacement, and the antimicrobial resistance–encoding region. Our results showed that STEC strains harboring ExPEC-associated virulence genes can include multiple serotypes and that the pR444_A plasmid can be acquired and mobilized by STEC strains.

Citrobacter rodentium Lysogenized with a Shiga Toxin-Producing Phage: A Murine Model for Shiga Toxin-Producing E. coli Infection

Shiga toxin-producing E. coli (STEC) is a common foodborne pathogen in developed countries. STEC generates "attaching and effacing" (AE) lesions on colonic epithelium, characterized by effacement of microvilli and the formation of actin "pedestals" beneath intimately attached bacteria. In addition, STEC are lysogenized with a phage that, upon induction, can produce potent Shiga toxins (Stx), potentially leading to both hemorrhagic colitis and hemolytic uremic syndrome. Investigation of the pathogenesis of this disease has been challenging because STEC does not readily colonize conventional mice.Citrobacter rodentium (CR) is a related mouse pathogen that also generates AE lesions. Whereas CR does not produce Stx, a murine model for STEC utilizes CR lysogenized with an E. coli-derived Stx phage, generating CR(Φstx), which both colonizes conventional mice and readily gives rise to systemic disease. We present here key methods for the use of CR(Φstx) infection as a highly predictable murine model for infection and disease by STEC. Importantly, we detail CR(Φstx) inoculation by feeding, determination of pathogen colonization, production of phage and toxin, and assessment of intestinal and renal pathology. These methods provide a framework for studying STEC-mediated systemic disease that may aid in the development of efficacious therapeutics.

Activation of the Nlrp3 Inflammasome Contributes to Shiga Toxin-Induced Hemolytic Uremic Syndrome in a Mouse Model

Objective

To explore the role of the Nlrp3 inflammasome activation in the development of hemolytic uremic syndrome (HUS) induced by Stx2 and evaluate the efficacy of small molecule Nlrp3 inhibitors in preventing the HUS.

Methods

Peritoneal macrophages (PMs) isolated from wild-type (WT) C57BL/6J mice and gene knockout mice (Nlrc4 ^-/-, Aim2 ^-/-, and Nlrp3 ^-/-) were treated with Stx2 in vitro and their IL-1β releases were measured. WT mice and Nlrp3 ^-/- mice were also treated with Stx2 in vivo by injection, and the biochemical indices (serum IL-1β, creatinine [CRE] and blood urea nitrogen [BUN]), renal injury, and animal survival were compared. To evaluate the effect of the Nlrp3 inhibitors in preventing HUS, WT mice were pretreated with different Nlrp3 inhibitors (MCC950, CY-09, Oridonin) before Stx2 treatment, and their biochemical indices and survival were compared with the WT mice without inhibitor pretreatment.

Results

When PMs were stimulated by Stx2 in vitro, IL-1β release in Nlrp3 ^-/- PMs was significantly lower compared to the other PMs. The Nlrp3 ^-/- mice treated by Stx2 in vivo, showed lower levels of the biochemical indices, alleviated renal injuries, and increased survival rate. When the WT mice were pretreated with the Nlrp3 inhibitors, both the biochemical indices and survival were significantly improved compared to those without inhibitor pretreatment, with Oridonin being most potent.

Conclusion

Nlrp3 inflammasome activation plays a vital role in the HUS development when mice are challenged by Stx2, and Oridonin is effective in preventing HUS.

Molecular Biology of Escherichia Coli Shiga Toxins' Effects on Mammalian Cells

Shiga toxins (Stxs), syn. Vero(cyto)toxins, are potent bacterial exotoxins and the principal virulence factor of enterohemorrhagic Escherichia coli (EHEC), a subset of Shiga toxin-producing E. coli (STEC). EHEC strains, e.g., strains of serovars O157:H7 and O104:H4, may cause individual cases as well as large outbreaks of life-threatening diseases in humans. Stxs primarily exert a ribotoxic activity in the eukaryotic target cells of the mammalian host resulting in rapid protein synthesis inhibition and cell death. Damage of endothelial cells in the kidneys and the central nervous system by Stxs is central in the pathogenesis of hemolytic uremic syndrome (HUS) in humans and edema disease in pigs. Probably even more important, the toxins also are capable of modulating a plethora of essential cellular functions, which eventually disturb intercellular communication. The review aims at providing a comprehensive overview of the current knowledge of the time course and the consecutive steps of Stx/cell interactions at the molecular level. Intervention measures deduced from an in-depth understanding of this molecular interplay may foster our basic understanding of cellular biology and microbial pathogenesis and pave the way to the creation of host-directed active compounds to mitigate the pathological conditions of STEC infections in the mammalian body.

Shiga toxin triggers endothelial and podocyte injury: the role of complement activation

Shiga toxin (Stx)-producing Escherichia coli (STEC) is the offending agent in post-diarrhea-associated hemolytic uremic syndrome (HUS), a disorder characterized by thrombocytopenia, microangiopathic hemolytic anemia, and acute kidney failure, with thrombi occluding the renal microvasculature. Endothelial dysfunction has been recognized as the trigger event in the development of microangiopathic processes. Glomerular endothelial cells are susceptible to the toxic effects of Stxs that, via nuclear factor kappa B (NF-κB) activation, induce the expression of genes encoding for adhesion molecules and chemokines, culminating in leukocyte adhesion and platelet thrombus formation on the activated endothelium. Complement activation via the alternative pathway has been seen in patients during the acute phase of STEC-associated HUS. Experimental evidence has highlighted the role of complement proteins in driving glomerular endothelium toward a thrombogenic phenotype. At the glomerular level, podocytes are also an important target of Stx-induced complement activation. Glomerular injury as a consequence of podocyte dysfunction and loss is thus a mechanism that might affect long-term renal outcomes in the disease. New approaches to targeting the complement system may be useful therapeutic options for patients with STEC-HUS.

Complement Activation Contributes to the Pathophysiology of Shiga Toxin-Associated Hemolytic Uremic Syndrome

Shiga toxin (Stx)-producing Escherichia coli (STEC) infections have become a threat to public health globally because of the severe illnesses that they can trigger, such as hemorrhagic colitis and the post-diarrheal hemolytic uremic syndrome (HUS), characterized by microangiopathic hemolytic anemia, thrombocytopenia, and acute kidney failure. Glomerular endothelial cells are primary targets of Stx which, after binding to its specific receptor globotriaosylceramide, upregulates proinflammatory proteins involved both in the recruitment and adhesion of leukocytes and thrombus formation at the site of endothelial injury. In this review, we discuss the role of complement activation in promoting glomerular microvascular dysfunction, providing evidence from experimental models and patients with STEC-HUS. Within the glomerulus, an important target for Stx-induced complement activation is the podocyte, a cell type that is in close contact with endothelial cells and participates in maintaining the filtration barrier. Recently, podocyte injury and loss have been indicated as potential risk factors for long-term renal sequelae in patients with STEC-HUS. Therapeutic approaches targeting the complement system, that may be useful options for patients with STEC-HUS, will also be discussed.

