In vitro effects of a high-molecular-weight heat-labile enterotoxin from enteroaggregative Escherichia coli

Plasmid-encoded toxin of Escherichia coli cleaves complement system proteins and inhibits complement-mediated lysis in vitro

Plasmid-encoded toxin (Pet) is an autotransporter protein of the serine protease autotransporters of Enterobacteriaceae (SPATE) family, important in the pathogenicity of Escherichia coli. The pet gene was initially found in the enteroaggregative E. coli (EAEC) virulence plasmid, pAA2. Although this virulence factor was initially described in EAEC, an intestinal E. coli pathotype, pet may also be present in other pathotypes, including extraintestinal pathogenic strains (ExPEC). The complement system is an important defense mechanism of the immune system that can be activated by invading pathogens. Proteases produced by pathogenic bacteria, such as SPATEs, have proteolytic activity and can cleave components of the complement system, promoting bacterial resistance to human serum. Considering these factors, the proteolytic activity of Pet and its role in evading the complement system were investigated. Proteolytic assays were performed by incubating purified components of the complement system with Pet and Pet S260I (a catalytic site mutant) proteins. Pet, but not Pet S260I, could cleave C3, C5 and C9 components, and also inhibited the natural formation of C9 polymers. Furthermore, a dose-dependent inhibition of ZnCl2-induced C9 polymerization in vitro was observed. E. coli DH5α survived incubation with human serum pre-treated with Pet. Therefore, Pet can potentially interfere with the alternative and the terminal pathways of the complement system. In addition, by cleaving C9, Pet may inhibit membrane attack complex (MAC) formation on the bacterial outer membrane. Thus, our data are suggestive of a role of Pet in resistance of E. coli to human serum.

Serine proteases autotransporter of Enterobacteriaceae: Structures, subdomains, motifs, functions, and targets

Serine protease autotransporters of Enterobacteriaceae (SPATE) constitute a superfamily of virulence factors, resembling the trypsin-like superfamily of serine proteases. SPATEs accomplish multiple functions associated to disease development of their hosts, which could be the consequence of SPATE cleavage of host cell components. SPATEs have been divided into class-1 and class-2 based on structural differences and biological effects, including similar substrate specificity, cytotoxic effects on cultured cells, and enterotoxin activity on intestinal tissues for class-1 SPATEs, whereas most class-2 SPATEs exhibit a lectin-like activity with a predilection to degrade a variety of mucins, including leukocyte surface O-glycoproteins and soluble host proteins, resulting in mucosal colonization and immune modulation. In this review, the structure of class-1 and class-2 are analyzed, making emphasis on their putative functional subdomains as well as a description of their function is provided, including prototypical mechanism of action.

A One Health Perspective for Defining and Deciphering Escherichia coli Pathogenic Potential in Multiple Hosts

E. coli is one of the most common species of bacteria colonizing humans and animals. The singularity of E. coli 's genus and species underestimates its multifaceted nature, which is represented by different strains, each with different combinations of distinct virulence factors. In fact, several E. coli pathotypes, or hybrid strains, may be associated with both subclinical infection and a range of clinical conditions, including enteric, urinary, and systemic infections. E. coli may also express DNA-damaging toxins that could impact cancer development. This review summarizes the different E. coli pathotypes in the context of their history, hosts, clinical signs, epidemiology, and control. The pathotypic characterization of E. coli in the context of disease in different animals, including humans, provides comparative and One Health perspectives that will guide future clinical and research investigations of E. coli infections.

Molecular Epidemiology of Enteroaggregative Escherichia coli (EAEC) Isolates of Hospitalized Children from Bolivia Reveal High Heterogeneity and Multidrug-Resistance

