Cloning of genes for production of Escherichia coli Shiga-like toxin type II

Investigation of the Causes of Shigatoxigenic Escherichia coli PCR Positive and Culture Negative Samples

Molecular methods may reveal the presence of pathogens in samples through the detection of specific target gene(s) associated with microorganisms, but often, the subsequent cultural isolation of the pathogen is not possible. This discrepancy may be related to low concentration of the cells, presence of dead cells, competitive microflora, injured cells and cells in a viable but non-culturable state, free DNA and the presence of free bacteriophages which can carry the target gene causing the PCR-positive/culture-negative results. Shiga-toxigenic Escherichia coli (STEC) was used as a model for studying this phenomenon, based on the phage-encoded cytotoxins genes (Stx family) as the detection target in samples through real-time qPCR. Stx phages can be integrated in the STEC chromosome or can be isolated as free particles in the environment. In this study, a combination of PCR with culturing was used for investigating the presence of the stx1 and stx2 genes in 155 ovine recto-anal junction swab samples (method (a)-PCR). Samples which were PCR-positive and culture-negative were subjected to additional analyses including detection of dead STEC cells (method (b)-PCR-PMA dye assay), presence of Stx phages (method (c)-plaque assays) and inducible integrated phages (method (d)-phage induction). Method (a) showed that even though 121 samples gave a PCR-positive result (78%), only 68 samples yielded a culturable isolate (43.9%). Among the 53 (34.2%) PCR-positive/culture-negative samples, 21 (39.6%) samples were shown to have STEC dead cells only, eight (15.1%) had a combination of dead cells and inducible stx phage, while two samples (3.8%) had a combination of dead cells, inducible phage and free stx phage, and a further two samples had Stx1 free phages only (3.8%). It was thus possible to reduce the samples with no explanation to 20 (37.7% of 53 samples), representing a further step towards an improved understanding of the STEC PCR-positive/culture-negative phenomenon.

Postinfectious Hemolytic Uremic Syndrome

Post-infectious hemolytic uremic syndrome (HUS) is caused by specific pathogens in patients with no identifiable HUS-associated genetic mutation or autoantibody. The majority of episodes is due to infections by Shiga toxin (Stx) producing Escherichia coli (STEC). This chapter reviews the epidemiology and pathogenesis of STEC-HUS, including bacterial-derived factors and host responses. STEC disease is characterized by hematological (microangiopathic hemolytic anemia), renal (acute kidney injury) and extrarenal organ involvement. Clinicians should always strive for an etiological diagnosis through the microbiological or molecular identification of Stx-producing bacteria and Stx or, if negative, serological assays. Treatment of STEC-HUS is supportive; more investigations are needed to evaluate the efficacy of putative preventive and therapeutic measures, such as non-phage-inducing antibiotics, volume expansion and anti-complement agents. The outcome of STEC-HUS is generally favorable, but chronic kidney disease, permanent extrarenal, mainly cerebral complication and death (in less than 5 %) occur and long-term follow-up is recommended. The remainder of this chapter highlights rarer forms of (post-infectious) HUS due to S. dysenteriae, S. pneumoniae, influenza A and HIV and discusses potential interactions between these pathogens and the complement system.

Clinical isolates of Shiga toxin 1a-producing Shigella flexneri with an epidemiological link to recent travel to Hispañiola

Shiga toxins (Stx) are cytotoxins involved in severe human intestinal disease. These toxins are commonly found in Shigella dysenteriae serotype 1 and Shiga-toxin-producing Escherichia coli; however, the toxin genes have been found in other Shigella species. We identified 26 Shigella flexneri serotype 2 strains isolated by public health laboratories in the United States during 2001-2013, which encode the Shiga toxin 1a gene (stx1a). These strains produced and released Stx1a as measured by cytotoxicity and neutralization assays using anti-Stx/Stx1a antiserum. The release of Stx1a into culture supernatants increased ≈100-fold after treatment with mitomycin C, suggesting that stx1a is carried by a bacteriophage. Infectious phage were found in culture supernatants and increased ≈1,000-fold with mitomycin C. Whole-genome sequencing of several isolates and PCR analyses of all strains confirmed that stx1a was carried by a lambdoid bacteriophage. Furthermore, all patients who reported foreign travel had recently been to Hispañiola, suggesting that emergence of these novel strains is associated with that region.

Prevalence of Shiga toxin-producing Shigella species isolated from French travellers returning from the Caribbean: an emerging pathogen with international implications

Shiga toxins (Stxs) are potent cytotoxins that inhibit host cell protein synthesis, leading to cell death. Classically, these toxins are associated with intestinal infections due to Stx-producing Escherichia coli or Shigella dysenteriae serotype 1, and infections with these strains can lead to haemolytic-uraemic syndrome. Over the past decade, there has been increasing recognition that Stx is produced by additional Shigella species. We recently reported the presence and expression of stx genes in Shigella flexneri 2a clinical isolates. The toxin genes were carried by a new stx-encoding bacteriophage, and infection with these strains correlated with recent travel to Haiti or the Dominican Republic. In this study, we further explored the epidemiological link to this region by utilizing the French National Reference Centre for Escherichia coli, Shigella and Salmonella collection to survey the frequency of Stx-producing Shigella species isolated from French travellers returning from the Caribbean. Approximately 21% of the isolates tested were found to encode and produce Stx. These isolates included strains of S. flexneri 2a, S. flexneri Y, and S. dysenteriae 4. All of the travellers who were infected with Stx-producing Shigella had recently travelled to Haiti, the Dominican Republic, or French Guiana. Furthermore, whole genome sequencing showed that the toxin genes were encoded by a prophage that was highly identical to the phage that we identified in our previous study. These findings demonstrate that this new stx-encoding prophage is circulating within that geographical area, has spread to other continents, and is capable of spreading to multiple Shigella serogroups.