Soluble CD40 Ligand and Oxidative Response Are Reciprocally Stimulated during Shiga Toxin-Associated Hemolytic Uremic Syndrome

Shiga toxin (Stx), produced by Escherichia coli, is the main pathogenic factor of diarrhea-associated hemolytic uremic syndrome (HUS), which is characterized by the obstruction of renal microvasculature by platelet-fibrin thrombi. It is well known that the oxidative imbalance generated by Stx induces platelet activation, contributing to thrombus formation. Moreover, activated platelets release soluble CD40 ligand (sCD40L), which in turn contributes to oxidative imbalance, triggering the release of reactive oxidative species (ROS) on various cellular types. The aim of this work was to determine if the interaction between the oxidative response and platelet-derived sCD40L, as consequence of Stx-induced endothelium damage, participates in the pathogenic mechanism during HUS. Activated human glomerular endothelial cells (HGEC) by Stx2 induced platelets to adhere to them. Although platelet adhesion did not contribute to endothelial damage, high levels of sCD40L were released to the medium. The release of sCD40L by activated platelets was inhibited by antioxidant treatment. Furthermore, we found increased levels of sCD40L in plasma from HUS patients, which were also able to trigger the respiratory burst in monocytes in a sCD40L-dependent manner. Thus, we concluded that platelet-derived sCD40L and the oxidative response are reciprocally stimulated during Stx2-associated HUS. This process may contribute to the evolution of glomerular occlusion and the microangiopathic lesions.

Baicalin Protects Mice from Lethal Infection by Enterohemorrhagic Escherichia coli

Shiga-like toxin-producing Escherichia coli (STEC) O157:H7 poses grave challenges to public health by its ability to cause severe colonic diseases and renal failure in both human and animals. Shiga-like toxins are the major pathogenic factor for some highly virulent E. coli expecially Shiga-like toxin 2. Conventional treatments such as antibiotics can facilitate the release of the toxin thus potentially exacerbate the diseases. Small molecule inhibitors and antibodies capable of neutralizing the toxins are the two major venues for the development of therapeutics against enterohemorrhagic serotype E. coli infection. While promising and potentially effective at clinical settings, these approaches need to overcome obstacles such as the limited routes of administration, responses from the host immune system, which are known to differ greatly among individuals. Our previous studies demonstrate that Baicalin (BAI), a flavonoid compound isolated from Scutellaria baicalensis protects against rStx2-induced cell cytotoxicity and also protects mice from lethal rStx2 challenges by inducing Stx2 to form inactive oligomers. In this manuscript, we present some exciting work showing that baicalin is an effective agent for therapeutic treatment of STEC O157:H7 infection.

Affinity-Based Screening of Tetravalent Peptides Identifies Subtype-Selective Neutralizers of Shiga Toxin 2d, a Highly Virulent Subtype, by Targeting a Unique Amino Acid Involved in Its Receptor Recognition

Shiga toxin (Stx), a major virulence factor of enterohemorrhagic Escherichia coli (EHEC), can be classified into two subgroups, Stx1 and Stx2, each consisting of various closely related subtypes. Stx2 subtypes Stx2a and Stx2d are highly virulent and linked with serious human disorders, such as acute encephalopathy and hemolytic-uremic syndrome. Through affinity-based screening of a tetravalent peptide library, we previously developed peptide neutralizers of Stx2a in which the structure was optimized to bind to the B-subunit pentamer. In this study, we identified Stx2d-selective neutralizers by targeting Asn16 of the B subunit, an amino acid unique to Stx2d that plays an essential role in receptor binding. We synthesized a series of tetravalent peptides on a cellulose membrane in which the core structure was exactly the same as that of peptides in the tetravalent library. A total of nine candidate motifs were selected to synthesize tetravalent forms of the peptides by screening two series of the tetravalent peptides. Five of the tetravalent peptides effectively inhibited the cytotoxicity of Stx2a and Stx2d, and notably, two of the peptides selectively inhibited Stx2d. These two tetravalent peptides bound to the Stx2d B subunit with high affinity dependent on Asn16. The mechanism of binding to the Stx2d B subunit differed from that of binding to Stx2a in that the peptides covered a relatively wide region of the receptor-binding surface. Thus, this highly optimized screening technique enables the development of subtype-selective neutralizers, which may lead to more sophisticated treatments of infections by Stx-producing EHEC.

Multiple-locus variable-number tandem repeat analysis for strain discrimination of non-O157 Shiga toxin-producing Escherichia coli

Non-O157 Shiga toxin-producing Escherichia coli (STEC) are foodborne pathogens of growing concern worldwide that have been associated with several recent multistate and multinational outbreaks of foodborne illness. Rapid and sensitive molecular-based bacterial strain discrimination methods are critical for timely outbreak identification and contaminated food source traceback. One such method, multiple-locus variable-number tandem repeat analysis (MLVA), is being used with increasing frequency in foodborne illness outbreak investigations to augment the current gold standard bacterial subtyping technique, pulsed-field gel electrophoresis (PFGE). The objective of this study was to develop a MLVA assay for intra- and inter-serogroup discrimination of six major non-O157 STEC serogroups-O26, O111, O103, O121, O45, and O145-and perform a preliminary internal validation of the method on a limited number of clinical isolates. The resultant MLVA scheme consists of ten variable number tandem repeat (VNTR) loci amplified in three multiplex PCR reactions. Sixty-five unique MLVA types were obtained among 84 clinical non-O157 STEC strains comprised of geographically diverse sporadic and outbreak related isolates. Compared to PFGE, the developed MLVA scheme allowed similar discrimination among serogroups O26, O111, O103, and O121 but not among O145 and O45. To more fully compare the discriminatory power of this preliminary MLVA method to PFGE and to determine its epidemiological congruence, a thorough internal and external validation needs to be performed on a carefully selected large panel of strains, including multiple isolates from single outbreaks.

Development of camelid single chain antibodies against Shiga toxin type 2 (Stx2) with therapeutic potential against Hemolytic Uremic Syndrome (HUS)

Shiga toxin (Stx)-producing Escherichia coli (STEC) infections are implicated in the development of the life-threatening Hemolytic Uremic Syndrome (HUS). Despite the magnitude of the social and economic problems caused by STEC infections, no licensed vaccine or effective therapy is presently available for human use. Single chain antibodies (VHH) produced by camelids exhibit several advantages in comparison with conventional antibodies, making them promising tools for diagnosis and therapy. In the present work, the properties of a recently developed immunogen, which induces high affinity and protective antibodies against Stx type 2 (Stx2), were exploited to develop VHHs with therapeutic potential against HUS. We identified a family of VHHs against the B subunit of Stx2 (Stx2B) that neutralize Stx2 in vitro at subnanomolar concentrations. One VHH was selected and was engineered into a trivalent molecule (two copies of anti-Stx2B VHH and one anti-seroalbumin VHH). The resulting molecule presented extended in vivo half-life and high therapeutic activity, as demonstrated in three different mouse models of Stx2-toxicity: a single i.v. lethal dose of Stx2, several i.v. incremental doses of Stx2 and intragastrical STEC infection. This simple antitoxin agent should offer new therapeutic options for treating STEC infections to prevent or ameliorate HUS outcome.