Enteroaggregative Escherichia coli (EAEC) is an emerging pathogen frequently associated with acute diarrhea in children and travelers to endemic regions. EAEC was found the most prevalent bacterial diarrheal pathogen from hospitalized Bolivian children less than five years of age with acute diarrhea from 2007 to 2010. Here, we further characterized the epidemiology of EAEC infection, virulence genes, and antimicrobial susceptibility of EAEC isolated from 414 diarrheal and 74 non-diarrheal cases. EAEC isolates were collected and subjected to a PCR-based virulence gene screening of seven virulence genes and a phenotypic resistance test to nine different antimicrobials. Our results showed that atypical EAEC (a-EAEC, AggR-negative) was significantly associated with diarrhea (OR, 1.62, 95% CI, 1.25 to 2.09, p < 0.001) in contrast to typical EAEC (t-EAEC, AggR-positive). EAEC infection was most prevalent among children between 7–12 months of age. The number of cases exhibited a biannual cycle with a major peak during the transition from warm to cold (April–June). Both typical and a-EAEC infections were graded as equally severe; however, t-EAEC harbored more virulence genes. aap, irp2 and pic were the most prevalent genes. Surprisingly, we detected 60% and 52.6% of multidrug resistance (MDR) EAEC among diarrheal and non-diarrheal cases. Resistance to ampicillin, sulfonamides, and tetracyclines was most common, being the corresponding antibiotics, the ones that are frequently used in Bolivia. Our work is the first study that provides comprehensive information on the high heterogenicity of virulence genes in t-EAEC and a- EAEC and the large prevalence of MDR EAEC in Bolivia.

Escherichia coli Pathobionts Associated with Inflammatory Bowel Disease

Gut bacteria play a key role in initiating and maintaining the inflammatory process in the gut tissues of inflammatory bowel disease (IBD) patients, by supplying antigens or other stimulatory factors that trigger immune cell activation. Changes in the composition of the intestinal microbiota in IBD patients compared to that in healthy controls and a reduced diversity of intestinal microbial species are linked to the pathogenesis of IBD. Adherent invasive Escherichia coli (AIEC) has been linked to Crohn's disease (CD) patients, while diffusely adherent E. coli (DAEC) has been associated with ulcerative colitis (UC). Bacteriological analysis of intestinal biopsy specimens and fecal samples from IBD patients shows an increased number of E. coli strains belonging to the B2 phylogenetic group, which are typically known as extraintestinal pathogenic E. coli (ExPEC). Results from studies of both cell cultures and animal models reveal pathogenic features of these E. coli pathobionts, which may link them to IBD pathogenesis. This suggests that IBD-associated E. coli strains play a facilitative role during IBD flares. In this review, we explain IBD-associated E. coli and its role in IBD pathogenesis.

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.

Virulence gene profiles and molecular genetic characteristics of diarrheagenic Escherichia coli from a hospital in western China

Background

Diarrheagenic Escherichia coli (DEC) is one of the most important etiological agents of diarrheal diseases. In this study we investigated the prevalence, virulence gene profiles, antimicrobial resistance, and molecular genetic characteristics of DEC at a hospital in western China.

Methods

A total of 110 Escherichia coli clinical isolates were collected from the First Affiliated Hospital of Chengdu Medical College from 2015 to 2016. Microbiological methods, PCR, antimicrobial susceptibility test, pulsed-field gel electrophoresis and multilocus sequence typing were used in this study.

Results

Molecular analysis of six DEC pathotype marker genes showed that 13 of the 110 E. coli isolates (11.82%) were DEC including nine (8.18%) diffusely adherent Escherichia coli (DAEC) and four (3.64%) enteroaggregative Escherichia coli (EAEC). The adherence genes fimC and fimH were present in all DAEC and EAEC isolates. All nine DAEC isolates harbored the virulence genes fyuA and irp2 and four (44.44%) also carried the hlyA and sat genes. The virulence genes fyuA, irp2, cnf1, hlyA, and sat were found in 100%, 100%, 75%, 50%, and 50% of EAEC isolates, respectively. In addition, all DEC isolates were multidrug resistant and had high frequencies of antimicrobial resistance. Molecular genetic characterization showed that the 13 DEC isolates were divided into 11 pulsed-field gel electrophoresis patterns and 10 sequence types.

Conclusions

To the best of our knowledge, this study provides the first report of DEC, including DAEC and EAEC, in western China. Our analyses identified the virulence genes present in E. coli from a hospital indicating their role in the isolated DEC strains’ pathogenesis. At the same time, the analyses revealed, the antimicrobial resistance pattern of the DEC isolates. Thus, DAEC and EAEC among the DEC strains should be considered a significant risk to humans in western China due to their evolved pathogenicity and antimicrobial resistance pattern.