Bacteria-phage interactions in natural environments

Phages are considered the most abundant and diverse biological entities on Earth and are notable not only for their sheer abundance, but also for their influence on bacterial hosts. In nature, bacteria-phage relationships are complex and have far-reaching consequences beyond particular pairwise interactions, influencing everything from bacterial virulence to eukaryotic fitness to the carbon cycle. In this review, we examine bacteria and phage distributions in nature first by highlighting biogeographic patterns and nonhost environmental influences on phage distribution, then by considering the ways in which phages and bacteria interact, emphasizing phage life cycles, bacterial responses to phage infection, and the complex patterns of phage host specificity. Finally, we discuss phage impacts on bacterial abundance, genetics, and physiology, and further aim to clarify distinctions between current theoretical models and point out areas in need of future research.

Shiga-toxin-producing Escherichia coli and haemolytic uraemic syndrome

Most cases of diarrhoea-associated haemolytic uraemic syndrome (HUS) are caused by Shiga-toxin-producing bacteria; the pathophysiology differs from that of thrombotic thrombocytopenic purpura. Among Shiga-toxin-producing Escherichia coli (STEC), O157:H7 has the strongest association worldwide with HUS. Many different vehicles, in addition to the commonly suspected ground (minced) beef, can transmit this pathogen to people. Antibiotics, antimotility agents, narcotics, and non-steroidal anti-inflammatory drugs should not be given to acutely infected patients, and we advise hospital admission and administration of intravenous fluids. Management of HUS remains supportive; there are no specific therapies to ameliorate the course. The vascular injury leading to HUS is likely to be well under way by the time infected patients seek medical attention for diarrhoea. The best way to prevent HUS is to prevent primary infection with Shiga-toxin-producing bacteria.

Escherichia coli O157:H7 Shiga toxin-encoding bacteriophages: integrations, excisions, truncations, and evolutionary implications

As it descended from Escherichia coli O55:H7, Shiga toxin (Stx)-producing E. coli (STEC) O157:H7 is believed to have acquired, in sequence, a bacteriophage encoding Stx2 and another encoding Stx1. Between these events, sorbitol-fermenting E. coli O157:H(-) presumably diverged from this clade. We employed PCR and sequence analyses to investigate sites of bacteriophage integration into the chromosome, using evolutionarily informative STEC to trace the sequence of acquisition of elements encoding Stx. Contrary to expectations from the two currently sequenced strains, truncated bacteriophages occupy yehV in almost all E. coli O157:H7 strains that lack stx(1) (stx(1)-negative strains). Two truncated variants were determined to contain either GTT or TGACTGTT sequence, in lieu of 20,214 or 18,895 bp, respectively, of the bacteriophage central region. A single-nucleotide polymorphism in the latter variant suggests that recombination in that element extended beyond the inserted octamer. An stx(2) bacteriophage usually occupies wrbA in stx(1)(+)/stx(2)(+) E. coli O157:H7, but wrbA is unexpectedly unoccupied in most stx(1)-negative/stx(2)(+) E. coli O157:H7 strains, the presumed progenitors of stx(1)(+)/stx(2)(+) E. coli O157:H7. Trimethoprim-sulfamethoxazole promotes the excision of all, and ciprofloxacin and fosfomycin significantly promote the excision of a subset of complete and truncated stx bacteriophages from the E. coli O157:H7 strains tested; bile salts usually attenuate excision. These data demonstrate the unexpected diversity of the chromosomal architecture of E. coli O157:H7 (with novel truncated bacteriophages and multiple stx(2) bacteriophage insertion sites), suggest that stx(1) acquisition might be a multistep process, and compel the consideration of multiple exogenous factors, including antibiotics and bile, when chromosome stability is examined.

Genetic profiling of enterohemorrhagic Escherichia coli strains in relation to clonality and clinical signs of infection

Sixty-seven human strains of enterohemorrhagic Escherichia coli (EHEC) (from patients with more or less severe symptoms) were serogrouped and arranged according to pulsed-field gel electrophoresis (PFGE) patterns. We used PCR to investigate the strains according to known or putative virulence factors, and associations with disease were studied. All EHEC strains with the same PFGE pattern belonged to the same serogroup. On the contrary, two serogroups (O157 and O8) included strains with different PFGE patterns. We found several different combinations of chromosomal and plasmid-borne determinants, encoding the putative virulence factors, among the strains. As judged from clinical symptoms, there was no marked difference in pathogenicity among the strains and their combinations of virulence traits. All strains of O157 had the genes coding for verocytotoxin (VT) 2, intimin (eaeA), E. coli hemolysin (E-hly), and secreted serine protease (espP). Among EHEC non-O157 strains, the genes coding for VT1 and VT2 were equally dispersed. EaeA positivity was just as common among VT1- as VT2-positive strains. Among the plasmid-borne determinants, E-hly and espP were the most common and E-hly might be a pathogenicity marker among EHEC non-O157 strains. The conclusion is that PFGE is a very useful tool in epidemiological studies. The EHEC plasmids are heterogeneous in their gene composition, with the four plasmid-borne determinants found in many combinations. There was no reliable correlation between chromosomal and plasmid-borne virulence factors and human disease.