Mass Spectrometry-Based Method of Detecting and Distinguishing Type 1 and Type 2 Shiga-Like Toxins in Human Serum

Shiga-like toxins (verotoxins) are responsible for the virulence associated with a variety of foodborne bacterial pathogens. Direct detection of toxins requires a specific and sensitive technique. In this study, we describe a mass spectrometry-based method of analyzing the tryptic decapeptides derived from the non-toxic B subunits. A gene encoding a single protein that yields a set of relevant peptides upon digestion with trypsin was designed. The (15)N-labeled protein was prepared by growing the expressing bacteria in minimal medium supplemented with (15)NH₄Cl. Trypsin digestion of the (15)N-labeled protein yields a set of (15)N-labeled peptides for use as internal standards to identify and quantify Shiga or Shiga-like toxins. We determined that this approach can be used to detect, quantify and distinguish among the known Shiga toxins (Stx) and Shiga-like toxins (Stx1 and Stx2) in the low attomole range (per injection) in complex media, including human serum. Furthermore, Stx1a could be detected and distinguished from the newly identified Stx1e in complex media. As new Shiga-like toxins are identified, this approach can be readily modified to detect them. Since intact toxins are digested with trypsin prior to analysis, the handling of intact Shiga toxins is minimized. The analysis can be accomplished within 5 h.

Shiga Toxins: Potent Poisons, Pathogenicity Determinants, and Pharmacological Agents

The Shiga toxins (Stxs), also known as Vero toxins and previously called Shiga-like toxins, are a family of potent protein synthesis inhibitors made by Shigella dysenteriae type 1 and some serogroups of Escherichia coli that cause bloody diarrhea in humans. Stxs act as virulence factors for both S. dysenteriae and E. coli and contribute to the disease process initiated by those organisms both directly and indirectly. A handful of methods exist for toxin purification, and the toxins can now even be purchased commercially. However, the Stxs are now classified as select agents, and specific rules govern the distribution of both the toxin and clones of the toxin. Toxin delivery into the host in S. dysenteriae type 1 is most likely aided by the invasiveness of that organism. Although the Stxs are made and produced by bacteria, they do not appear to act against either their host organism or other bacteria under normal circumstances, most likely because the A subunit is secreted from the cytoplasm as soon as it is synthesized and because the holotoxin cannot enter intact bacterial cells. The effectiveness of antibiotic therapy in patients infected with Stx-producing E. coli (STEC) such as O157:H7 as well as the potential risks of such treatment are areas of controversy. Several studies indicate that the course of the diarrhea stage of the disease is unaltered by antibiotic treatment. Several groups anticipate that a therapy that targets the Stxs is an important component of trying to alleviate disease caused by Stx-producing bacteria.

Reduced Toxicity of Shiga Toxin (Stx) Type 2c in Mice Compared to Stx2d Is Associated with Instability of Stx2c Holotoxin

Shiga toxin (Stx) is an AB₅ ribotoxin made by Stx-producing Escherichia coli (STEC). These organisms cause diarrhea, hemorrhagic colitis and the hemolytic uremic syndrome. STEC make two types of Stxs, Stx1 and/or Stx2. Stx2 has one prototype (a) and six subtypes (b–g), but only STEC that make Stx2a, and/or Stx2c, or Stx2d are associated with severe disease. However, Stx2c is about 10-fold less toxic than Stx2d in vivo despite only two amino acid differences in the A subunit at positions 291 and 297. We made mutations at these two sites to create intermediate toxins between Stx2c and Stx2d, and determined the 50% cytotoxic dose on Vero cells before and after heat treatment, and the 50% lethal dose in mice of the toxins. We found that serine 291 was associated with increased toxicity in vivo and that either amino acid change from that in Stx2c to that in Stx2d increased heat stability. We also assessed the secondary structure of Stx2c and Stx2d by circular dichroism (CD) spectroscopy. The CD studies suggest that Stx2c has a less-ordered secondary structure than Stx2d. We conclude that both amino acids at positions 291 and 297 in Stx2c contribute to its decreased stability and in vivo toxicity compared to Stx2d.

Diversity of Shiga Toxin-Producing Escherichia coli (STEC) O26:H11 Strains Examined via stx Subtypes and Insertion Sites of Stx and EspK Bacteriophages

Shiga toxin-producing Escherichia coli (STEC) is a food-borne pathogen that may be responsible for severe human infections. Only a limited number of serotypes, including O26:H11, are involved in the majority of serious cases and outbreaks. The main virulence factors, Shiga toxins (Stx), are encoded by bacteriophages. Seventy-four STEC O26:H11 strains of various origins (including human, dairy, and cattle) were characterized for their stx subtypes and Stx phage chromosomal insertion sites. The majority of food and cattle strains possessed the stx(1a) subtype, while human strains carried mainly stx(1a) or stx(2a). The wrbA and yehV genes were the main Stx phage insertion sites in STEC O26:H11, followed distantly by yecE and sbcB. Interestingly, the occurrence of Stx phages inserted in the yecE gene was low in dairy strains. In most of the 29 stx-negative E. coli O26:H11 strains also studied here, these bacterial insertion sites were vacant. Multilocus sequence typing of 20 stx-positive or stx-negative E. coli O26:H11 strains showed that they were distributed into two phylogenetic groups defined by sequence type 21 (ST21) and ST29. Finally, an EspK-carrying phage was found inserted in the ssrA gene in the majority of the STEC O26:H11 strains but in only a minority of the stx-negative E. coli O26:H11 strains. The differences in the stx subtypes and Stx phage insertion sites observed in STEC O26:H11 according to their origin might reflect that strains circulating in cattle and foods are clonally distinct from those isolated from human patients.

Shiga toxin-producing Escherichia coli O157, O26 and O111 in cattle faeces and hides in Italy

INTRODUCTION

Ruminants are regarded as the natural reservoir for Shiga toxin-producing Escherichia coli (STEC), especially of serogroup O157.

MATERIALS AND METHODS

During 2011 and 2012, 320 samples (160 faecal samples from the rectum and 160 hide samples from the brisket area) were collected from 160 cattle at slaughter in Northern Italy during warm months (May to October). Cattle were reared in different farms and their age at slaughter ranged between nine months and 15 years, most of them being culled cattle (median age: six years; average age: 4.6 years). Samples were tested by immunomagnetic-separation technique for E coli O157 and O26 and by a screening PCR for stx genes followed by cultural detection of STEC. The virulence genes stx1, stx2, eae, and e-hlyA were detected and among stx2-positive isolates the presence of the stx2a and stx2c variants was investigated.

RESULTS

Twenty-one of 160 cattle (13.1 per cent; 95 per cent CI 8.3 to 19.4 per cent) were found to be faecal carriers of STEC. STEC O157 was found in 10 (6.3 per cent) samples, STEC O26 in six (3.8 per cent) and STEC O111 in one (0.6 per cent). Four isolates (2.5 per cent) were O not determined (OND). Six out of 160 (3.8 per cent; 95 per cent CI 1.4 to 8.0 per cent) hide samples were positive for STEC; four hides (2.5 per cent) were contaminated by STEC O157 and two (1.3 per cent) by STEC O26. In three cattle (1.9 per cent) STEC from both faeces and hides were detected. Among STEC O157, 87.5 per cent of them carried the stx2c gene and 12.5 per cent carried both stx1 and stx2c genes. No O157 isolate harboured stx2a variant. STEC O26 and O111 carried the stx1 gene only. One OND strain carried both the stx2a and stx2c genes.