Genomic and functional characterisation of two Enterococcus strains isolated from Cotija cheese and their potential role in ripening

Enterococcus spp. are present in the native microbiota of many traditional fermented foods. Their ability to produce antibacterial compounds, mainly against Listeria monocytogenes, has raised interest recently. However, there is scarce information about their proteolytic and lipolytic potential, and their biotechnological application is currently limited because enterococcal strains have been related to nosocomial infections. In this work, next-generation sequencing and optimised bioinformatic pipelines were used to annotate the genomes of two Enterococcus strains-one E. faecium and one E. faecalis-isolated from the Mexican artisanal ripened Cotija cheese. A battery of genes involved in their proteolytic system was annotated. Genes coding for lipases, esterases and other enzymes whose final products contribute to cheese aroma and flavour were identified as well. As for the production of antibacterial compounds, several peptidoglycan hydrolase- and bacteriocin-coding genes were identified in both genomes experimentally and by bioinformatic analyses. E. faecalis showed resistance to aminoglycosides and E. faecium to aminoglycosides and macrolides, as predicted by the genome functional annotation. No pathogenicity islands were found in any of the strains, although traits such as the ability of biofilm formation and cell aggregation were observed. Finally, a comparative genomic analysis was able to discriminate between the food strains isolated and nosocomial strains. In summary, pathogenic strains are resistant to a wide range of antibiotics and contain virulence factors that cause host damage; in contrast, food strains display less antibiotic resistance, include genes that encode class II bacteriocins and express virulence factors associated with host colonisation rather than invasion.

A simple electroelution method for rapid protein purification: isolation and antibody production of alpha toxin from Clostridium septicum

Clostridium septicum produces a number of diseases in human and farm animals which, in most of the cases, are fatal without clinical intervention. Alpha toxin is an important agent and the unique lethal virulent factor produced by Clostridium septicum. This toxin is haemolytic, highly lethal and necrotizing activities but is being used as an antigen to develop animal vaccines. The aim of this study was to isolate the alpha toxin of Clostridium septicum and produce highly specific antibodies against it. In this work, we have developed a simple and efficient method for alpha toxin purification, based on electroelution that can be used as a time-saving method for purifying proteins. This technique avoids contamination by other proteins that could appear during other protein purification techniques such chromatography. The highly purified toxin was used to produce polyclonal antibodies. The specificity of the antibodies was tested by western blot and these antibodies can be applied to the quantitative determination of alpha toxin by slot blot.

Genetic Virulence Profile of Enteroaggregative Escherichia coli Strains Isolated from Danish Children with Either Acute or Persistent Diarrhea

Enteroaggregative Escherichia coli (EAEC) is frequently found in diarrheal stools worldwide. It has been associated with persistent diarrhea, weight loss, and failure to thrive in children living in developing countries. A number of important EAEC virulence genes are identified; however, their roles in acute and persistent diarrhea have not been previously investigated. The aim of this study was to identify specific EAEC virulence genes associated with duration and type of diarrhea in Danish children. We aimed to improve the current diagnostics of EAEC and enable targeting of strains with an expected severe disease course. Questionnaires answered by parents provided information regarding duration of diarrhea and presence of blood or mucus. A total of 295 EAEC strains were collected from children with acute (≤7 days) and persistent diarrhea (≥14 days) and were compared by using multiplex PCR targeting the genes sat, sepA, pic, sigA, pet, astA, aatA, aggR, aaiC, aap, agg3/4C, ORF3, aafA, aggA, agg3A, agg4A, and agg5A. Furthermore, the distribution of EAEC genes in strains collected from cases of bloody, mucoid, and watery diarrhea was investigated. The classification and regression tree analysis (CART) was applied to investigate the relationship between EAEC virulence genes and diarrheal duration and type. Persistent diarrhea was associated with strains lacking the pic gene (p = 0.002) and with the combination of the genes pic, sat, and absence of the aggA gene (p = 0.05). Prolonged diarrhea was associated with the combination of the genes aatA and astA (p = 0.03). Non-mucoid diarrhea was associated with strains lacking the aatA gene (p = 0.004). Acute diarrhea was associated with the genes aggR, aap, and aggA by individual odds ratios. Resistance toward gentamicin and ciprofloxacin was observed in 7.5 and 3% of strains, respectively. Multi-drug resistance was observed in 38% of strains. Genetic host factors have been associated with an increased risk of EAEC-associated disease. Therefore, we investigated a panel of risk factors in two groups of children—EAEC-positive and EAEC-negative—to identify additional factors predisposing to disease. The duration of breastfeeding was positively correlated with the likelihood of belonging to the EAEC-negative group of children.

