Subtypes of the plasmid-encoded serine protease EspP in Shiga toxin-producing Escherichia coli: distribution, secretion, and proteolytic activity

First molecular characterization of Escherichia coli O157:H7 isolates from clinical samples in Paraguay using whole-genome sequencing

Escherichia coli O157:H7 is a foodborne pathogen implicated in numerous outbreaks worldwide that has the ability to cause extra-intestinal complications in humans. The Enteropathogens Division of the Central Public Health Laboratory (CPHL) in Paraguay is working to improve the genomic characterization of Shiga toxin-producing E. coli (STEC) to enhance laboratory-based surveillance and investigation of foodborne disease outbreaks. Whole genome sequencing (WGS) is proposed worldwide to be used in the routine laboratory as a high-resolution tool that allows to have all the results in a single workflow. This study aimed to carry out for the first time, the genomic characterization by WGS of nine STEC O157:H7 strains isolated from human samples in Paraguay. We were able to identify virulence and resistance mechanisms, MLST subtype, and even establish the phylogenetic relationships between isolates. Furthermore, we detected the presence of strains belonging to hypervirulent clade 8 in most of the isolates studied.

IgG Binds Escherichia coli Serine Protease EspP and Protects Mice From E. coli O157:H7 Infection

Shiga toxin-producing Escherichia coli O157:H7 is a virulent strain causing severe gastrointestinal infection, hemolytic uremic syndrome and death. To date there are no specific therapies to reduce progression of disease. Here we investigated the effect of pooled immunoglobulins (IgG) on the course of disease in a mouse model of intragastric E. coli O157:H7 inoculation. Intraperitoneal administration of murine IgG on day 3, or both on day 3 and 6, post-inoculation improved survival and decreased intestinal and renal pathology. When given on both day 3 and 6 post-inoculation IgG treatment also improved kidney function in infected mice. Murine and human commercially available IgG preparations bound to proteins in culture filtrates from E. coli O157:H7. Bound proteins were extracted from membranes and peptide sequences were identified by mass spectrometry. The findings showed that murine and human IgG bound to E. coli extracellular serine protease P (EspP) in the culture filtrate, via the IgG Fc domain. These results were confirmed using purified recombinant EspP and comparing culture filtrates from the wild-type E. coli O157:H7 strain to a deletion mutant lacking espP. Culture filtrates from wild-type E. coli O157:H7 exhibited enzymatic activity, specifically associated with the presence of EspP and demonstrated as pepsin cleavage, which was reduced in the presence of murine and human IgG. EspP is a virulence factor previously shown to promote colonic cell injury and the uptake of Shiga toxin by intestinal cells. The results presented here suggest that IgG binds to EspP, blocks its enzymatic activity, and protects the host from E. coli O157:H7 infection, even when given post-inoculation.

Serine Protease Autotransporters of the Enterobacteriaceae (SPATEs): Out and About and Chopping It Up

Autotransporters are secreted proteins with multiple functions produced by a variety of Gram-negative bacteria. In Enterobacteriaceae, a subgroup of these autotransporters are the SPATEs (serine protease autotransporters of Enterobacteriaceae). SPATEs play a crucial role in survival and virulence of pathogens such as Escherichia coli and Shigella spp. and contribute to intestinal and extra-intestinal infections. These high molecular weight proteases are transported to the external milieu by the type Va secretion system and function as proteases with diverse substrate specificities and biological functions including adherence and cytotoxicity. Herein, we provide an overview of SPATEs and discuss recent findings on the biological roles of these secreted proteins, including proteolysis of substrates, adherence to cells, modulation of the immune response, and virulence in host models. In closing, we highlight recent insights into the regulation of expression of SPATEs that could be exploited to understand fundamental SPATE biology.

Enterohemorrhagic E. coli (EHEC)-Secreted Serine Protease EspP Stimulates Electrogenic Ion Transport in Human Colonoid Monolayers

One of the characteristic manifestations of Shiga-toxin-producing Escherichia coli (E. coli) infection in humans, including EHEC and Enteroaggregative E. coli O104:H4, is watery diarrhea. However, neither Shiga toxin nor numerous components of the type-3 secretion system have been found to independently elicit fluid secretion. We used the adult stem-cell-derived human colonoid monolayers (HCM) to test whether EHEC-secreted extracellular serine protease P (EspP), a member of the serine protease family broadly expressed by diarrheagenic E. coli can act as an enterotoxin. We applied the Ussing chamber/voltage clamp technique to determine whether EspP stimulates electrogenic ion transport indicated by a change in short-circuit current (Isc). EspP stimulates Isc in HCM. The EspP-stimulated Isc does not require protease activity, is not cystic fibrosis transmembrane conductance regulator (CFTR)-mediated, but is partially Ca²⁺-dependent. EspP neutralization with a specific antibody reduces its potency in stimulating Isc. Serine Protease A, secreted by Enteroaggregative E. coli, also stimulates Isc in HCM, but this current is CFTR-dependent. In conclusion, EspP stimulates colonic CFTR-independent active ion transport and may be involved in the pathophysiology of EHEC diarrhea. Serine protease toxins from E. coli pathogens appear to serve as enterotoxins, potentially significantly contributing to watery diarrhea.

Variation in the Distribution of Putative Virulence and Colonization Factors in Shiga Toxin-Producing Escherichia coli Isolated from Different Categories of Cattle

Shiga toxin-producing Escherichia coli (STEC) are pathogens of significant public health concern. Several studies have confirmed that cattle are the main reservoir of STEC in Argentina and other countries. Although Shiga toxins represent the primary virulence factors of STEC, the adherence and colonization of the gut are also important in the pathogenesis of the bacteria. The aim of this study was to analyze and to compare the presence of putative virulence factors codified in plasmid -katP, espP, subA, stcE- and adhesins involved in colonization of cattle -efa1, iha- in 255 native STEC strains isolated from different categories of cattle from different production systems. The most prevalent gene in all strains was espP, and the less prevalent was stcE. katP was highly detected in strains isolated from young and rearing calves (33.3%), while subA was predominant in those isolated from adults (71.21%). Strains from young calves showed the highest percentage of efa1 (72.46%), while iha showed a high distribution in strains from rearing calves and adults (87.04 and 98.48% respectively). It was observed that espP and iha were widely distributed throughout all strains, whereas katP, stcE, and efa1 were more associated with the presence of eae and subA with the eae-negative strains. A great proportion of eae-negative strains were isolated from adults -dairy and grazing farms- and from rearing calves -dairy and feedlot-, while mostly of the eae-positive strains were isolated from dairy young calves. Data exposed indicate a correlation between the category of the animal and the production systems with the presence or absence of several genes implicated in adherence and virulence of STEC.