CONCLUSIONS

This study shows that STEC O157 from cattle can harbour the stx2c variant, which is associated with haemolytic uraemic syndrome in humans, and that cattle hides may be a source of human pathogenic STEC O157 and O26 in the slaughterhouse environment.

The inhibitory effect of colloidal bismuth hydroxide gel on Escherichia coli O157:H7 and on the activity of Shiga toxins

BACKGROUND

Shiga toxin-producing Escherichia coli (STEC) is the causative agent of hemolytic uremic syndrome (HUS). Colloidal bismuth hydroxide gel (CBHG) is an anti-diarrheal and antisecretory compound, which does not inhibit gastrointestinal motility and reaches an in vivo gut concentration of 10.8 mg/ml of bismuth. Its action on bacteria has not been studied. We analyzed its inhibitory effects on STEC, as well as the deactivation of the Shiga toxin (Stx) and its ability to block the spread of genes encoding Stx. We determined a minimum inhibitory concentration and bactericidal concentration for the STEC O157:H7 strain (EDL933), with CBHG and Chobet® bismuth cream with pectin (CBCHP). We analyzed its effect on Stx by means of cytotoxicity assay and ELISA, as well as its effect on the free 933 W Stx phage.

RESULTS

Effect on the EDL933 strain: CBHG: MIC 10 mg/ml of bismuth.

CBCHP

MIC 6 mg/ml and MBC 15 mg/ml of bismuth. Effect on EDL933 virulence factors: significant decrease in active Stx and 933 W Stx phage titer. ELISA did not find significant differences with treatment.

CONCLUSIONS

The results obtained may be useful in the development of new therapeutic strategies based on the use of CBHG to prevent or improve the prognosis of HUS, as it can be used to control STEC infections.

Overview and Historical Perspectives

In this overview, we describe the history of Shiga toxin (Stx)-producing Escherichia coli (STEC) in two phases. In phase one, between 1977 and 2011, we learned that E. coli could produce Shiga toxin and cause both hemorrhagic colitis and the hemolytic-uremic syndrome in humans and that the prototype STEC-E. coli O157:H7-adheres to and effaces intestinal epithelial cells by a mechanism similar to that of enteropathogenic E. coli. We also recognized that the genes for Stx are typically encoded on a lysogenic phage; that STEC O157:H7 harbors a large pathogenicity island that encodes the elements needed for the characteristic attaching and effacing lesion; and that the most severe cases of human disease are linked to production of Stx type 2a, not Stx type 1a. Phase two began with a large food-borne outbreak of hemorrhagic colitis and hemolytic-uremic syndrome in Germany in 2011. That outbreak was caused by a novel strain consisting of enteroaggregative E. coli O104:H4 transduced by a Stx2a-converting phage. From this outbreak we learned that any E. coli strain that can adhere tightly to the human bowel (either by a biofilm-like mechanism as in E. coli O104:H4 or by an attaching and effacing mechanism as in E. coli O157:H7) can cause severe diarrheal and systemic illness when it acquires the capacity to produce Stx2a. This overview provides the basis for the review of current information regarding these fascinating and complex pathogens.

Glycolipid binding preferences of Shiga toxin variants

The major virulence factor of Shiga toxin producing E. coli, is Shiga toxin (Stx), an AB5 toxin that consists of a ribosomal RNA-cleaving A-subunit surrounded by a pentamer of receptor-binding B subunits. The two major isoforms, Stx1 and Stx2, and Stx2 variants (Stx2a-h) significantly differ in toxicity. The exact reason for this toxicity difference is unknown, however different receptor binding preferences are speculated to play a role. Previous studies used enzyme linked immunosorbent assay (ELISA) to study binding of Stx1 and Stx2a toxoids to glycolipid receptors. Here, we studied binding of holotoxin and B-subunits of Stx1, Stx2a, Stx2b, Stx2c and Stx2d to glycolipid receptors globotriaosylceramide (Gb3) and globotetraosylceramide (Gb4) in the presence of cell membrane components such as phosphatidylcholine (PC), cholesterol (Ch) and other neutral glycolipids. In the absence of PC and Ch, holotoxins of Stx2 variants bound to mixtures of Gb3 with other glycolipids but not to Gb3 or Gb4 alone. Binding of all Stx holotoxins significantly increased in the presence of PC and Ch. Previously, Stx2a has been shown to form a less stable B-pentamer compared to Stx1. However, its effect on glycolipid receptor binding is unknown. In this study, we showed that even in the absence of the A-subunit, the B-subunits of both Stx1 and Stx2a were able to bind to the glycolipids and the more stable B-pentamer formed by Stx1 bound better than the less stable pentamer of Stx2a. B-subunit mutant of Stx1 L41Q, which shows similar stability as Stx2a B-subunits, lacked glycolipid binding, suggesting that pentamerization is more critical for binding of Stx1 than Stx2a.

Humoral immune response to Shiga Toxin 2 (Stx2) among Brazilian urban children with hemolytic uremic syndrome and healthy controls

Background

Shiga toxin (Stx)-producing Escherichia coli (STEC) infection is associated with hemolytic uremic syndrome (HUS), the main cause of acute renal failure in early childhood. Stx is essential in the pathogenesis of HUS, which has been mostly related to Stx2-producing isolates. Very limited data exist on the immune response to STEC in the Brazilian population. In this study, the prevalence of immunoglobulin G (IgG) antibodies to Stx2 was investigated in sera of children diagnosed with HUS and of healthy children in the city of São Paulo, Brazil.

Methods

IgG-antibody reactivity to Stx2 was determined by immunoblotting (WB) and enzyme-linked immunosorbent assay (ELISA) in sera from 13 children with HUS aged 8 months to 6 years and 54 healthy urban children aged 5 months to 7 years.

Results

A positive immune response to the A and B subunits of Stx2 was observed in 46.1% HUS patients and in 16.6% healthy individuals by WB. All HUS patients and 62.9% healthy children showed IgG antibodies to the Stx2 A subunit. The frequency of antibodies to both subunits or only to the A subunit of Stx2 was significantly higher in HUS patients than controls (p < 0.05). Also, the mean OD value obtained by ELISA was higher in that group. Considering children’s age, the frequency of reactivity to either the A subunit or both subunits of Stx2 was considerably higher in HUS children up to three years old compared to controls in the same age range. Moreover, in almost 37% of healthy children, no immune response to Stx2 was detected independently of the child’s age.

Conclusions

The seroepidemiolgy of anti-Stx2 antibodies was described for the first time in healthy children and children with HUS in Brazil. The percentage of individuals showing antibodies against Stx2 was higher among HUS patients than controls, and in spite of the low number of notified HUS cases, STEC strains are circulating in our settings. In addition, the results obtained also corroborated previous data on the increased sensitivity and specificity of WB compared to toxin-based enzyme immunoassays.