Genetic profiling of enterohemorrhagic Escherichia coli strains in relation to clonality and clinical signs of infection

Sixty-seven human strains of enterohemorrhagic Escherichia coli (EHEC) (from patients with more or less severe symptoms) were serogrouped and arranged according to pulsed-field gel electrophoresis (PFGE) patterns. We used PCR to investigate the strains according to known or putative virulence factors, and associations with disease were studied. All EHEC strains with the same PFGE pattern belonged to the same serogroup. On the contrary, two serogroups (O157 and O8) included strains with different PFGE patterns. We found several different combinations of chromosomal and plasmid-borne determinants, encoding the putative virulence factors, among the strains. As judged from clinical symptoms, there was no marked difference in pathogenicity among the strains and their combinations of virulence traits. All strains of O157 had the genes coding for verocytotoxin (VT) 2, intimin (eaeA), E. coli hemolysin (E-hly), and secreted serine protease (espP). Among EHEC non-O157 strains, the genes coding for VT1 and VT2 were equally dispersed. EaeA positivity was just as common among VT1- as VT2-positive strains. Among the plasmid-borne determinants, E-hly and espP were the most common and E-hly might be a pathogenicity marker among EHEC non-O157 strains. The conclusion is that PFGE is a very useful tool in epidemiological studies. The EHEC plasmids are heterogeneous in their gene composition, with the four plasmid-borne determinants found in many combinations. There was no reliable correlation between chromosomal and plasmid-borne virulence factors and human disease.

Human neutrophils and their products induce Shiga toxin production by enterohemorrhagic Escherichia coli

The Shiga toxins (Stx) are critical virulence factors for Escherichia coli O157:H7 and other serotypes of enterohemorrhagic E. coli (EHEC). These potent toxins are encoded in the genomes of temperate lambdoid bacteriophages. We recently demonstrated that induction of the resident Stx2-encoding prophage in an O157:H7 clinical isolate is required for toxin production by this strain. Since several factors produced by human cells, including hydrogen peroxide (H2O2), are capable of inducing lambdoid prophages, we hypothesized that such molecules might also induce toxin production by EHEC. Here, we studied whether H2O2 and also human neutrophils, an important endogenous source of H2O2, induced Stx2 expression by an EHEC clinical isolate. Both H2O2 and neutrophils were found to augment Stx2 production, raising the possibility that these agents may lead to prophage induction in vivo and thereby contribute to EHEC pathogenesis.

A genomic island, termed high-pathogenicity island, is present in certain non-O157 Shiga toxin-producing Escherichia coli clonal lineages

Shiga toxin-producing Escherichia coli (STEC) strains cause a wide spectrum of diseases in humans. In this study, we tested 206 STEC strains isolated from patients for potential virulence genes including stx, eae, and enterohemorrhagic E. coli hly. In addition, all strains were examined for the presence of another genetic element, the high-pathogenicity island (HPI). The HPI was first described in pathogenic Yersinia species and encodes the pesticin receptor FyuA and the siderophore yersiniabactin. The HPI was found in the genome of distinct clonal lineages of STEC, including all 31 eae-positive O26:H11/H(-) strains and 7 of 12 eae-negative O128:H2/H(-) strains. In total, the HPI was found in 56 (27.2%) of 206 STEC strains. However, it was absent from the genome of all 37 O157:H7/H(-), 14 O111:H(-), 13 O103:H2, and 13 O145:H(-) STEC isolates, all of which were positive for eae. Polypeptides encoded by the fyuA gene located on the HPI could be detected by using immunoblot analysis in most of the HPI-positive STEC strains, suggesting the presence of a functional yersiniabactin system. The HPI in STEC was located next to the tRNA gene asnT. In contrast to the HPI of other pathogenic enterobacteria, the HPI of O26 STEC strains shows a deletion at its left junction, leading to a truncated integrase gene int. We conclude from this study that the Yersinia HPI is disseminated among certain clonal subgroups of STEC strains. The hypothesis that the HPI in STEC contributes to the fitness of the strains in certain ecological niches rather than to their pathogenic potential is discussed.

Regulation of the Shiga-like toxin II operon in Escherichia coli

Investigations of the regulation of the bacteriophage-encoded Shiga-like toxin II (SLT-II) in Escherichia coli demonstrated that bacteriophages exhibit a regulatory impact on toxin production by two mechanisms. Firstly, replication of the toxin-converting bacteriophages brings about an increase in toxin production due to concomitant multiplication of toxin gene copies. Secondly, an influence of a phage-encoded regulatory molecule was demonstrated by using low-copy-number plasmid pADR-28, carrying a translational gene fusion between the promoter and proximal portion of slt-IIA and the structural gene for bacterial alkaline phosphatase (phoA). PhoA activity, reflecting the slt-II promoter activity, was significantly enhanced in E. coli strains which and been lysogenized with an SLT-I or SLT-II-converting bacteriophage (H-19B or 933W, respectively) or bacteriophage lambda. Both mechanisms are dependent on bacteriophage induction and hence are recA dependent. Moreover, the study revealed that the DNA-binding protein H-NS has a regulatory impact on both bacteriophage-mediated SLT-II synthesis and the activity of the slt-II promoter of plasmid pADR-28. While a slight impact of growth temperature on SLT-II expression was observed, no impact of either osmolarity, pH, oxygen tension, acetates, iron level, or utilized carbon source could be demonstrated.

Gene toxin patterns of Escherichia coli isolated from diseased and healthy piglets

DNA probes specific for genes coding for the heat stable enterotoxins (ST-I and ST-II), the heat labile enterotoxins (LT-I and LT-II), Shiga like cytotoxins and for the enterohemorrhagic factor (EHF), were used to examined 150 fecal Escherichia coli isolates from both diarrheic and healthy piglets. Thirty seven percent of the isolates hybridized with the LT-I probe, seventy one of them did so with the LT-II probe, while seventy six percent of the isolates possessed genes coding for the heat stable enterotoxins. No SLT-I positive hybridization was found and four percent of the isolates possessed SLT-II genes.