Host cell interactions of outer membrane vesicle-associated virulence factors of enterohemorrhagic Escherichia coli O157: Intracellular delivery, trafficking and mechanisms of cell injury

Outer membrane vesicles (OMVs) are important tools in bacterial virulence but their role in the pathogenesis of infections caused by enterohemorrhagic Escherichia coli (EHEC) O157, the leading cause of life-threatening hemolytic uremic syndrome, is poorly understood. Using proteomics, electron and confocal laser scanning microscopy, immunoblotting, and bioassays, we investigated OMVs secreted by EHEC O157 clinical isolates for virulence factors cargoes, interactions with pathogenetically relevant human cells, and mechanisms of cell injury. We demonstrate that O157 OMVs carry a cocktail of key virulence factors of EHEC O157 including Shiga toxin 2a (Stx2a), cytolethal distending toxin V (CdtV), EHEC hemolysin, and flagellin. The toxins are internalized by cells via dynamin-dependent endocytosis of OMVs and differentially separate from vesicles during intracellular trafficking. Stx2a and CdtV-B, the DNase-like CdtV subunit, separate from OMVs in early endosomes. Stx2a is trafficked, in association with its receptor globotriaosylceramide within detergent-resistant membranes, to the Golgi complex and the endoplasmic reticulum from where the catalytic Stx2a A1 fragment is translocated to the cytosol. CdtV-B is, after its retrograde transport to the endoplasmic reticulum, translocated to the nucleus to reach DNA. CdtV-A and CdtV-C subunits remain OMV-associated and are sorted with OMVs to lysosomes. EHEC hemolysin separates from OMVs in lysosomes and targets mitochondria. The OMV-delivered CdtV-B causes cellular DNA damage, which activates DNA damage responses leading to G2 cell cycle arrest. The arrested cells ultimately die of apoptosis induced by Stx2a and CdtV via caspase-9 activation. By demonstrating that naturally secreted EHEC O157 OMVs carry and deliver into cells a cocktail of biologically active virulence factors, thereby causing cell death, and by performing first comprehensive analysis of intracellular trafficking of OMVs and OMV-delivered virulence factors, we provide new insights into the pathogenesis of EHEC O157 infections. Our data have implications for considering O157 OMVs as vaccine candidates.

Enterohemorrhagic Escherichia coli (EHEC) O157, the leading EHEC group causing diarrhea and the life-threatening hemolytic uremic syndrome in humans, produce several virulence factors which play distinct roles in the pathogenesis of these diseases. However, the mechanisms of their secretion and host cell injury are poorly understood. We show here that EHEC O157 strains isolated from patients shed nanostructures termed outer membrane vesicles (OMVs) which contain major EHEC O157 virulence factors including Shiga toxin 2a (Stx2a), cytolethal distending toxin V (CdtV), EHEC hemolysin, and flagellin. The OMVs are taken up by human intestinal epithelial and renal and brain microvascular endothelial cells, which are the major targets during EHEC O157 infections, and deliver the virulence factors intracellularly. Inside cells the virulence factors separate from OMVs and are transported via different pathways to their target compartments including the cytosol (Stx2a), nucleus (CdtV-B subunit), and mitochondria (EHEC hemolysin). Cells exposed to EHEC O157 OMVs develop G2 cell cycle arrest induced by CdtV-B-mediated DNA damage. This is followed by apoptotic cell death triggered by Stx2a and CdtV via caspase-9 activation. OMVs thus serve as novel tools of EHEC O157-mediated host injury and are quite likely involved in the pathogenesis of human diseases.

Plasmids from Shiga Toxin-Producing Escherichia coli Strains with Rare Enterohemolysin Gene (ehxA) Subtypes Reveal Pathogenicity Potential and Display a Novel Evolutionary Path

Most Shiga toxin-producing Escherichia coli (STEC) strains associated with severe disease, such as hemolytic-uremic syndrome (HUS), carry large enterohemolysin-encoding (ehxA) plasmids, e.g., pO157 and pO103, that contribute to STEC clinical manifestations. Six ehxA subtypes (A through F) exist that phylogenetically cluster into eae-positive (B, C, F), a mix of eae-positive (E) and eae-negative (A), and a third, more distantly related, cluster of eae-negative (D) STEC strains. While subtype B, C, and F plasmids share a number of virulence traits that are distinct from those of subtype A, sequence data have not been available for subtype D and E plasmids. Here, we determined and compared the genetic composition of four subtype D and two subtype E plasmids to establish their evolutionary relatedness among ehxA subtypes and define their potential role in pathogenicity. We found that subtype D strains carry one exceptionally large plasmid (>200 kbp) that carries a variety of virulence genes that are associated with enterotoxigenic and enterohemorrhagic E. coli, which, quite possibly, enables these strains to cause disease despite being food isolates. Our data offer further support for the hypothesis that this subtype D plasmid represents a novel virulence plasmid, sharing very few genetic features with other plasmids; we conclude that these plasmids have evolved from a different evolutionary lineage than the plasmids carrying the other ehxA subtypes. In contrast, the 50-kbp plasmids of subtype E (pO145), although isolated from HUS outbreak strains, carried only few virulence-associated determinants, suggesting that the clinical presentation of subtype E strains is largely a result of chromosomally encoded virulence factors.