Shigella: A Highly Virulent and Elusive Pathogen

Despite a significant decrease in Shigella-related mortality, shigellosis continues to carry a significant burden of disease worldwide, particularly in Asia and Africa. Shigella is a highly virulent pathogen comprised of four major species with numerous subtypes. Shigella dysenteriae and Shigella flexneri infections are predominant in resource-limited settings. Clinical presentations range from mild watery diarrhea to severe dysentery with systemic complications such as electrolyte imbalance, seizures and hemolytic uremic syndrome. S. dysenteriae subtype 1, the producer of Shiga toxin, causes the most severe illness and highest mortality. Susceptible strains of Shigella may be effectively treated with inexpensive oral antibiotics such as ampicillin or trimethoprim-sulfamethoxazole. Unfortunately, multidrug resistant strains have emerged that have rendered most antibiotics, including fluoroquinolones and extended-spectrum cephalosporins, ineffective. Management and prevention of shigellosis represents a major public health challenge. The development of an effective vaccine is urgently needed to decrease its global impact.

Production of hybrid-IgG/IgA plantibodies with neutralizing activity against Shiga toxin 1

Shiga toxin 1 (Stx1) is a virulence factor of enterohemorrhagic Escherichia coli, such as the O157:H7 strain. In the intestines, secretory IgA (SIgA) is a major component of the immune defense against pathogens and toxins. To form SIgA, the production of dimeric IgA that retains biological activity is an important step. We previously established hybrid-IgG/IgA having variable regions of the IgG specific for the binding subunit of Stx1 (Stx1B) and the heavy chain constant region of IgA. If hybrid-IgG/IgA cDNAs can be expressed in plants, therapeutic or preventive effects may be expected in people eating those plants containing a “plantibody”. Here, we established transgenic Arabidopsis thaliana expressing dimeric hybrid-IgG/IgA. The heavy and light chain genes were placed under the control of a bidirectional promoter and terminator of the chlorophyll a/b-binding protein of Arabidopsis thaliana (expression cassette). This expression cassette and the J chain gene were subcloned into a single binary vector, which was then introduced into A. thaliana by means of the Agrobacterium method. Expression and assembly of the dimeric hybrid-IgG/IgA in plants were revealed by ELISA and immunoblotting. The hybrid-IgG/IgA bound to Stx1B and inhibited Stx1B binding to Gb₃, as demonstrated by ELISA. When Stx1 holotoxin was pre-treated with the resulting plantibody, the cytotoxicity of Stx1 was inhibited. The toxin neutralization was also demonstrated by means of several assays including Stx1-induced phosphatidylserine translocation on the plasma membrane, caspase-3 activation and 180 base-pair DNA ladder formation due to inter-nucleosomal cleavage. These results indicate that edible plants containing hybrid-IgG/IgA against Stx1B have the potential to be used for immunotherapy against Stx1-caused food poisoning.

stx2vhaIs the Dominant Genotype of Shiga Toxin-ProducingEscherichia coliO157:H7 Isolated from Patients and Domestic Animals in Three Regions of China

Shiga toxin-producing Escherichia coli(STEC) O157: H7 strains were isolated from domestic animals and patients from Xuzhou City, Jiangsu Province, China and the bordering Anhui and Henan Provinces and were examined for the stx genotype. Of 390 strains, 277 were identified as genotype stx2vha ; 41, stx2 ; 51, stx2-stx1 ; 1, stx2-stx2vha-stx1 ; 5, stx2-stx2vha ; and 15 were un-typeable. Of the 277 stx2vha-bearing isolates, 116 were isolated from goats; 42, cattle; 38, hens, and 35 from pigs. The study shows stx2vha is the dominant genotype and goats are an important reservoir.

A single VHH-based toxin-neutralizing agent and an effector antibody protect mice against challenge with Shiga toxins 1 and 2

Shiga toxin-producing Escherichia coli (STEC) is a major cause of severe food-borne disease worldwide, and two Shiga toxins, Stx1 and Stx2, are primarily responsible for the serious disease consequence, hemolytic-uremic syndrome (HUS). Here we report identification of a panel of heavy-chain-only antibody (Ab) V(H) (VHH) domains that neutralize Stx1 and/or Stx2 in cell-based assays. VHH heterodimer toxin-neutralizing agents containing two linked Stx1-neutralizing VHHs or two Stx2-neutralizing VHHs were generally much more potent at Stx neutralization than a pool of the two-component monomers tested in cell-based assays and in vivo mouse models. We recently reported that clearance of toxins can be promoted by coadministering a VHH-based toxin-neutralizing agent with an antitag monoclonal antibody (MAb), called the "effector Ab," that indirectly decorates each toxin molecule with four Ab molecules. Decoration occurs because the Ab binds to a common epitopic tag present at two sites on each of the two VHH heterodimer molecules that bind to each toxin molecule. Here we show that coadministration of effector Ab substantially improved the efficacy of Stx toxin-neutralizing agents to prevent death or kidney damage in mice following challenge with Stx1 or Stx2. A single toxin-neutralizing agent consisting of a double-tagged VHH heterotrimer--one Stx1-specific VHH, one Stx2-specific VHH, and one Stx1/Stx2 cross-specific VHH--was effective in preventing all symptoms of intoxication from Stx1 and Stx2 when coadministered with effector Ab. Overall, the availability of simple, defined, recombinant proteins that provide cost-effective protection against HUS opens up new therapeutic approaches to managing disease.

Binding of Pk-trisaccharide analogs of globotriaosylceramide to Shiga toxin variants

The two major forms of Shiga toxin, Stx1 and Stx2, use the glycolipid globotriaosylceramide (Gb3) as their cellular receptor. Stx1 primarily recognizes the Pk-trisaccharide portion and has three Pk binding sites per B monomer. The Stx2a subtype requires glycolipid residues in addition to Pk. We synthesized analogs of Pk to examine the binding preferences of Stx1 and Stx2 subtypes a to d. Furthermore, to determine how many binding sites must be engaged, the Pk analogues were conjugated to biotinylated mono- and biantennary platforms, allowing for the display of two to four Pk analogues per streptavidin molecule. Stx binding to Pk analogues immobilized on streptavidin-coated plates was assessed by enzyme-linked immunosorbent assay (ELISA). Stx1, but not the Stx2 subtypes, bound to native Pk. Stx2a and Stx2c bound to the Pk analog with a terminal GalNAc (NAc-Pk), while Stx1, Stx2b, and Stx2d did not bind to this analog. Interestingly, the purified Stx2d B subunit bound to NAc-Pk, suggesting that the A subunit of Stx2d interferes with binding. Disaccharide analogs (Galα1-4Gal, GalNAcα1-4Gal, and Galα1-4GalNAc) did not support the binding of any of the Stx forms, indicating that the trisaccharide is necessary for binding. Studies with monoantennary and biantennary analogs and mixtures suggest that Stx1, Stx2a, and Stx2c need to engage at least three Pk analogues for effective binding. To our knowledge, this is the first study examining the minimum number of Pk analogs required for effective binding and the first report documenting the role of the A subunit in influencing Stx2 binding.