Comparison of the relative toxicities of Shiga-like toxins type I and type II for mice

In earlier studies using a streptomycin-treated mouse model of infection caused by enterohemorrhagic Escherichia coli (EHEC), animals fed Shiga-like toxin type II (SLT-II)-producing strains developed acute renal cortical necrosis and died, while mice fed Shiga-like toxin type I (SLT-I)-producing clones did not die (E. A. Wadolkowski, L. M. Sung, J. A. Burris, J. E. Samuel, and A. D. O'Brien, Infect. Immun. 58:3959-3965, 1990). To examine the bases for the differences we noted between the two toxins in the murine infection model, we injected mice with purified toxins and carried out histopathological examinations. Despite the genetic and structural similarities between the two toxins, SLT-II had a 50% lethal dose (LD50) which was approximately 400 times lower than that of SLT-I when injected intravenously or intraperitoneally into mice. Histopathologic examination of toxin-injected mice revealed that detectable damage was limited to renal cortical tubule epithelial cells. Passive administration of anti-SLT-II antibodies protected mice from SLT-II-mediated kidney damage and death. Immunofluorescence staining of normal murine kidney sections incubated with purified SLT-I or SLT-II demonstrated that both toxins bound to cortical tubule and medullary duct epithelial cells. Compared with SLT-I, SLT-II was more heat and pH stable, suggesting that SLT-II is a relatively more stable macromolecule. Although both toxins bound to globotriaosylceramide, SLT-I bound with a higher affinity in a solid-phase binding assay. Differences in enzymatic activity between the two toxins were not detected. These data suggest that structural/functional differences between the two toxins, possibly involving holotoxin stability and/or receptor affinity, may contribute to the differential LD50s in mice.

Cloning and sequencing of two new Verotoxin 2 variant genes of Escherichia coli isolated from cases of human and bovine diarrhea

We cloned and sequenced two new Verotoxin 2 (VT2) variant genes: one from an Escherichia coli strain from a case of bovine diarrhea and the other from an E. coli strain from a patient with diarrhea. The nucleotide and amino acid sequences of these two genes were highly homologous with, but distinct from those of the VT2, VT2vha, VT2vhb, SLT-IIv (VT2vp1) and SLT-IIva (VT2vp2) genes. Their nucleotide sequences were much more closely homologous to that of VT2vh than to that of VT2vp. Search for these two new genes in other Verocytotoxin-producing E. coli strains resulted in the isolation of 2 strains carrying one of the new VT2 variant genes, one strain from Tokyo and the other from Canada.

Detection of virulence factors in culturable Escherichia coli isolates from water samples by DNA probes and recovery of toxin-bearing strains in minimal o-nitrophenol-beta-D-galactopyranoside-4-methylumbelliferyl-beta-D-g luc uronide media

A total of 449 Escherichia coli isolates in treated and raw water sources were submitted to DNA-DNA hybridization using seven different DNA probes to detect homology to sequences that code for Shiga-like toxins I and II; heat-stabile and heat-labile toxins, adherence factors EAF and eae, and the fimbrial antigen of entero-hemorrhagic E. coli. Fifty-nine (13%) of the isolates demonstrated homology with one or more specific DNA probes. More than 50% of the isolates in treated water were not recovered in MMO-4-methylumbelliferyl-beta-D-glucuronide media designed for detection of this indicator.

Serotype O157:H7 Escherichia coli from bovine and meat sources

Serotype O157:H7 Escherichia coli strains from several different bovine and meat (beef) sources were studied to determine the diversity of their virulence properties and to compare their plasmid characteristics. Eighteen strains from cattle feces, 2 from water buffalo feces, 3 from beef samples, and 2 from feces of human hemolytic uremic syndrome cases were examined. All of these strains hybridized with the CVD419 DNA probe which identifies serotype O157:H7 and many other serotypes of verocytotoxin-producing E. coli. Of 15 bovine strains that hybridized with two verocytotoxin DNA probes, 8 hybridized with both verocytotoxin 1 (VT1) and VT2 probes, 5 hybridized with only the VT2 probe, and 2 hybridized with only the VT1 probe. This distribution was similar to that reported for O157:H7 E. coli isolated from humans. All three beef isolates hybridized with both VT1 and VT2 probes. All strains that hybridized with the VT probes were positive in the verocytotoxin assay, and all probe-negative strains were negative in the assay. All the strains possessed large plasmids with molecular sizes ranging from 53 to 64 MDa. Fifteen of the 20 cattle and water buffalo strains had one or more additional small plasmids. Restriction patterns resulting from HindIII, SmaI, and BamHI digestions of the large plasmids were used to compare all possible pairs of five different single plasmid-bearing strains from different countries (Egypt, England, and the United States). The restriction patterns of these strains were distinct, and the mean coefficients of similarity for these comparisons ranged from 71 to 91%, indicating a moderate degree of genetic diversity. This diversity and the presence of multiple plasmids in many bovine and human O157:H7 strains reinforce the usefulness of plasmid analysis in future studies. Only four of the 20 bovine strains and 1 of the 3 beef strains possessed the capability for adherence to HEp-2 and Intestine 407 cells in the presence of mannose, indicating that in vitro expression of localized adherence is not a universal property of O157:H7 strains of bovine origin.

Two copies of Shiga-like toxin II-related genes common in enterohemorrhagic Escherichia coli strains are responsible for the antigenic heterogeneity of the O157:H- strain E32511

Thirty-two clinical isolates of Shiga-like toxin (SLT)-producing Escherichia coli associated with single cases or outbreaks of bloody diarrhea, hemorrhagic colitis, the hemolytic uremic syndrome, or edema disease of swine were examined for multiple copies of genes belonging to the slt-I or slt-II toxin families. Five of 19 strains that were known to produce SLT-II or to hybridize to slt-II-specific probes by colony blot were found by Southern hybridization to contain two copies of toxin genes related to slt-II. The genes for two toxins closely related to slt-II were cloned from one of the isolates, Escherichia coli O157:H- strain E32511. One copy of the operon was found to be essentially identical to slt-II; it differed from slt-II by only one nucleotide base. This single nucleotide difference did not affect the predicted amino acid sequence. The predicted amino acid sequence of the A subunit of the second operon was identical to that of SLT-II, but the predicted amino acid sequence of the B subunit was identical to that of the B2F1 toxin VT2ha. We designated this second operon slt-IIc. Neutralization assays using several monoclonal antibodies and polyclonal antiserum prepared against SLT-II showed that SLT-IIc was antigenically related to but distinct from SLT-II.