IMPORTANCE

Bacterial plasmids are known to be key agents of change in microbial populations, promoting the dissemination of various traits, such as drug resistance and virulence. This study determined the genetic makeup of virulence plasmids from rare enterohemolysin subtype D and E Shiga toxin-producing E. coli strains. We demonstrated that ehxA subtype D plasmids represent a novel E. coli virulence plasmid, and although subtype D plasmids were derived from nonclinical isolates, they encoded a variety of virulence determinants that are associated with pathogenic E. coli In contrast, subtype E plasmids, isolated from strains recovered from severely ill patients, carry only a few virulence determinants. The results of this study reemphasize the plasticity and vast diversity among E. coli plasmids. This work demonstrates that, although E. coli strains of certain serogroups may not be frequently associated with disease, they should not be underestimated in protecting human health and food safety.

EspP, an Extracellular Serine Protease from Enterohemorrhagic E. coli, Reduces Coagulation Factor Activities, Reduces Clot Strength, and Promotes Clot Lysis

BACKGROUND

EspP (E. coli secreted serine protease, large plasmid encoded) is an extracellular serine protease produced by enterohemorrhagic E. coli (EHEC) O157:H7, a causative agent of diarrhea-associated Hemolytic Uremic Syndrome (D+HUS). The mechanism by which EHEC induces D+HUS has not been fully elucidated.

OBJECTIVES

We investigated the effects of EspP on clot formation and lysis in human blood.

METHODS

Human whole blood and plasma were incubated with EspP(WT )at various concentrations and sampled at various time points. Thrombin time (TT), prothrombin time (PT), and activated partial thromboplastin time (aPTT), coagulation factor activities, and thrombelastgraphy (TEG) were measured.

RESULTS AND CONCLUSIONS

Human whole blood or plasma incubated with EspP(WT) was found to have prolonged PT, aPTT, and TT. Furthermore, human whole blood or plasma incubated with EspP(WT) had reduced activities of coagulation factors V, VII, VIII, and XII, as well as prothrombin. EspP did not alter the activities of coagulation factors IX, X, or XI. When analyzed by whole blood TEG, EspP decreased the maximum amplitude of the clot, and increased the clot lysis. Our results indicate that EspP alters hemostasis in vitro by decreasing the activities of coagulation factors V, VII, VIII, and XII, and of prothrombin, by reducing the clot strength and accelerating fibrinolysis, and provide further evidence of a functional role for this protease in the virulence of EHEC and the development of D+HUS.

Enterohemorrhagic Escherichia coli reduce mucus and intermicrovillar bridges in human stem cell-derived colonoids

BACKGROUND AND AIMS

Enterohemorrhagic E. coli (EHEC) causes over 70,000 episodes of foodborne diarrhea annually in the USA. The early sequence of events which precede life-threatening hemorrhagic colitis and hemolytic uremic syndrome are not fully understood due to the initial asymptomatic phase of the disease and the lack of a suitable animal model. The aim of this study was to determine the initial molecular events in the interaction between EHEC and human colonic epithelium.

METHODS

Human colonoids derived from adult proximal colonic stem cells were developed into monolayers to study EHEC-epithelial interactions. Monolayer confluency and differentiation were monitored by transepithelial electrical resistance (TER) measurements. The monolayers were apically infected with EHEC and the progression of epithelial damage over time was assessed using biochemical and imaging approaches.

RESULTS

Human colonoid cultures recapitulate the differential protein expression patterns characteristic of the crypt and surface colonocytes. Mucus-producing differentiated colonoid monolayers are preferentially colonized by EHEC. Upon colonization, EHEC forms characteristic attaching and effacing lesions on the apical surface of colonoid monolayers. Mucin 2, a main component of colonic mucus, and protocadherin 24 (PCDH24), a microvillar resident protein, are targeted by EHEC at early stages of infection. The EHEC secreted serine protease, EspP, initiates brush border damage through PCDH24 reduction.

CONCLUSIONS

Human colonoid monolayers are a relevant pathophysiological model which allows the study of early molecular events during enteric infections. Colonoid monolayers provide access to both apical and basolateral surfaces, thus providing an advantage over 3D cultures to study host-pathogen interactions in a controllable and tractable manner. EHEC reduces colonic mucus and affects the brush border cytoskeleton in the absence of commensal bacteria.

Structural and functional characterization of cleavage and inactivation of human serine protease inhibitors by the bacterial SPATE protease EspPα from enterohemorrhagic E. coli

EspPα and EspI are serine protease autotransporters found in enterohemorrhagic Escherichia coli. They both belong to the SPATE autotransporter family and are believed to contribute to pathogenicity via proteolytic cleavage and inactivation of different key host proteins during infection. Here, we describe the specific cleavage and functional inactivation of serine protease inhibitors (serpins) by EspPα and compare this activity with the related SPATE EspI. Serpins are structurally related proteins that regulate vital protease cascades, such as blood coagulation and inflammatory host response. For the rapid determination of serpin cleavage sites, we applied direct MALDI-TOF-MS or ESI-FTMS analysis of coincubations of serpins and SPATE proteases and confirmed observed cleavage positions using in-gel-digest of SDS-PAGE-separated degradation products. Activities of both serpin and SPATE protease were assessed in a newly developed photometrical assay using chromogenic peptide substrates. EspPα cleaved the serpins α1-protease inhibitor (α1-PI), α1-antichymotrypsin, angiotensinogen, and α2-antiplasmin. Serpin cleavage led to loss of inhibitory function as demonstrated for α1-PI while EspPα activity was not affected. Notably, EspPα showed pronounced specificity and cleaved procoagulatory serpins such as α2-antiplasmin while the anticoagulatory antithrombin III was not affected. Together with recently published research, this underlines the interference of EspPα with hemostasis or inflammatory responses during infection, while the observed interaction of EspI with serpins is likely to be not physiologically relevant. EspPα-mediated serpin cleavage occurred always in flexible loops, indicating that this structural motif might be required for substrate recognition.

Biochemical characterization of the SPATE members EspPα and EspI

The activity of serine proteases is influenced by their substrate specificity as well as by the physicochemical conditions. Here, we present the characterization of key biochemical features of the two SPATE members EspPα and EspI from Shiga-toxin producing Escherichia coli (STEC) and enterohemorrhagic E. coli (EHEC). Both proteases show high activity at conditions mimicking the human blood stream. Optimal activities were observed at slightly alkaline pH and low millimolar concentrations of the divalent cations Ca²⁺ and Mg²⁺ at physiological temperatures indicating a function in the human host. Furthermore, we provide the first cleavage profile for EspI demonstrating pronounced specificity of this protease.