Shiga toxins decrease enterohaemorrhagic Escherichia coli survival within Acanthamoeba castellanii

Enterohaemorrhagic Escherichia coli (EHEC) are zoonotic pathogens transmitted to humans through contaminated water or bovine products. One of the strategies used by pathogenic bacteria to survive in aquatic environments is using free-living amoebae as hosts. Acanthamoeba castellanii is an amoeba known to host several waterborne pathogens. This study investigates the survival of EHEC with A. castellanii, which could contribute to its spread and transmission to humans. We used a gentamicin protection assay as well as fluorescence and electron microscopy to monitor the intra-amoebae survival of EHEC O157:H7 over 24 h. The results showed that EHEC were able to survive within A. castellanii and that this survival was reduced by Shiga toxins (Stx) produced by EHEC. A toxic effect mediated by Stx was demonstrated by amoebae mortality and LDH release during co-culture of EHEC and amoeba. This work describes the ability of EHEC to survive within A. castellanii, and this host-pathogen interaction is partially controlled by the Stx. Thus, this ubiquitous amoeba could represent an environmental niche for EHEC survival and transmission.

Molecular and antimicrobial susceptibility analyses distinguish clinical from bovine Escherichia coli O157 strains

A population-based study combining (i) antimicrobial, (ii) genetic, and (iii) virulence analyses with molecular evolutionary analyses revealed segregative characteristics distinguishing human clinical and bovine Escherichia coli O157 strains from western Canada. Human (n = 50) and bovine (n = 50) strains of E. coli O157 were collected from Saskatchewan and Manitoba in 2006 and were analyzed by using the six-marker lineage-specific polymorphism assay (LSPA6), antimicrobial susceptibility analysis, the colicin assay, plasmid and virulence profiling including the eae, ehxA, espA, iha, stx1, stx2, stx2c, stx2d, stx2d-activatable, stx2e, and stx2f virulence-associated genes, and structure analyses. Multivariate logistic regression and Fisher's exact test strongly suggested that antimicrobial susceptibility was the most distinctive characteristic (P = 0.00487) associated with human strains. Among all genetic, virulence, and antimicrobial determinants, resistance to tetracycline (P < 0.000) and to sulfisoxazole (P < 0.009) were the most strongly associated segregative characteristics of bovine E. coli O157 strains. Among 11 virulence-associated genes, stx2c showed the strongest association with E. coli O157 strains of bovine origin. LSPA6 genotyping showed the dominance of the lineage I genotype among clinical (90%) and bovine (70%) strains, indicating the importance of lineage I in O157 epidemiology and ecology. Population structure analysis revealed that the more-diverse bovine strains came from a unique group of strains characterized by a high degree of antimicrobial resistance and high frequencies of lineage II genotypes and stx2c variants. These findings imply that antimicrobial resistance generated among bovine strains of E. coli O157 has a large impact on the population of this human pathogen.

Modulation of the enterohemorrhagic E. coli virulence program through the human gastrointestinal tract

Enteric pathogens must not only survive passage through the gastrointestinal tract but must also coordinate expression of virulence determinants in response to localized microenvironments with the host. Enterohemorrhagic Escherichia coli (EHEC), a serious food and waterborne human pathogen, is well equipped with an arsenal of molecular factors that allows it to survive passage through the gastrointestinal tract and successfully colonize the large intestine. This review will explore how EHEC responds to various environmental cues associated with particular microenvironments within the host and how it employs these cues to modulate virulence factor expression, with a view to developing a conceptual framework for understanding modulation of EHEC’s virulence program in response to the host. In vitro studies offer significant insights into the role of individual environmental cues but in vivo studies using animal models as well as data from natural infections will ultimately provide a more comprehensive picture of the highly regulated virulence program of this pathogen.

Prophage induction is enhanced and required for renal disease and lethality in an EHEC mouse model

Enterohemorrhagic Escherichia coli (EHEC), particularly serotype O157:H7, causes hemorrhagic colitis, hemolytic uremic syndrome, and even death. In vitro studies showed that Shiga toxin 2 (Stx2), the primary virulence factor expressed by EDL933 (an O157:H7 strain), is encoded by the 933W prophage. And the bacterial subpopulation in which the 933W prophage is induced is the producer of Stx2. Using the germ-free mouse, we show the essential role 933W induction plays in the virulence of EDL933 infection. An EDL933 derivative with a single mutation in its 933W prophage, resulting specifically in that phage being uninducible, colonizes the intestines, but fails to cause any of the pathological changes seen with the parent strain. Hence, induction of the 933W prophage is the primary event leading to disease from EDL933 infection. We constructed a derivative of EDL933, SIVET, with a biosensor that specifically measures induction of the 933W prophage. Using this biosensor to measure 933W induction in germ-free mice, we found an increase three logs greater than was expected from in vitro results. Since the induced population produces and releases Stx2, this result indicates that an activity in the intestine increases Stx2 production.

Facing glycosphingolipid-Shiga toxin interaction: dire straits for endothelial cells of the human vasculature

The two major Shiga toxin (Stx) types, Stx1 and Stx2, produced by enterohemorrhagic Escherichia coli (EHEC) in particular injure renal and cerebral microvascular endothelial cells after transfer from the human intestine into the circulation. Stxs are AB(5) toxins composed of an enzymatically active A subunit and the pentameric B subunit, which preferentially binds to the glycosphingolipid globotriaosylceramide (Gb3Cer/CD77). This review summarizes the current knowledge on Stx-caused cellular injury and the structural diversity of Stx receptors as well as the initial molecular interaction of Stxs with the human endothelium of different vascular beds. The varying lipoforms of Stx receptors and their spatial organization in lipid rafts suggest a central role in different modes of receptor-mediated endocytosis and intracellular destiny of the toxins. The design and development of tailored Stx neutralizers targeting the oligosaccharide-toxin recognition event has become a very real prospect to ameliorate or prevent life-threatening renal and neurological complications.

Modification in media composition to obtain secretory production of STxB-based vaccines using Escherichia coli

Shiga toxin B-subunit (STxB) from Shigella dysenteriae targets in vivo antigen to cancer cells, dendritic cells (DC) and B cells, which preferentially express the globotriaosylceramide (Gb3) receptor. This pivotal role has encouraged scientists to investigate fusing STxB with other clinical antigens. Due to the challenges of obtaining a functional soluble form of the recombinant STxB, such as formation of inclusion bodies during protein expression, scientists tend to combine STxB with vaccine candidates rather than using their genetically fused forms. In this work, we fused HPV16 E7 as a vaccine candidate to the recombinantly-produced STxB. To minimize the formation of inclusion bodies, we investigated a number of conditions during the expression procedure. Then various strategies were used in order to obtain high yield of soluble recombinant protein from E. coli which included the use of different host strains, reduction of cultivation temperature, as well as using different concentrations of IPTG and different additives (Glycin, Triton X-100, ZnCl(2)). Our study demonstrated the importance of optimizing incubation parameters for recombinant protein expression in E. coli; also showed that the secretion production can be achieved over the course of a few hours when using additives such as glycine and Triton X-100. Interestingly, it was shown that when the culture mediums were supplemented by additives, there was an inverse ratio between time of induction (TOI) and the level of secreted protein at lower temperatures. This study determines the optimal conditions for high yield soluble E7-STxB expression and subsequently facilitates reaching a functionally soluble form of STxB-based vaccines, which can be considered as a potent vaccine candidate for cervical cancer.