Mapping the minimal contiguous gene segment that encodes functionally active Shiga-like toxin II

Shiga-like toxin type II (SLT-II) is one of two antigenically distinct cytotoxins produced by enterohemorrhagic Escherichia coli that are believed to play a central role in the pathogenesis of enterohemorrhagic E. coli-induced disease. SLT-II is a bipartite toxin with an enzymatically active A subunit that inhibits protein synthesis and an oligomeric B subunit that binds to the glycolipid globotriaosylceramide on eukaryotic cells. In this study, functional boundaries of the slt-II operon were mapped. Mutant proteins lacking the last four amino acids from the carboxy terminus of the 70-amino-acid mature SLT-II B polypeptide had no cytotoxic activity. However, when only two amino acids were removed from the carboxy terminus of the B subunit, the cytotoxic activity of the holotoxin was not altered drastically. Furthermore, a 21-amino-acid extension to the carboxy terminus of the SLT-II B polypeptide was tolerated with a minimum reduction in cytotoxic activity of the holotoxin. Deletion of the region coding for amino acids 3 through 18 of the 296-amino-acid mature SLT-II A polypeptide resulted in complete ablation of the cytotoxic activity of the holotoxin as well as abolition of the enzymatic activity of the A subunit. Thus, it appears that both 5'- and 3'-terminal coding sequences are essential for function of the slt-II operon.

Identification of three amino acid residues in the B subunit of Shiga toxin and Shiga-like toxin type II that are essential for holotoxin activity

Shiga toxin of Shigella dysenteriae type I and Shiga-like toxins I and II (SLT-I and SLT-II, respectively) of enterohemorrhagic Escherichia coli are functionally similar protein cytotoxins. These toxin molecules have a bipartite molecular structure which consists of an enzymatically active A subunit that inhibits protein synthesis in eukaryotic cells and an oligomeric B subunit that binds to globotriaosylceramide glycolipid receptors on eukaryotic cells. Regionally directed chemical mutagenesis of the B subunit of SLT-II was used to identify amino acids in the B subunit that are critical for SLT-II holotoxin cytotoxic activity. Three noncytotoxic mutants were isolated, and their mutations were mapped. The substitutions of arginine with cysteine at codon 32, alanine with threonine at codon 42, and glycine with aspartic acid at codon 59 in the 70-amino-acid mature SLT-II B polypeptide resulted in the complete abolition of cytotoxicity. The analogous arginine, alanine, and glycine residues were conserved at codons 33, 43, and 60 in the 69-amino-acid mature B polypeptide of Shiga toxin. Comparable mutations induced in the B-subunit gene of Shiga toxin by oligonucleotide-directed, site-specific mutagenesis resulted in drastically decreased cytotoxicity (10(3)- to 10(6)-fold) as compared with that of wild-type Shiga toxin. The mutant SLT-II and Shiga toxin B subunits were characterized for stability, receptor binding, immunoreactivity, and ability to be assembled into holotoxin.

Acute renal tubular necrosis and death of mice orally infected with Escherichia coli strains that produce Shiga-like toxin type II

Escherichia coli O157:H7 strains have been implicated as etiologic agents in food-borne outbreaks of hemorrhagic colitis and the hemolytic-uremic syndrome. A prototype E. coli O157:H7 strain, designated 933, produces Shiga-like toxin I (SLT-I) and SLT-II and harbors a 60-MDa plasmid. In a previous study, streptomycin-treated mice were fed 933 together with a derivative cured of the 60-MDa plasmid (designated 933cu). Strain 933cu colonized poorly, but in approximately one-third of the animals, an isolate of 933cu was obtained from the feces that had regained the ability to colonize well. This isolate, designated 933cu-rev, killed all of the animals when fed alone to mice. In this investigation, two types of experiments were done to assess whether SLT-I, SLT-II, or both contributed to the death of mice fed 933cu-rev. (i) Mice were pretreated with monoclonal antibodies to SLT-I, SLT-II, SLT-I and SLT-II, or cholera toxin (as a control) before infection with 933cu-rev. (ii) Mice were fed either an E. coli K-12 strain carrying cloned SLT-I genes or the same K-12 strain carrying cloned SLT-II genes. The results of both types of experiments indicated that the deaths of the orally infected mice were due solely to SLT-II. Extensive histological and selected electron microscopic examinations of various tissues from the infected animals suggested that death was due to acute renal cortical tubular necrosis consistent with toxic renal damage. These data indicate a critical role for SLT-II, but not SLT-I, in renal damage associated with E. coli O157:H7 infection of streptomycin-treated mice.