Diarrheagenic Escherichia coli

Most Escherichia coli strains live harmlessly in the intestines and rarely cause disease in healthy individuals. Nonetheless, a number of pathogenic strains can cause diarrhea or extraintestinal diseases both in healthy and immunocompromised individuals. Diarrheal illnesses are a severe public health problem and a major cause of morbidity and mortality in infants and young children, especially in developing countries. E. coli strains that cause diarrhea have evolved by acquiring, through horizontal gene transfer, a particular set of characteristics that have successfully persisted in the host. According to the group of virulence determinants acquired, specific combinations were formed determining the currently known E. coli pathotypes, which are collectively known as diarrheagenic E. coli. In this review, we have gathered information on current definitions, serotypes, lineages, virulence mechanisms, epidemiology, and diagnosis of the major diarrheagenic E. coli pathotypes.

Epidemiology and clinical manifestations of enteroaggregative Escherichia coli

Enteroaggregative Escherichia coli (EAEC) represents a heterogeneous group of E. coli strains. The pathogenicity and clinical relevance of these bacteria are still controversial. In this review, we describe the clinical significance of EAEC regarding patterns of infection in humans, transmission, reservoirs, and symptoms. Manifestations associated with EAEC infection include watery diarrhea, mucoid diarrhea, low-grade fever, nausea, tenesmus, and borborygmi. In early studies, EAEC was considered to be an opportunistic pathogen associated with diarrhea in HIV patients and in malnourished children in developing countries. In recent studies, associations with traveler's diarrhea, the occurrence of diarrhea cases in industrialized countries, and outbreaks of diarrhea in Europe and Asia have been reported. In the spring of 2011, a large outbreak of hemolytic-uremic syndrome (HUS) and hemorrhagic colitis occurred in Germany due to an EAEC O104:H4 strain, causing 54 deaths and 855 cases of HUS. This strain produces the potent Shiga toxin along with the aggregative fimbriae. An outbreak of urinary tract infection associated with EAEC in Copenhagen, Denmark, occurred in 1991; this involved extensive production of biofilm, an important characteristic of the pathogenicity of EAEC. However, the heterogeneity of EAEC continues to complicate diagnostics and also our understanding of pathogenicity.

Cytokeratin 8 is an epithelial cell receptor for Pet, a cytotoxic serine protease autotransporter of Enterobacteriaceae

The group of proteins known as serine protease autotransporters of Enterobacteriaceae (SPATE) is a growing family of serine proteases secreted to the external milieu by the type V secretion system. Pet toxin and some other SPATE belong to the class 1 cytotoxic SPATE, which have comparable protease strength on fodrin. Pet is internalized and is directed to its intracellular substrate by retrograde transport. However, the epithelial cell receptor for Pet has yet to be identified. We show that Pet has affinity for the epithelial cell surface until the saturation of the binding sites at 100 nM Pet. Affinity column assays and matrix-assisted laser desorption ionization–time of flight (MALDI-TOF) analysis identified a cytokeratin (CK8) which directly binds to Pet, and both proteins colocalized on the cell surface. Interestingly, CK8 is not present in kidney cell lines, which are not susceptible to Pet. Inhibition experiments by using anti-CK8 and ck8 small interfering RNA (siRNA) blocked the cytotoxic effect induced by Pet, while exogenous CK8 expression in kidney cells made them susceptible to Pet intoxication. Recombinant CK8 showed a Pet-binding pattern similar to that seen by using fixed cells. Remarkably, Pet colocalized with CK8 and clathrin at early times (receptor-mediated endocytosis), and subsequently, Pet colocalized with CK8 and Rab5b in the early endosomes. These data support the idea that CK8 is an important receptor for Pet on epithelial cells for starting its cytotoxic effects. These data suggest that therapeutics that block Pet-CK8 interaction may improve outcome of diseases caused by Pet-secreting Enterobacteriaceae such as enteroaggregative Escherichia coli.