Characterization of urinary tract infection-associated Shiga toxin-producing Escherichia coli

Enterohemorrhagic Escherichia coli (EHEC), a subgroup of Shiga toxin (Stx)-producing E. coli (STEC), is a leading cause of diarrhea and hemolytic-uremic syndrome (HUS) in humans. However, urinary tract infections (UTIs) caused by this microorganism but not associated with diarrhea have occasionally been reported. We geno- and phenotypically characterized three EHEC isolates obtained from the urine of hospitalized patients suffering from UTIs. These isolates carried typical EHEC virulence markers and belonged to HUS-associated E. coli (HUSEC) clones, but they lacked virulence markers typical of uropathogenic E. coli. One isolate exhibited a localized adherence (LA)-like pattern on T24 urinary bladder epithelial cells. Since the glycosphingolipids (GSLs) globotriaosylceramide (Gb3Cer) and globotetraosylceramide (Gb4Cer) are well-known receptors for Stx but also for P fimbriae, a major virulence factor of extraintestinal pathogenic E. coli (ExPEC), the expression of Gb3Cer and Gb4Cer by T24 cells and in murine urinary bladder tissue was examined by thin-layer chromatography and mass spectrometry. We provide data indicating that Stxs released by the EHEC isolates bind to Gb3Cer and Gb4Cer isolated from T24 cells, which were susceptible to Stx. All three EHEC isolates expressed stx genes upon growth in urine. Two strains were able to cause UTI in a murine infection model and could not be outcompeted in urine in vitro by typical uropathogenic E. coli isolates. Our results indicate that despite the lack of ExPEC virulence markers, EHEC variants may exhibit in certain suitable hosts, e.g., in hospital patients, a uropathogenic potential. The contribution of EHEC virulence factors to uropathogenesis remains to be further investigated.

Comparative proteomic analysis of the membrane proteins of two Haemophilus parasuis strains to identify proteins that may help in habitat adaptation and pathogenesis

Background

Haemophilus parasuis is the causative agent of Glässer’s disease characterized by polyserositis, arthritis, and meningitis in pig, leading to serious economic loss. Despite many years of study, virulence factors and the mechanisms of the entire infection process remain largely unclear. So two-dimensional gel electrophoresis and mass spectrometry were used to search for distinctions at the membrane protein expression level between two H. parasuis isolates aimed at uncovering some proteins potentially involved in habitat adaption and pathogenesis.

Results

A comparative proteomic approach combining two-dimensional gel electrophoresis with mass spectrometry and tandem mass spectrometry was employed to explore the differences among membrane proteomes of a virulent Haemophilus parasuis strain isolated from the lung of a diseased pig and an avirulent strain isolated from the nasal swab of a healthy pig. Differentially expressed protein spots identified by mass spectrometry were annotated and analyzed by bioinformatic interpretation. The mRNA level was determined by quantitative real-time PCR. Proteins representing diverse functional activities were identified. Among them, the tonB-dependent siderophore receptor was a new discovery highlighted for its activity in iron uptake. In addition, periplasmic serine protease and putrescine/spermidine ABC transporter substrate-binding protein were given focus because of their virulence potential. This study revealed that the differentially expressed proteins were important in either the habitat adaption or pathogenesis of H. parasuis.

Conclusions

The outcome demonstrated the presence of some proteins which raise the speculation for their importance in helping in habitat adaption or pathogenesis within the host.

Heteropathogenic virulence and phylogeny reveal phased pathogenic metamorphosis in Escherichia coli O2:H6

Extraintestinal pathogenic and intestinal pathogenic (diarrheagenic) Escherichia coli differ phylogenetically and by virulence profiles. Classic theory teaches simple linear descent in this species, where non-pathogens acquire virulence traits and emerge as pathogens. However, diarrheagenic Shiga toxin-producing E. coli (STEC) O2:H6 not only possess and express virulence factors associated with diarrheagenic and uropathogenic E. coli but also cause diarrhea and urinary tract infections. These organisms are phylogenetically positioned between members of an intestinal pathogenic group (STEC) and extraintestinal pathogenic E. coli. STEC O2:H6 is, therefore, a 'heteropathogen,' and the first such hybrid virulent E. coli identified. The phylogeny of these E. coli and the repertoire of virulence traits they possess compel consideration of an alternate view of pathogen emergence, whereby one pathogroup of E. coli undergoes phased metamorphosis into another. By understanding the evolutionary mechanisms of bacterial pathogens, rational strategies for counteracting their detrimental effects on humans can be developed.

Recent advances in understanding enteric pathogenic Escherichia coli

Although Escherichia coli can be an innocuous resident of the gastrointestinal tract, it also has the pathogenic capacity to cause significant diarrheal and extraintestinal diseases. Pathogenic variants of E. coli (pathovars or pathotypes) cause much morbidity and mortality worldwide. Consequently, pathogenic E. coli is widely studied in humans, animals, food, and the environment. While there are many common features that these pathotypes employ to colonize the intestinal mucosa and cause disease, the course, onset, and complications vary significantly. Outbreaks are common in developed and developing countries, and they sometimes have fatal consequences. Many of these pathotypes are a major public health concern as they have low infectious doses and are transmitted through ubiquitous mediums, including food and water. The seriousness of pathogenic E. coli is exemplified by dedicated national and international surveillance programs that monitor and track outbreaks; unfortunately, this surveillance is often lacking in developing countries. While not all pathotypes carry the same public health profile, they all carry an enormous potential to cause disease and continue to present challenges to human health. This comprehensive review highlights recent advances in our understanding of the intestinal pathotypes of E. coli.