Transcription of the Shiga-like toxin type II and Shiga-like toxin type II variant operons of Escherichia coli

Shiga-like toxin type II (SLT-II) and Shiga-like toxin type II variant (SLT-IIv) are cytotoxins produced by certain strains of Escherichia coli. Nucleotide sequence analyses had revealed that the structural genes for the A subunit and B subunit of SLT-II or SLT-IIv are arranged in an operon. Primer extension and S1 nuclease protection analyses identified a promoter for the slt-II operon 118 bases upstream of the slt-IIA gene. The slt-IIv promoter was demonstrated to be identical to the slt-II promoter. The slt-II and slt-IIv promoters differed significantly from the previously characterized Shiga toxin (stx) and Shiga-like toxin type 1 (slt-I) promoters. The transcriptional efficiencies of the stx and slt-II promoters were compared in fusions to the chloramphenicol acetyltransferase gene, and constitutive expression of the slt-II promoter was found to be equivalent to derepressed expression of the stx promoter. In contrast to the stx and slt-I promoters, the slt-II and slt-IIv promoters did not contain sequences for binding of the Fur repressor protein, and SLT-II production was not determined by iron levels in the media in various E. coli strains with wild-type or mutant ferric uptake regulation (fur) alleles. Northern (RNA) blot analysis demonstrated a single mRNA transcript for the slt-II operon, and further analysis of the slt-II operon by primer extension did not reveal an independent promoter for the B subunit gene. A putative rho-independent transcription terminator was identified 274 bases downstream of slt-IIB. These data indicated that the slt-II and slt-IIv operons differ from the stx/slt-I operon in regulation of their transcription by iron. Whether these regulatory differences enable the type I and type II groups of Shiga-like toxins to perform different roles in the pathogenesis of infectious diseases remains to be established.

Modification of the glycolipid-binding specificity of vero cytotoxin by polymyxin B and other cyclic amphipathic peptides

Polymyxin B, an amphipathic cyclic decapeptide produced by Bacillus polymyxa, is routinely used in the extraction of the components from the periplasmic space of gram-negative bacteria. Vero cytotoxin 1 (VT1) is an Escherichia coli-elaborated subunit toxin which binds to the glycolipid globotriosylceramide (Gal-alpha 1-4-Gal beta 1-4-Glc-ceramide [Gb3]) and has been strongly implicated in the etiology of the hemolytic uremic syndrome and hemorrhagic colitis. We now show by in vitro glycolipid-binding assays that in the presence of low concentrations of polymyxin B, globotetraosylceramide (GalNAc beta 1-3Gal alpha 1-4Gal beta 1-4Glc-ceramide [Gb4]) is also recognized by both the VT1 B (binding) subunit and holotoxin. Melittin, a 26-amino-acid cyclic peptide of similar amphipathic nature, produced the same effect, whereas a hydrophobic blocking agent did not. Triton X-100 did not increase binding of VT1 to Gb4 but prevented glycolipid binding in toto at concentrations above 0.5%. Caution is therefore advised in the analysis of VT1 glycolipid binding in the presence of amphipathic peptides.

Comparison of the glycolipid receptor specificities of Shiga-like toxin type II and Shiga-like toxin type II variants

The antigenically distinct Shiga-like toxins (SLTs) SLT-1 and SLT-II are cytotoxic for both Vero and HeLa cells and use Gal alpha 1-4Gal beta 1-4Glc beta 1-1Cer (Gb3) molecules as functional receptors. SLT-II-related variants SLT-IIvp and SLT-IIvh, produced by a porcine isolate and a human isolate, respectively, are cytotoxic for Vero but not HeLa cells. To investigate the basis for these differences in cytotoxic specificity among SLTs, the nature of the receptor for the SLT-II variants was examined. First, the patterns of binding of SLT-II and the SLT-II variants to Gb3 receptor analogs Gal alpha 1-4Gal-bovine serum albumin and Gal alpha 1-4Gal beta 1-4Glc-bovine serum albumin were compared. SLT-IIvp bound the trisaccharide neoglycoprotein preferentially, while SLT-IIvh bound both analogs equally but with less affinity than did SLT-II. Next, the glycolipids to which the SLT-II variants bound in Vero and HeLa cells were identified by thin-layer chromatography. SLT-IIvp bound to Gb3, GalNAc beta 1-3Gal alpha 1-4Gal beta 1-4Glc beta 1-1Cer (Gb4), and Gal beta 1-3GalNAc beta 1-3Gal alpha 1-4Gal beta 1-4Glc beta 1-1Cer (Gb5) in Vero cells but only Gb3 in HeLa cells. However, SLT-IIvh bound to Gal alpha 1-4Gal beta 1-1Cer (Gb2) and Gb3 in HeLa cells but only Gb3 in Vero cells. In addition, hybrid toxins (SLT-IIvp subunit A with SLT-II subunit B or SLT-II subunit A with SLT-IIvp subunit B) were used to show that the receptor specificities of the SLTs was B subunit specific. These differences in receptor specificities are important in vivo, as evidenced by a 400-fold difference in the 50% lethal doses of purified SLT-IIvp and SLT-II (200 versus 0.5 ng, respectively) for mice. These data indicate that SLT-II-cytotoxic variants can occur as a consequence of differences in receptor specificity and affinity.

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.

In vivo formation of hybrid toxins comprising Shiga toxin and the Shiga-like toxins and role of the B subunit in localization and cytotoxic activity

Shiga toxin, Shiga-like toxin I (SLT-I) and Shiga-like toxin II (SLT-II) are cell-associated cytotoxins that kill both Vero cells and HeLa cells, whereas Shiga-like toxin II variant (SLT-IIv) is an extracellular cytotoxin that is more cytotoxic for Vero cells than for HeLa cells. The basis for these differences in cytotoxin localization and host cell specificity were examined in this study. The A and B subunit genes of Shiga toxin and the SLTs were recombined by two methods so that hybrid toxins would be formed in vivo. Complementation of heterologous subunits was accomplished by cloning the individual A and B subunit genes of SLT-I, SLT-II, and SLT-IIv on plasmid vectors of different incompatibility groups so that they could be maintained in double transformants of Escherichia coli. In addition, six operon fusions were constructed so that the A and B subunit genes of Shiga toxin, SLT-II, and SLT-IIv could be expressed as a single operon. The activities of the hybrid cytotoxins were assessed in three ways: (i) level of cytotoxicity, (ii) ratio of HeLa to Vero cell cytotoxicity, and (iii) ratio of extracellular to cell-associated cytotoxicity. Neither the A subunit of Shiga toxin nor SLT-I associated with a heterologous B subunit to form an active cytotoxin. However, in all other cases the hybrid molecules formed by subunit complementation or operon fusion were cytotoxic. Furthermore, the cytotoxic specificity and localization of the hybrid cytotoxins always corresponded to the activities of the native toxin possessing the same B subunit.