Receptor-ligand binding is one mechanism by which cells sense and respond to external cues. Receptors may also be utilized by toxins to mediate their own internalization. Pet toxin is secreted by enteroaggregative Escherichia coli, an organism that causes persistent diarrhea in children, traveler’s diarrhea, and acute and persistent diarrhea in patients with HIV. Pet is a member of the family of serine protease autotransporters of Enterobacteriaceae (SPATE). SPATE in different pathogens are virulence factors, and Pet belongs to the class 1 cytotoxic SPATE, which have comparable protease strength on their biological substrate, fodrin (a cytoskeletal protein important for maintaining cell viability). To cleave fodrin, Pet enters the cells by clathrin-mediated endocytosis. This mechanism includes receptor-mediated endocytosis (a receptor-ligand complex triggers the endocytosis). We show that CK8 is an important receptor for Pet on epithelial cells and that it may be useful for identifying molecules that block the interaction of CK8 with Pet.

Autotransporter secretion: varying on a theme

Autotransporters are widely distributed among Gram-negative bacteria. They can have a large variety of functions and many of them have a role in virulence. They are synthesized as large precursors with an N-terminal signal sequence that mediates transport across the inner membrane via the Sec machinery and a translocator domain that mediates the transport of the connected passenger domain across the outer membrane to the bacterial cell surface. Like integral outer membrane proteins, the translocator domain folds in a β-barrel structure and requires the Bam machinery for its insertion into the outer membrane. After transport across the outer membrane, the passenger may stay connected via the translocator domain to the bacterial cell surface or it is proteolytically released into the extracellular milieu. Based on the size of the translocator domain and its position relative to the passenger in the precursor, autotransporters are divided into four sub-categories. We review here the current knowledge of the biogenesis, structure and function of various autotransporters.

Prevalence of enteroaggregative Escherichia coli and its virulence-related genes in a case-control study among children from north-eastern Brazil

Enteroaggregative Escherichia coli (EAEC) is an important agent that causes endemic and epidemic diarrhoeal diseases worldwide. Several EAEC virulence-related genes (VRGs) have been described but their role in the clinical outcome of infection is not completely defined. This study investigated the prevalence of EAEC and potential associations of its VRGs with risk of or protection from diarrhoeal diseases in children from urban communities in north-eastern Brazil. The case-control study included 166 children, who had their stools evaluated for the EAEC diagnostic genes (aaiC and aatA) using PCR. Positive samples were further analysed by multiplex PCR and identified 18 VRGs. EAEC was found in the same proportion in both groups (41%). The plasmid-borne gene encoding a hexosyltransferase homologue (capU) was the most frequently detected (89.6%), followed by dispersin protein (aap, 58.2%) and EAEC HilA homologue (eilA, 57.8%). The AAF/III fimbrial subunit (agg3A) gene was observed at lower frequency (1.5%). Plasmid-encoded toxin (pet) or AAF/II fimbrial subunit (aafA) was associated significantly with disease. AAF/IV fimbrial subunit (agg4A) or hypothetical plasmid-encoded haemolysin (orf61) was detected significantly more in controls than in children with diarrhoea. In addition, one set of genes in combination, aaiC and agg3/4C but lacking agg4A and orf61, was associated with diarrhoea cases; and another one, orf61 in the absence of pet and aafA, was correlated with control children. These data confirm a high prevalence, endemicity and heterogeneity of EAEC strains in the developing urban areas of north-eastern Brazil. Statistical correlation between cases and controls was seen with either isolated or combined sets of genes, suggesting that the pathophysiology of EAEC infection involves a complex and dynamic modulation of several VRGs.