Enterohemorrhagic Escherichia coli as causes of hemolytic uremic syndrome in the Czech Republic

BACKGROUND

Enterohemorrhagic Escherichia coli (EHEC) cause diarrhea-associated hemolytic uremic syndrome (D+ HUS) worldwide, but no systematic study of EHEC as the causative agents of HUS was performed in the Czech Republic. We analyzed stools of all patients with D+ HUS in the Czech Republic between 1998 and 2012 for evidence of EHEC infection. We determined virulence profiles, phenotypes, antimicrobial susceptibilities and phylogeny of the EHEC isolates.

METHODOLOGY/PRINCIPAL FINDINGS

Virulence loci were identified using PCR, phenotypes and antimicrobial susceptibilities were determined using standard procedures, and phylogeny was assessed using multilocus sequence typing. During the 15-year period, EHEC were isolated from stools of 39 (69.4%) of 56 patients. The strains belonged to serotypes [fliC types] O157:H7/NM[fliC(H7)] (50% of which were sorbitol-fermenting; SF), O26:H11/NM[fliC(H11)], O55:NM[fliC(H7)], O111:NM[fliC(H8)], O145:H28[fliC(H28)], O172:NM[fliC(H25)], and Orough:NM[fliC(H250]. O26:H11/NM[fliC(H11)] was the most common serotype associated with HUS (41% isolates). Five stx genotypes were identified, the most frequent being stx(2a) (71.1% isolates). Most strains contained EHEC-hlyA encoding EHEC hemolysin, and a subset (all SF O157:NM and one O157:H7) harbored cdt-V encoding cytolethal distending toxin. espPα encoding serine protease EspPα was found in EHEC O157:H7, O26:H11/NM, and O145:H28, whereas O172:NM and Orough:NM strains contained espPγ. All isolates contained eae encoding adhesin intimin, which belonged to subtypes β (O26), γ (O55, O145, O157), γ2/θ (O111), and ε (O172, Orough). Loci encoding other adhesins (efa1, lpfA(O26), lpfA(O157OI-141), lpfA(O157OI-154), iha) were usually associated with particular serotypes. Phylogenetic analysis demonstrated nine sequence types (STs) which correlated with serotypes. Of these, two STs (ST660 and ST1595) were not found in HUS-associated EHEC before.

CONCLUSIONS/SIGNIFICANCE

EHEC strains, including O157:H7 and non-O157:H7, are frequent causes of D+ HUS in the Czech Republic. Identification of unusual EHEC serotypes/STs causing HUS calls for establishment of an European collection of HUS-associated EHEC, enabling to study properties and evolution of these important pathogens.

Bacterial serine proteases secreted by the autotransporter pathway: classification, specificity, and role in virulence

Serine proteases exist in eukaryotic and prokaryotic organisms and have emerged during evolution as the most abundant and functionally diverse group. In Gram-negative bacteria, there is a growing family of high molecular weight serine proteases secreted to the external milieu by a fascinating and widely employed bacterial secretion mechanism, known as the autotransporter pathway. They were initially found in Neisseria, Shigella, and pathogenic Escherichia coli, but have now also been identified in Citrobacter rodentium, Salmonella, and Edwardsiella species. Here, we focus on proteins belonging to the serine protease autotransporter of Enterobacteriaceae (SPATEs) family. Recent findings regarding the predilection of serine proteases to host intracellular or extracellular protein-substrates involved in numerous biological functions, such as those implicated in cytoskeleton stability, autophagy or innate and adaptive immunity, have helped provide a better understanding of SPATEs' contributions in pathogenesis. Here, we discuss their classification, substrate specificity, and potential roles in pathogenesis.

Prevalence, biogenesis, and functionality of the serine protease autotransporter EspP

Enterohemorrhagic E. coli (EHEC) causes severe diseases in humans worldwide. One of its virulence factors is EspP, which belongs to the serine protease autotransporters of Enterobacteriaceae (SPATE) family. In this review we recapitulate the current data on prevalence, biogenesis, structural properties and functionality. EspP has been used to investigate mechanistic details of autotransport, and recent studies indicate that this transport mechanism is not autonomous but rather dependent on additional factors. Currently, five subtypes have been identified (EspPα-EspPε), with EspPα being associated with highly virulent EHEC serotypes and isolates from patients with severe disease. EspPα has been shown to degrade major proteins of the complement cascade, namely C3 and C5 and probably interferes with hemostasis by cleavage of coagulation factor V. Furthermore, EspPα is believed to contribute to biofilm formation perhaps by polymerization to rope-like structures. Together with the proteolytic activity, EspPα might ameliorate host colonization and interfere with host response.

Identification and mechanism of evolution of new alleles coding for the AIDA-I autotransporter of porcine pathogenic Escherichia coli

Autotransporters are a large family of virulence factors of Gram-negative bacterial pathogens. The autotransporter adhesin involved in diffuse adherence (AIDA-I) is an outer membrane protein of Escherichia coli, which allows binding to epithelial cells as well as the autoaggregation of bacteria. AIDA-I is glycosylated by a specific heptosyltransferase encoded by the aah gene that forms an operon with the aidA gene. aidA is highly prevalent in strains that cause disease in pigs. Nevertheless, there are only two published whole-length sequences for this gene. In this study, we sequenced the aah and aidA genes of 24 aidA-positive porcine strains harboring distinct virulence factor profiles. We compared the obtained sequences and performed phylogenetic and pulsed-field electrophoresis analyses. Our results suggest that there are at least 3 different alleles for aidA, which are associated with distinct virulence factor profiles. The genes are found on high-molecular-weight plasmids and seem to evolve via shuffling mechanisms, with one of the sequences showing evidence of genetic recombination. Our work suggests that genetic plasticity allows the evolution of aah-aidA alleles that are selected during pathogenesis.

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

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

Crystal structure of the passenger domain of the Escherichia coli autotransporter EspP

Autotransporters represent a large superfamily of known and putative virulence factors produced by Gram-negative bacteria. They consist of an N-terminal "passenger domain" responsible for the specific effector functions of the molecule and a C-terminal "β-domain" responsible for translocation of the passenger across the bacterial outer membrane. Here, we present the 2.5-Å crystal structure of the passenger domain of the extracellular serine protease EspP, produced by the pathogen Escherichia coli O157:H7 and a member of the serine protease autotransporters of Enterobacteriaceae (SPATEs). Like the previously structurally characterized SPATE passenger domains, the EspP passenger domain contains an extended right-handed parallel β-helix preceded by an N-terminal globular domain housing the catalytic function of the protease. Of note, however, is the absence of a second globular domain protruding from this β-helix. We describe the structure of the EspP passenger domain in the context of previous results and provide an alternative hypothesis for the function of the β-helix within SPATEs.