Hybridization of Escherichia coli producing Shiga-like toxin I, Shiga-like toxin II, and a variant of Shiga-like toxin II with synthetic oligonucleotide probes

Synthetic oligonucleotides, constructed from the nucleotide sequences of genes coding for the A subunit of Shiga-like toxin (SLT) I and the B subunit of SLT-II, were used as probes at different degrees of stringency to identify Escherichia coli producing different types of SLTs. At 45 degrees C, the A-I oligonucleotide probe hybridized with E. coli producing SLT-I, SLT-II, and variant of SLT-II (SLT-IIv). At 53 degrees C, only SLT-I-producing E. coli hybridized with this probe. At 45 degrees C, the B-II oligonucleotide probe hybridized with SLT-II- and SLT-IIv-producing E. coli. At 53 degrees C, this probe hybridized with only SLT-II-producing E. coli. The A-I and B-II oligonucleotide probes were subsequently tested for hybridization with 73 SLT-producing E. coli and 49 non-SLT-producing E. coli isolated in Asia and Canada. At 45 degrees C, the A-I oligomer had a sensitivity of 97% and a specificity of 100% in identifying SLT-producing E. coli. At 53 degrees C, the A-I oligonucleotide probe had a sensitivity of 92% and a specificity of 91% in identifying E. coli containing genes encoding SLT-I. At 45 degrees C, the B-II oligonucleotide had a 100% sensitivity and 97% specificity in identifying E. coli that hybridized with the SLT-II probe. Of 17 E. coli that hybridized only with the SLT-II probe, 10 did not hybridize with the B-II oligonucleotide at 53 degrees C. All 10 isolates were cytotoxic to Vero cells but not to HeLa cells, confirming that the B-II oligonucleotide probe used at 53 degrees C will differentiate isolates producing SLT-II and SLT-IIv.

Evaluation of oligonucleotide probes for identification of shiga-like-toxin-producing Escherichia coli

Four synthetic oligonucleotide probes representing different regions of the Shiga-like toxin I (SLT-I) structural genes and one oligonucleotide derived from the SLT-II gene of Escherichia coli serotype O157:H7 strain 933 were examined for the identification of E. coli strains that produce cytotoxins for Vero or HeLa cells. E. coli strains that synthesize SLT-I alone or O157:H7 isolates that coexpress SLT-I and SLT-II hybridized with all four probes that were complementary to the SLT-I genes, suggesting that they have toxin genes with great homology in all the regions examined. In colony hybridization tests, these oligonucleotide probes did not react with E. coli strains that were nontoxigenic for Vero cells or that produced cytotoxins belonging to the SLT-II family. The probe derived from the slt-IIA gene distinguished E. coli strains that produced SLT-II alone from SLT-I-producing strains and hybridized to all E. coli O157:H7 strains that produced both SLT-I and SLT-II. Using two of these oligonucleotide probes that were complementary to slt-IA or slt-IIA sequences, we identified 50 of 52 cytotoxin-producing strains, whereas none of 416 nontoxigenic E. coli strains was reactive. The colony blot hybridization with the oligonucleotide probes described here can serve as a specific and sensitive test with potential diagnostic value.

Determination by DNA hybridization of Shiga-like-toxin-producing Escherichia coli in children with diarrhea in Thailand

Specific DNA probes were used to identify Shiga-like-toxin (SLT) I- and II-producing Escherichia coli from children less than 5 years of age with bloody diarrhea, in infants with diarrhea, and in controls of the same age without diarrhea in Thailand. At one hospital, SLT-producing E. coli was identified in 4 (7%) of 54 children with bloody diarrhea from whom other enteric pathogens were not identified and from 3 (6%) of 50 children without diarrhea. In the positive specimens, SLT-producing E. coli constituted only 0.3 to 4% of the 100 to 300 colonies on the replica blots. Non-toxin-encoding 933J and 933W bacteriophagelike DNA sequences were detected by colony hybridization with E. coli isolates from 18 (33%) of 54 children with bloody diarrhea and 23 (46%) of 50 controls. At another hospital, SLT-producing E. coli was not identified in 115 infants with diarrhea and 119 controls without diarrhea. One infant with diarrhea was infected with E. coli O76:H7 that hybridized with the enterohemorrhagic E. coli probe but not with the SLT probes. E. coli producing SLT I or SLT II was isolated in small numbers from a similar proportion of Thai children with bloody diarrhea in whom no other enteric pathogen was identified and from controls without diarrhea.