Enteroaggregative Escherichia coli, a heterogenous, underestimated and under-diagnosed E. coli pathotype in Iran

The main features of enteroaggregative Escherichia coli (EAEC) pathogenesis include attachment of bacteria to the intestinal mucosa, production of various toxins and cytotoxins, and stimulation of mucosal inflammation. 'Virulence' genes encode these features. Comparison of different EAEC isolates has shown that the virulence gene content of these isolates varies considerably. The heterogeneity of EAEC strains was concluded from the results obtained from the volunteer as well as other studies. Although the underlying mechanism behind the apparent increase in O104:H4 virulence is not known, several bacterial factors have been implicated. In this review, the known virulence factors involved in pathogenesis of EAEC pathotype are summarized.

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.

Genome sequence analyses of two isolates from the recent Escherichia coli outbreak in Germany reveal the emergence of a new pathotype: Entero-Aggregative-Haemorrhagic Escherichia coli (EAHEC)

The genome sequences of two Escherichia coli O104:H4 strains derived from two different patients of the 2011 German E. coli outbreak were determined. The two analyzed strains were designated E. coli GOS1 and GOS2 (German outbreak strain). Both isolates comprise one chromosome of approximately 5.31 Mbp and two putative plasmids. Comparisons of the 5,217 (GOS1) and 5,224 (GOS2) predicted protein-encoding genes with various E. coli strains, and a multilocus sequence typing analysis revealed that the isolates were most similar to the entero-aggregative E. coli (EAEC) strain 55989. In addition, one of the putative plasmids of the outbreak strain is similar to pAA-type plasmids of EAEC strains, which contain aggregative adhesion fimbrial operons. The second putative plasmid harbors genes for extended-spectrum β-lactamases. This type of plasmid is widely distributed in pathogenic E. coli strains. A significant difference of the E. coli GOS1 and GOS2 genomes to those of EAEC strains is the presence of a prophage encoding the Shiga toxin, which is characteristic for enterohemorrhagic E. coli (EHEC) strains. The unique combination of genomic features of the German outbreak strain, containing characteristics from pathotypes EAEC and EHEC, suggested that it represents a new pathotype Entero-Aggregative-Haemorrhagic Escherichiacoli (EAHEC).

Effects of the plasmid-encoded toxin of enteroaggregative Escherichia coli on focal adhesion complexes

Enteroaggregative Escherichia coli (EAEC) is an emerging diarrheal pathogen. Many EAEC strains produce the plasmid-encoded toxin (Pet), which exerts cytotoxic effects on human intestinal tissue. Pet-intoxicated HEp-2 cells exhibit rounding and detachment from the substratum, accompanied by loss of F-actin stress fibers and condensation of the spectrin-containing membrane cytoskeleton. Although studies suggest that Pet directly cleaves spectrin, it is not known whether this is the essential mode of action of the toxin. In addition, the effects of Pet on cytoskeletal elements other than actin and spectrin have not been reported. Here, we demonstrate by immunofluorescence that upon Pet intoxication, HEp-2 and HT29 cells lose focal adhesion complexes (FAC), a process that includes the redistribution of focal adhesion kinase (FAK), α-actinin, paxillin, vinculin, F-actin, and spectrin itself. This redistribution was coupled with the depletion of phosphotyrosine labeling at FACs. Immunoblotting and immunoprecipitation experiments revealed that FAK was tyrosine dephosphorylated, before the redistribution of FAK and spectrin. Moreover, phosphatase inhibition blocked cell retraction, suggesting that tyrosine dephosphorylation is an event that precedes FAK cleavage. Finally, we show that in vitro tyrosine-dephosphorylated FAK was susceptible to Pet cleavage. These data suggest that mechanisms other than spectrin redistribution occur during Pet intoxication.

Enteroaggregative Escherichia coli plasmid-encoded toxin

Plasmid-encoded toxin (Pet) is secreted by enteroaggregative Escherichia coli (EAEC), a pathotype of diarrhogenic E. coli. EAEC infection is an important cause of diarrhea in outbreak and nonoutbreak settings in developing and developed countries. EAEC secretes Pet by using the type V secretion system. Mature secreted Pet is a serine protease and its eukaryotic target is the actin-binding protein alpha-fodrin. When Pet cleaves alpha-fodrin in the target cell cytosol, the organization of the actin cytoskeleton is disrupted. The loss of actin filament structure results in cell rounding and detachment from the substratum. This article summarizes the long trip of Pet during its biogenesis, its interaction with epithelial cells, intracellular trafficking and mechanism of action.