Enterohaemorrhagic Escherichia coli haemolysin is cleaved and inactivated by serine protease EspPα

The haemolysin from enterohaemorrhagic Escherichia coli (EHEC-Hly) and the serine protease EspPα are putative virulence factors of EHEC. We investigated the interplay between these secreted factors and demonstrate that EspPα cleaves the 107 kDa large EHEC-Hly. Degradation was observed when purified EspPα was added to a growing culture of an EHEC-Hly-expressing strain, with isolated proteins and with coexpressing strains, and was independent of the EHEC serotype. EHEC-Hly breakdown occurred as a multistage process with the formation of characteristic fragments with relative molecular masses of ∼82 kDa and/or ∼84 kDa and ∼34 kDa. The initial cleavage occurred in the N-terminal hydrophobic domain of EHEC-Hly between Leu²³⁵ and Ser²³⁶ and abolished its haemolytic activity. In a cellular infection system, the cytolytic potential of EHEC-Hly-secreting recombinant strains was abolished when EspPα was coexpressed. EHEC in contact with human intestinal epithelial cells simultaneously upregulated their EHEC-Hly and EspP indicating that both molecules might interact under physiological conditions. We propose the concept of bacterial effector molecule interference (BEMI), reflecting the concerted interplay of virulence factors. Interference between effector molecules might be an additional way to regulate virulence functions and increases the complexity of monomolecular phenotypes.

EspP, a serine protease of enterohemorrhagic Escherichia coli, impairs complement activation by cleaving complement factors C3/C3b and C5

Hemolytic-uremic syndrome (HUS) is a life-threatening disorder characterized by hemolytic anemia, thrombocytopenia, and renal insufficiency. It is caused mainly by infections with enterohemorrhagic Escherichia coli (EHEC). Recently, Shiga toxin 2, the best-studied virulence factor of EHEC, was reported to interact with complement, implying that complement may be involved in the pathogenesis of EHEC-induced HUS. The aim of the present study was to investigate whether or not the serine protease EspP, an important virulence factor of EHEC, interacts with complement proteins. EspP did not have any effect on the integrity of factor H or factor I. However, EspP was shown to cleave purified C3/C3b and C5. Cleavage of the respective complement proteins also occurred in normal human serum (NHS) as a source of C3/C3b or C5 or when purified complement proteins were added to the supernatant of an EspP-producing wild-type strain. Edman degradation allowed unequivocal mapping of all three main C3b fragments but not of the three main C5 fragments. Complement activation was significantly downregulated in all three pathways for C5-depleted serum to which C5, preincubated with EspP, was added (whereas C5 preincubated with an EspP mutant was able to fully reconstitute complement activation). This indicates that EspP markedly destroyed the functional activity, as measured by a commercial total complement enzyme-linked immunosorbent assay (Wieslab). Downregulation of complement by EspP in vivo may influence the colonization of EHEC bacteria in the gut or the disease severity of HUS.

Serine protease autotransporters of enterobacteriaceae (SPATEs): biogenesis and function

Serine Protease Autotransporters of Enterobacteriaceae (SPATEs) constitute a large family of proteases secreted by Escherichia coli and Shigella. SPATEs exhibit two distinct proteolytic activities. First, a C-terminal catalytic site triggers an intra-molecular cleavage that releases the N-terminal portion of these proteins in the extracellular medium. Second, the secreted N-terminal domains of SPATEs are themselves proteases; each contains a canonical serine-protease catalytic site. Some of these secreted proteases are toxins, eliciting various effects on mammalian cells. Here, we discuss the biogenesis of SPATEs and their function as toxins.

Molecular analysis of virulence profiles and Shiga toxin genes in food-borne Shiga toxin-producing Escherichia coli

In this study, 75 Shiga toxin (Stx)-producing Escherichia coli (STEC) strains originating from foods (n = 73) and drinking water (n = 2) were analyzed for their stx genotype, as well as for further chromosome-, phage-, and plasmid-encoded virulence factors. A broad spectrum of stx genes was detected. Fifty-three strains (70.7%) contained stx(2) or stx(2) variants, including stx(2d), mucus-activatable stx(2d), stx(2e), and stx(2g). Seven strains (9.3%) harbored stx(1) or stx(1c), and 15 strains (20.0%) carried both stx(2) and/or stx(2) variants and stx(1) or stx(1c). Beside stx, the most abundant accessory virulence markers in STEC food isolates were iha (57.3%), ehxA (40.0%), espP (28.0%), and subAB (25.3%). Only four strains were eae positive; three of these belonged to the serogroups O26, O103, and O157 and contained a typical enterohemorrhagic E. coli virulence spectrum. The results of this study show that a number of STEC strains that occur in foods appear to be pathogenic for humans, based on their virulence profiles. Analysis of stx subtypes and detection of additional virulence factors in eae-negative strains may help to better assess the risk of such strains for causing human infection.

Structure and function relationship of the autotransport and proteolytic activity of EspP from Shiga toxin-producing Escherichia coli

BACKGROUND

The serine protease autotransporter EspP is a proposed virulence factor of Shiga toxin-producing Escherichia coli (STEC). We recently distinguished four EspP subtypes (EspPalpha, EspPbeta, EspPgamma, and EspPdelta), which display large differences in transport and proteolytic activities and differ widely concerning their distribution within the STEC population. The mechanisms underlying these functional variations in EspP subtypes are, however, unknown.