Comparison of three different methods for detection of Shiga toxin-producing Escherichia coli in a tertiary pediatric care center

Shiga toxin-producing Escherichia coli (STEC) is a well-known cause of sporadic and epidemic food-borne gastroenteritis. A low infectious dose, approximately 10 microorganisms, is sufficient to cause disease that may lead to hemolytic-uremic syndrome. The objective of this study was to compare the performances of an in-house real-time PCR, a commercial enzyme immunoassay (EIA) (Premier EHEC; Meridian Bioscience), and culture on sorbitol MacConkey agar for the detection of STEC in a tertiary care pediatric hospital. Of 632 stool samples tested, 21 were positive for STEC. All were detected by PCR, 6 were detected by EIA, and only 5 O157 STEC isolates were identified by culture. Among the 15 specimens falsely negative by EIA, there were 9 Stx1, 2 Stx2, and 4 Stx1 and Stx2 STEC isolates. The latter group included 2 O157 STEC isolates that would have been missed if only EIA had been performed. To our knowledge, this is the first prospective study performed in a pediatric hospital which demonstrates the superiority of PCR over EIA for the detection of STEC. We conclude that PCR is specific and more sensitive than EIA. PCR should be considered for routine use in clinical settings where molecular detection facilities are available. Its lower limit of detection, equivalent to the infectious dose, is an obvious advantage for patient care and public health surveillance.

Public health value of next-generation DNA sequencing of enterohemorrhagic Escherichia coli isolates from an outbreak

In 2009, an outbreak of enterohemorrhagic Escherichia coli (EHEC) on an open farm infected 93 persons, and approximately 22% of these individuals developed hemolytic-uremic syndrome (HUS). Genome sequencing was used to investigate outbreak-derived animal and human EHEC isolates. Phylogeny based on the whole-genome sequence was used to place outbreak isolates in the context of the overall E. coli species and the O157:H7 sequence type 11 (ST11) subgroup. Four informative single nucleotide polymorphisms (SNPs) were identified and used to design an assay to type 122 other outbreak isolates. The SNP phylogeny demonstrated that the outbreak strain was from a lineage distinct from previously reported O157:H7 ST11 EHEC and was not a member of the hypervirulent clade 8. The strain harbored determinants for two Stx2 verotoxins and other putative virulence factors. When linked to the epidemiological information, the sequence data indicate that gross contamination of a single outbreak strain occurred across the farm prior to the first clinical report of HUS. The most likely explanation for these results is that a single successful strain of EHEC spread from a single introduction through the farm by clonal expansion and that contamination of the environment (including the possible colonization of several animals) led ultimately to human cases.

A novel murine infection model for Shiga toxin-producing Escherichia coli

Enterohemorrhagic E. coli (EHEC) is an important subset of Shiga toxin-producing (Stx-producing) E. coli (STEC), pathogens that have been implicated in outbreaks of food-borne illness and can cause intestinal and systemic disease, including severe renal damage. Upon attachment to intestinal epithelium, EHEC generates "attaching and effacing" (AE) lesions characterized by intimate attachment and actin rearrangement upon host cell binding. Stx produced in the gut transverses the intestinal epithelium, causing vascular damage that leads to systemic disease. Models of EHEC infection in conventional mice do not manifest key features of disease, such as AE lesions, intestinal damage, and systemic illness. In order to develop an infection model that better reflects the pathogenesis of this subset of STEC, we constructed an Stx-producing strain of Citrobacter rodentium, a murine AE pathogen that otherwise lacks Stx. Mice infected with Stx-producing C. rodentium developed AE lesions on the intestinal epithelium and Stx-dependent intestinal inflammatory damage. Further, the mice experienced lethal infection characterized by histopathological and functional kidney damage. The development of a murine model that encompasses AE lesion formation and Stx-mediated tissue damage will provide a new platform upon which to identify EHEC alterations of host epithelium that contribute to systemic disease.

Enterohemorrhagic E. coli (EHEC) pathogenesis

Enterohemorrhagic Escherichia coli (EHEC) serotype O157:H7 is a human pathogen responsible for outbreaks of bloody diarrhea and hemolytic uremic syndrome (HUS) worldwide. Conventional antimicrobials trigger an SOS response in EHEC that promotes the release of the potent Shiga toxin that is responsible for much of the morbidity and mortality associated with EHEC infection. Cattle are a natural reservoir of EHEC, and approximately 75% of EHEC outbreaks are linked to the consumption of contaminated bovine-derived products. This review will discuss how EHEC causes disease in humans but is asymptomatic in adult ruminants. It will also analyze factors utilized by EHEC as it travels through the bovine gastrointestinal (GI) tract that allow for its survival through the acidic environment of the distal stomachs, and for its ultimate colonization in the recto-anal junction (RAJ). Understanding the factors crucial for EHEC survival and colonization in cattle will aid in the development of alternative strategies to prevent EHEC shedding into the environment and consequent human infection.

A single-step purification and molecular characterization of functional Shiga toxin 2 variants from pathogenic Escherichia coli

A one-step affinity chromatography method was developed to purify Shiga toxin 2 variants (Stx2) Stx2a, Stx2c, Stx2d and Stx2g from bacterial culture supernatants. Analysis of the purified Stx2 variants by denaturing gel electrophoresis revealed 32 kDa and 7 kDa protein bands, corresponding to the Stx2A- and B-subunits, respectively. However, native gel electrophoresis indicated that purified Stx2c and Stx2d were significantly higher in molecular weight than Stx2a and Stx2g. In a cytotoxicity assay with Hela cells, the 50% cytotoxic dose of Stx2a and Stx2g were 100 pg and 10 pg, respectively, but 1 ng each for Stx2c and Stx2d. Interestingly, analysis of the 50% inhibitory dose in a cell-free translational system from rabbit reticulocyte lysates indicated that Stx2g had a lower capacity to inhibit protein synthesis than the other Stx2 variants. The cytotoxicities in Hela cells were neutralized with an anti-Stx2B antibody and were denatured at 80 °C for 1 h. These findings demonstrated that Stx2 variants exhibited different toxicities, holotoxin structure, and stabilities using distinct systems for assessing toxin activities. The development of a simple method for purification of Stx2 variants will enable further studies of Stx2-mediated toxicity in various model systems.

Regulation of cytokine and chemokine expression by the ribotoxic stress response elicited by Shiga toxin type 1 in human macrophage-like THP-1 cells

Shiga toxins (Stxs) are cytotoxins produced by the enteric pathogens Shigella dysenteriae serotype 1 and Shiga toxin-producing Escherichia coli (STEC). Stxs bind to a membrane glycolipid receptor, enter cells, and undergo retrograde transport to ultimately reach the cytosol, where the toxins exert their protein synthesis-inhibitory activity by depurination of a single adenine residue from the 28S rRNA component of eukaryotic ribosomes. The depurination reaction activates the ribotoxic stress response, leading to signaling via the mitogen-activated protein kinase (MAPK) pathways (Jun N-terminal protein kinase [JNK], p38, and extracellular signal-regulated kinase [ERK]) in human epithelial, endothelial, and myeloid cells. We previously showed that treatment of human macrophage-like THP-1 cells with Stxs resulted in increased cytokine and chemokine expression. In the present study, we show that individual inactivation of ERK, JNK, and p38 MAPKs using pharmacological inhibitors in the presence of Stx1 resulted in differential regulation of the cytokines tumor necrosis factor alpha and interleukin-1β (IL-1β) and chemokines IL-8, growth-regulated protein-β, macrophage inflammatory protein-1α (MIP-1α), and MIP-1β. THP-1 cells exposed to Stx1 upregulate the expression of select dual-specificity phosphatases (DUSPs), enzymes that dephosphorylate and inactivate MAPKs in mammalian cells. In this study, we confirmed DUSP1 protein production by THP-1 cells treated with Stx1. DUSP1 inhibition by triptolide showed that ERK and p38 phosphorylation is regulated by DUSP1, while JNK phosphorylation is not. Inhibition of p38 MAPK signaling blocked the ability of Stx1 to induce DUSP1 mRNA expression, suggesting that an autoregulatory signaling loop may be activated by Stxs. Thus, Stxs appear to be capable of eliciting signals which both activate and deactivate signaling for increased cytokine/chemokine production in human macrophage-like cells.