Quantitative analysis and partial characterization of cytotoxin production by Salmonella strains

The pathogenesis of the wide-spectrum human disease caused by Salmonella species is poorly understood. Cytotoxin production by other enteric pathogens has been increasingly investigated recently, and data are accumulating regarding the role of cytotoxins in enteric infections and hemolytic uremic syndrome. We studied the cytotoxic activity of 131 Salmonella strains of the major serotypes, including 94 strains of Salmonella enteritidis, 12 strains of Salmonella typhi, and 25 strains of Salmonella choleraesuis. Cytotoxicity was quantitatively determined in sonic extracts by a [3H]thymidine-labeled HeLa cell assay. All Salmonella strains examined showed some degree of cytotoxic activity. The geometric means +/- standard deviations of the amounts of cytotoxin produced (50% cytotoxic dose per milligram of bacterial protein) were 27 +/- 2 for S. typhi, 65 +/- 2 for S. enteritidis, and 117 +/- 2 for S. choleraesuis. Analysis of variance showed that the differences in cytotoxin production by the three species were significant (P less than 0.001). No significant differences were found between stool isolates and invasive strains of the same species. Neutralization studies showed that the cytotoxins produced by all Salmonella strains were immunologically distinct from Shiga toxin and the closely related Shiga-like toxins produced by Escherichia coli. DNA hybridization studies with DNA probes for Shiga-like toxins of types I and II showed no hybridization. In each species the cytotoxin was heat labile and sensitive to trypsin treatment, which indicated that its active component was probably protein in nature. Upon ultrafiltration with Amicon membranes and gel filtration chromatography, cytotoxic activity was found in the molecular weight range of 56,000 to 78,000. Our findings indicate that salmonellae produce cytotoxin(s) that may play a role in the manifestations of the various species.

Cloning and sequencing of a Shiga-like toxin type II variant from Escherichia coli strain responsible for edema disease of swine

A Shiga-like toxin type II variant (SLT-IIv) is produced by strains of Escherichia coli responsible for edema disease of swine and is antigenically related to Shiga-like toxin type II (SLT-II) of enterohemorrhagic E. coli. However, SLT-IIv is only active against Vero cells, whereas SLT-II is active against both Vero and HeLa cells. The structural genes for SLT-IIv were cloned from E. coli S1191, and the nucleotide sequence was determined and compared with those of other members of the Shiga toxin family. The A subunit genes for SLT-IIv and SLT-II were highly homologous (94%), whereas the B subunit genes were less homologous (79%). The SLT-IIv genes were more distantly related (55 to 60% overall homology) to the genes for Shiga toxin of Shigella dysenteriae type 1 and the nearly identical Shiga-like toxin type I (SLT-I) of enterohemorrhagic E. coli. (These toxins are referred to together as Shiga toxin/SLT-I.) The A subunit of SLT-IIv, like those of other members of this toxin family, had regions of homology with the plant lectin ricin. SLT-IIv did not bind to galactose-alpha 1-4-galactose conjugated to bovine serum albumin, which is an analog of the eucaryotic cell receptor for Shiga toxin/SLT-I and SLT-II. These findings support the hypothesis that SLT-IIv binds to a different cellular receptor than do other members of the Shiga toxin family but has a similar mode of intracellular action. The organization of the SLT-IIv operon was similar to that of other members of the Shiga toxin family. Iron did not suppress SLT-IIv or SLT-II production, in contrast with its effect on Shiga toxin/SLT-I. Therefore, the regulation of synthesis of SLT-IIv and SLT-II differs from that of Shiga toxin/SLT-I.

Affinity purification and characterization of Shiga-like toxin II and production of toxin-specific monoclonal antibodies

Shiga-like toxin (SLT-II) was purified to apparent homogeneity from Escherichia coli K-12 strain NM522 containing the cloned toxin genes on recombinant plasmid pEB1. Purification was accomplished by a series of column chromatography techniques: anion-exchange, chromatofocusing, cation-exchange, and monoclonal antibody affinity chromatography. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis of the pure toxin showed that SLT-II consisted of A and B subunits with apparent molecular weights of 32,000 and 10,200 +/- 800, respectively. A band of molecular weight 25,000 was also observed after sodium dodecyl sulfate-polyacrylamide gel electrophoresis and identified as the A1 subunit by Western immunoblot analysis with toxin-specific monoclonal antibodies (MAbs). The pI of the purified toxin was 5.2. Approximately 1 pg of pure SLT-II, but was not neutralized by polyclonal antibodies or MAbs to SLT-I. Five hybridomas against SLT-II were produced (BC5 BB12, DC1 EH5, EA5 BA3, ED5 DF3, and GB6 BA4). Culture supernatant fluids containing MAbs from these hybridomas did not neutralize the cytotoxicity of SLT-I or Shiga toxin. Western blot analysis showed that two MAbs (MAb DC1 EH5 and MAb GB6 BA4) recognized the A and A1 subunits of SLT-II and three MAbs (MAb BC5 BB12, MAb EA5 BA3, and MAb ED5 DF3) recognized the B subunit of SLT-II. MAb BC5 BB12 was used to prepare an affinity column for toxin purification.

DNA probes for Shiga-like toxins I and II and for toxin-converting bacteriophages

A set of DNA probes has been developed to study the genes for Shiga-like toxins (SLT) and the bacteriophage from which these toxin genes were isolated. Under stringent conditions of hybridization (80 to 90% homology), these probes detect strains containing (i) SLT I-related genes, (ii) SLT II-related genes, (iii) phage sequences from the SLT I-converting phage H19A/933J, and (iv) phage sequences from the SLT II-converting phage 933W. Strain characterization by hybridization with the toxin gene probes was as accurate as methods that used toxin-specific antibody to determine toxin synthesis. Screening of different gram-negative bacteria with the toxin probes revealed that only two species carry sequences related to the SLT genes, Escherichia coli and Shigella dysenteriae 1. These results indicated that the lower levels of toxin activity observed in shigellae other than S. dysenteriae 1 are due to a gene(s) that is genetically distinct from that which encodes Shiga toxin. Analysis of enterotoxigenic, enteroinvasive, enteropathogenic, and enterohemorrhagic E. coli indicated that SLT genes are found primarily in the enterohemorrhagic E. coli strain group. Use of both the toxin and the phage probes has identified a variety of genotypic combinations of phage and toxin sequences which differ from those observed for the original toxin-converting phage isolates, for E. coli O157:H7 strain 933, and for E. coli O26:H11 strain H19.