METHODOLOGY/PRINCIPAL FINDINGS

The structural basis of proteolytic and autotransport activity was investigated using transposon-based linker scanning mutagenesis, site-directed mutagenesis and structure-function analysis derived from homology modelling of the EspP passenger domain. Transposon mutagenesis of the passenger domain inactivated autotransport when pentapeptide linker insertions occurred in regions essential for overall correct folding or in a loop protruding from the beta-helical core. Loss of proteolytic function was limited to mutations in Domain 1 in the N-terminal third of the EspP passenger. Site-directed mutagenesis demonstrated that His(127), Asp(156) and Ser(263) in Domain 1 form the catalytic triad of EspP.

CONCLUSIONS/SIGNIFICANCE

Our data indicate that in EspP i) the correct formation of the tertiary structure of the passenger domain is essential for efficient autotransport, and ii) an elastase-like serine protease domain in the N-terminal Domain 1 is responsible for the proteolytic phenotype. Lack of stabilizing interactions of Domain 1 with the core structure of the passenger domain ablates proteolytic activity in subtypes EspPbeta and EspPdelta.

Phenotypic and genotypic traits of Shiga toxin-negative E. coli O157:H7/H(-) bovine and porcine strains

Enterohemorrhagic E. coli (EHEC) O157:H7/H(-) (nonmotile) exist as Shiga toxin gene (stx)-positive and stx-negative variants in patients and the environment. We analyzed molecular characteristics, phenotypes, and the phylogenetic background of three stx-negative E. coli O157:H7/H(-) strains isolated from cattle and a pig and compared them with those of human EHEC and stx-negative E. coli O157:H7/H(-). All three animal strains contained fliCH7 and two contained eae. One eae-positive strain (O157:H(-)) was sorbitol-fermenting (SF) and the other (O157:H7) was non-sorbitol-fermenting (nSF). These two strains shared a spectrum of non-stx putative virulence and fitness genes with human nSF and SF EHEC and stx-negative E. coli O157:H7/H(-) and belonged, similar to the vast majority of human isolates, to sequence type (ST) 11 in multilocus sequence typing. In contrast, the eae-negative O157:H7 animal isolate, which was SF, differed in spectrum of virulence genes and also differed phylogenetically (ST717) from the two eae-positive strains and the human EHEC and stx-negative E. coli O157:H7/H(-). In contrast to efforts with human stx-negative E. coli O157:H(-), attempts to transduce the two stx-negative/eae-positive animal O157:H7/H(-) strains with stx(2)-encoding phages from human SF and nSF EHEC O157:H7/H(-) failed, despite the animal strains having intact loci where such phages integrate in human EHEC O157 (wrbA and yecE). The role of animal stx-negative/eae-positive and stx-negative/eae-negative E. coli O157:H7/H(-) in their natural source and in human infections needs further investigation.

Intrahost genome alterations in enterohemorrhagic Escherichia coli

Bacterial chromosomes are not fixed molecules; they evolve over the course of infections in human beings. During infection, a variety of strong selective pressures are exerted on the pathogen. The resulting genetic changes that occur in intestinal pathogens might influence clinical outcome and have an impact on diagnosis and epidemiology. Enterohemorrhagic Escherichia coli (EHEC) is a good example of this process. These zoonotic pathogens cause diarrhea, bloody diarrhea, and hemolytic uremic syndrome in human beings, whereas in their natural habitat they mostly are asymptomatic colonizers. Thus, EHEC must be able to quickly adapt from one milieu to another. The greatest challenge it might face is to infect human beings--profound chromosomal changes occur during the brief period that EHEC passes through the human gastrointestinal tract, leading to gains and losses of virulence determinants. The intensive study of human enteric factors that induce or modulate pathogen chromosome instability could provide important information about host-microbial interactions.

Shiga toxin, cytolethal distending toxin, and hemolysin repertoires in clinical Escherichia coli O91 isolates

Shiga toxin (Stx)-producing Escherichia coli (STEC) strains of serogroup O91 are the most common human pathogenic eae-negative STEC strains. To facilitate diagnosis and subtyping of these pathogens, we genotypically and phenotypically characterized 100 clinical STEC O91 isolates. Motile strains expressed flagellar antigens H8 (1 strain), H10 (2 strains), H14 (52 strains), and H21 (20 strains) or were H nontypeable (Hnt) (10 strains); 15 strains were nonmotile. All nonmotile and Hnt strains possessed the fliC gene encoding the flagellin subunit of the H14 antigen (fliC(H14)). Most STEC O91 strains possessed enterohemorrhagic E. coli hlyA and expressed an enterohemolytic phenotype. Among seven stx alleles identified, stx(2dact), encoding mucus- and elastase-activatable Stx2d, was present solely in STEC O91:H21, whereas most strains of the other serotypes possessed stx(1). Moreover, only STEC O91:H21 possessed the cdt-V cluster, encoding cytolethal distending toxin V; the toxin was regularly expressed and was lethal to human microvascular endothelial cells. Infection with STEC O91:H21 was associated with hemolytic-uremic syndrome (P = 0.0015), whereas strains of the other serotypes originated mostly in patients with nonbloody diarrhea. We conclude that STEC O91 clinical isolates belong to at least four lineages that differ by H antigens/fliC types, stx genotypes, and non-stx putative virulence factors, with accumulation of virulence determinants in the O91:H21 lineage. Isolation of STEC O91 from patients' stools on enterohemolysin agar and the rapid initial subtyping of these isolates using fliC genotyping facilitate the identification of these emerging pathogens in clinical and epidemiological studies and enable prediction of the risk of a severe clinical outcome.

Enterohemorrhagic Escherichia coli O157:H7 gene expression profiling in response to growth in the presence of host epithelia

Background

The pathogenesis of enterohemorrhagic Escherichia coli (EHEC) O157∶H7 infection is attributed to virulence factors encoded on multiple pathogenicity islands. Previous studies have shown that EHEC O157∶H7 modulates host cell signal transduction cascades, independent of toxins and rearrangement of the cytoskeleton. However, the virulence factors and mechanisms responsible for EHEC-mediated subversion of signal transduction remain to be determined. Therefore, the purpose of this study was to first identify differentially regulated genes in response to EHEC O157∶H7 grown in the presence of epithelial cells, compared to growth in the absence of epithelial cells (that is, growth in minimal essential tissue culture medium alone, minimal essential tissue culture medium in the presence of 5% CO₂, and Penassay broth alone) and, second, to identify EHEC virulence factors responsible for pathogen modulation of host cell signal transduction.