Shiga toxin-producing Escherichia coli O104:H4: a new challenge for microbiology

In 2011, Germany experienced the largest outbreak with a Shiga toxin-producing Escherichia coli (STEC) strain ever recorded. A series of environmental and trace-back and trace-forward investigations linked sprout consumption with the disease, but fecal-oral transmission was also documented. The genome sequences of the pathogen revealed a clonal outbreak with enteroaggregative E. coli (EAEC). Some EAEC virulence factors are carried on the virulence plasmid pAA. From an unknown source, the epidemic strains acquired a lambdoid prophage carrying the gene for the Shiga toxin. The resulting strains therefore possess two different mobile elements, a phage and a plasmid, contributing essential virulence genes. Shiga toxin is released by decaying bacteria in the gut, migrates through the intestinal barrier, and is transported via the blood to target organs, like the kidney. In a mouse model, probiotic bifidobacteria interfered with transport of the toxin through the gut mucosa. Researchers explored bacteriophages, bacteriocins, and low-molecular-weight inhibitors against STEC. Randomized controlled clinical trials of enterohemorrhagic E. coli (EHEC)-associated hemolytic uremic syndrome (HUS) patients found none of the interventions superior to supportive therapy alone. Antibodies against one subtype of Shiga toxin protected pigs against fatal neurological infection, while treatment with a toxin receptor decoy showed no effect in a clinical trial. Likewise, a monoclonal antibody directed against a complement protein led to mixed results. Plasma exchange and IgG immunoadsoprtion ameliorated the condition in small uncontrolled trials. The epidemic O104:H4 strains were resistant to all penicillins and cephalosporins but susceptible to carbapenems, which were recommended for treatment.

Structure-dependent pseudoreceptor intracellular traffic of adamantyl globotriaosyl ceramide mimics

The verotoxin (VT) (Shiga toxin) receptor globotriaosyl ceramide (Gb(3)), mediates VT1/VT2 retrograde transport to the endoplasmic reticulum (ER) for cytosolic A subunit access to inhibit protein synthesis. Adamantyl Gb(3) is an amphipathic competitive inhibitor of VT1/VT2 Gb(3) binding. However, Gb(3)-negative VT-resistant CHO/Jurkat cells incorporate adaGb(3) to become VT1/VT2-sensitive. CarboxyadaGb(3), urea-adaGb(3), and hydroxyethyl adaGb(3), preferentially bound by VT2, also mediate VT1/VT2 cytotoxicity. VT1/VT2 internalize to early endosomes but not to Golgi/ER. AdabisGb(3) (two deacyl Gb(3)s linked to adamantane) protects against VT1/VT2 more effectively than adaGb(3) without incorporating into Gb(3)-negative cells. AdaGb(3) (but not hydroxyethyl adaGb(3)) incorporation into Gb(3)-positive Vero cells rendered punctate cell surface VT1/VT2 binding uniform and subverted subsequent Gb(3)-dependent retrograde transport to Golgi/ER to render cytotoxicity (reduced for VT1 but not VT2) brefeldin A-resistant. VT2-induced vacuolation was maintained in adaGb(3)-treated Vero cells, but vacuolar membrane VT2 was lost. AdaGb(3) destabilized membrane cholesterol and reduced Gb(3) cholesterol stabilization in phospholipid liposomes. Cholera toxin GM1-mediated Golgi/ER targeting was unaffected by adaGb(3). We demonstrate the novel, lipid-dependent, pseudoreceptor function of Gb(3) mimics and their structure-dependent modulation of endogenous intracellular Gb(3) vesicular traffic.

Interaction of ricin and Shiga toxins with ribosomes

Ricin and Shiga toxins designated as ribosome inactivating proteins (RIPs) are RNA N-glycosidases that depurinate a specific adenine (A₄₃₂₄ in rat 28S rRNA) in the conserved α-sarcin/ricin loop of the large rRNA, inhibiting protein synthesis. Evidence obtained from a number of studies suggests that interaction with ribosomal proteins plays an important role in the catalytic activity and ribosome specificity of RIPs. This review summarizes the recent developments in identification of the ribosomal proteins that interact with ricin and Shiga toxins and the principles governing these interactions.

Specific egg yolk immunoglobulin as a new preventive approach for Shiga-toxin-mediated diseases

Shiga toxins (Stxs) are involved in the development of severe systemic complications associated with enterohemorrhagic Escherichia coli (EHEC) infection. Various neutralizing agents against Stxs are under investigation for management of EHEC infection. In this study, we immunized chickens with formalin-inactivated Stx-1 or Stx-2, and obtained immunoglobulin Y (IgY) from the egg yolk. Anti-Stx-1 IgY and anti-Stx-2 IgY recognized the corresponding Stx A subunit and polymeric but not monomeric B subunit. Anti-Stx-1 IgY and anti-Stx-2 IgY suppressed the cytotoxicity of Stx-1 and Stx-2 to HeLa 229 cells, without cross-suppressive activity. The suppressive activity of these IgY was abrogated by pre-incubation with the corresponding recombinant B subunit, which suggests that the antibodies directed to the polymeric B subunits were predominantly involved in the suppression. In vivo, the intraperitoneal or intravenous administration of these IgY rescued mice from death caused by intraperitoneal injection of the corresponding toxin at a lethal dose. Moreover, oral administration of anti-Stx-2 IgY reduced the mortality of mice infected intestinally with EHEC O157:H7. Our results therefore suggest that anti-Stx IgY antibodies may be considered as preventive agents for Stx-mediated diseases in EHEC infection.

An Escherichia coli O157-specific engineered pyocin prevents and ameliorates infection by E. coli O157:H7 in an animal model of diarrheal disease

AvR2-V10.3 is an engineered R-type pyocin that specifically kills Escherichia coli O157, an enteric pathogen that is a major cause of food-borne diarrheal disease. New therapeutics to counteract E. coli O157 are needed, as currently available antibiotics can exacerbate the consequences of infection. We show here that orogastric administration of AvR2-V10.3 can prevent or ameliorate E. coli O157:H7-induced diarrhea and intestinal inflammation in an infant rabbit model of infection when the compound is administered either in a postexposure prophylactic regimen or after the onset of symptoms. Notably, administration of AvR2-V10.3 also reduces bacterial carriage and fecal shedding of this pathogen. Our findings support the further development of pathogen-specific R-type pyocins as a way to treat enteric infections.