Methodology/Principal Findings

Overnight cultures of EHEC O157∶H7 were incubated for 6 hr at 37°C in the presence or absence of confluent epithelial (HEp-2) cells. Total RNA was then extracted and used for microarray analyses (Affymetrix E. coli Genome 2.0 gene chips). Relative to bacteria grown in each of the other conditions, EHEC O157∶H7 cultured in the presence of cultured epithelial cells displayed a distinct gene-expression profile. A 2.0-fold increase in the expression of 71 genes and a 2.0-fold decrease in expression of 60 other genes were identified in EHEC O157∶H7 grown in the presence of epithelial cells, compared to bacteria grown in media alone.

Conclusion/Significance

Microarray analyses and gene deletion identified a protease on O-island 50, gene Z1787, as a potential virulence factor responsible for mediating EHEC inhibition of the interferon (IFN)-γ-Jak1,2-STAT-1 signal transduction cascade. Up-regulated genes provide novel targets for use in developing strategies to interrupt the infectious process.

Molecular subtyping and distribution of the serine protease from shiga toxin-producing Escherichia coli among atypical enteropathogenic E. coli strains

Atypical enteropathogenic Escherichia coli (aEPEC) and Shiga toxin-producing E. coli (STEC) were examined to determine the prevalence and sequence of espP, which encodes a serine protease. These analyses indicated shared espP sequence types between the two E. coli pathotypes and thus provide further insights into the evolution of aEPEC and STEC.

Detection and characterization of the fimbrial sfp cluster in enterohemorrhagic Escherichia coli O165:H25/NM isolates from humans and cattle

The sfp cluster, encoding Sfp fimbriae and located in the large plasmid of sorbitol-fermenting (SF) enterohemorrhagic Escherichia coli (EHEC) O157 (pSFO157), has been considered a unique characteristic of this organism. We discovered and then characterized the sfp cluster in EHEC O165:H25/NM (nonmotile) isolates of human and bovine origin. All seven strains investigated harbored a complete sfp cluster (carrying sfpA, sfpH, sfpC, sfpD, sfpJ, sfpF, and sfpG) of 6,838 bp with >99% nucleotide sequence homology to the sfp cluster of SF EHEC O157:NM. The sfp cluster in EHEC O165:H25/NM strains was located in an approximately 80-kb (six strains) or approximately 120-kb (one strain) plasmid which differed in structure, virulence genes, and sfp flanks from pSFO157. All O165:H25/NM strains belonged to the same multilocus sequence type (ST119) and were only distantly phylogenetically related to SF EHEC O157:NM (ST11). The highly conserved sfp cluster in different clonal backgrounds suggests that this segment was acquired independently by EHEC O165:H25 and SF EHEC O157:NM. Its presence in an additional EHEC serotype extends the diagnostic utility of PCR targeting sfpA as an easy and efficient approach to seek EHEC in patients' stools. The reasons for the convergence of pathogenic EHEC strains on a suite of virulence loci remain unknown.

Treatment and outcome of Shiga-toxin-associated hemolytic uremic syndrome (HUS)

Hemolytic uremic syndrome (HUS) is the most common cause of acute renal failure in childhood and the reason for chronic renal replacement therapy. It leads to significant morbidity and mortality during the acute phase. In addition to acute morbidity and mortality, long-term renal and extrarenal complications can occur in a substantial number of children years after the acute episode of HUS. The most common infectious agents causing HUS are enterohemorrhagic Escherichia coli (EHEC)-producing Shiga toxin (and belonging to the serotype O157:H7) and several non-O157:H7 serotypes. D(+) HUS is an acute disease characterized by prodromal diarrhea followed by acute renal failure. The classic clinical features of HUS include the triad of microangiopathic hemolytic anemia, thrombocytopenia, and acute renal failure. HUS mortality is reported to be between 3% and 5%, and death due to HUS is nearly always associated with severe extrarenal disease, including severe central nervous system (CNS) involvement. Approximately two thirds of children with HUS require dialysis therapy, and about one third have milder renal involvement without the need for dialysis therapy. General management of acute renal failure includes appropriate fluid and electrolyte management, antihypertensive therapy if necessary, and initiation of renal replacement therapy when appropriate. The prognosis of HUS depends on several contributing factors. In general "classic" HUS, induced by EHEC, has an overall better outcome. Totally different is the prognosis in patients with atypical and particularly recurrent HUS. However, patients with severe disease should be screened for genetic disorders of the complement system or other underlying diseases.

Anaerobic conditions promote expression of Sfp fimbriae and adherence of sorbitol-fermenting enterohemorrhagic Escherichia coli O157:NM to human intestinal epithelial cells

The sfp gene cluster, unique to sorbitol-fermenting (SF) enterohemorrhagic Escherichia coli (EHEC) O157:NM strains, encodes fimbriae that mediate mannose-resistant hemagglutination in laboratory E. coli strains but are not expressed in wild-type SF EHEC O157:NM strains under standard laboratory conditions. We investigated whether Sfp fimbriae are expressed under conditions that mimic the intestinal environment and whether they contribute to the adherence of SF EHEC O157:NM strains to human intestinal epithelial cells. The transcription of sfpA (encoding the major fimbrial subunit) was upregulated in all strains investigated, and all expressed SfpA and possessed fimbriae that reacted with an anti-SfpA antibody when the strains were grown on solid media under anaerobic conditions. Sfp expression was absent under aerobic conditions and in liquid media. Sfp upregulation under anaerobic conditions was significantly higher on blood agar and a medium simulating the colonic environment than on a medium simulating the ileal environment (P < 0.05). The induction of Sfp fimbriae in SF E. coli O157:NM strains correlates with increased adherence to Caco-2 and HCT-8 cells. Our data indicate that the expression of Sfp fimbriae in SF E. coli O157:NM strains is induced under conditions resembling those of the natural site of infection and that Sfp fimbriae may contribute to the adherence of the organisms to human intestinal epithelium.