Pic, an autotransporter protein secreted by different pathogens in the Enterobacteriaceae family, is a potent mucus secretagogue

A hallmark of enteroaggregative Escherichia coli (EAEC) infection is a formation of biofilm, which comprises a mucus layer with immersed bacteria in the intestines of patients. While studying the mucinolytic activity of Pic in an in vivo system, rat ileal loops, we surprisingly found that EAEC induced hypersecretion of mucus, which was accompanied by an increase in the number of mucus-containing goblet cells. Interestingly, an isogenic pic mutant (EAEC Δpic) was unable to cause this mucus hypersecretion. Furthermore, purified Pic was also able to induce intestinal mucus hypersecretion, and this effect was abolished when Pic was heat denatured. Site-directed mutagenesis of the serine protease catalytic residue of Pic showed that, unlike the mucinolytic activity, secretagogue activity did not depend on this catalytic serine protease motif. Other pathogens harboring the pic gene, such as Shigella flexneri and uropathogenic E. coli (UPEC), also showed results similar to those for EAEC, and construction of isogenic pic mutants of S. flexneri and UPEC confirmed this secretagogue activity. Thus, Pic mucinase is responsible for one of the pathophysiologic features of the diarrhea mediated by EAEC and the mucoid diarrhea induced by S. flexneri.

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.

The majority of enteroaggregative Escherichia coli strains produce the E. coli common pilus when adhering to cultured epithelial cells

Enteroaggregative Escherichia coli (EAEC) have emerged as a significant worldwide cause of chronic diarrhea in the pediatric population and in HIV patients. The vast majority of EAEC strains do not produce the aggregative adherence fimbriae I-III (AAFs) so far reported and thus, what adherence factors are present in these strains remains unknown. Here, we investigated the prevalence of the chromosomal E. coli common pilus (ECP) genes and ECP production amongst 130 EAEC strains of diverse origin as well as the role of ECP in EAEC adherence. Through multiplex PCR analysis we found that 96% of EAEC strains contained the ecpA structural pilin gene whereas only 3.1% and 5.4% were positive for AAF fimbrial genes aggA or aafA, respectively. Among the ecpA(+) strains, 63% produced ECP when adhering to cultured epithelial cells. An ecpA mutant derived from prototypic strain 042 (AAF/II(+)) was not altered in adherence suggesting that the AAF/II, and not ECP, plays a major role in this strain. In contrast, strain 278-1 (AAF(-)) deleted of the ecpA gene was significantly reduced in adherence to cultured epithelial cells. In all, these data indicate a potential role of ECP in adherence for EAEC strains lacking the known AAFs and that in association with other adhesive determinants, ECP may contribute to their survival and persistence within the host and in the environment.

Host-Toxin Interactions Involving EspC and Pet, Two Serine Protease Autotransporters of the Enterobacteriaceae

EspC and Pet are toxins secreted by the diarrheagenic enteropathogenic and enteroaggregative Escherichia coli pathotypes, respectively. Both toxins have a molecular mass around 110 kDa and belong to the same protein family called Serine Protease Autotransporters of the Enterobacteriaceae (SPATE). Furthermore, both toxins act within the cytosol of intoxicated epithelial cells to disrupt the architecture of the actin cytoskeleton. This cytopathic and enterotoxic effect results from toxin cleavage of the actin-binding protein fodrin, although the two toxins recognize different cleavage sites on fodrin. EspC and Pet also have dramatically different mechanisms of entering the target cell which appear dependent upon the E. coli pathotype. In this review, we compare/contrast EspC and Pet in regards to their mode of delivery into the target cell, their effects on fodrin and the actin cytoskeleton, and their possible effects on the physiology of the intestinal epithelial cell.

A host-specific factor is necessary for efficient folding of the autotransporter plasmid-encoded toxin

Autotransporters are the most common virulence factors secreted from Gram-negative pathogens. Until recently, autotransporter folding and outer membrane translocation were thought to be self-mediated events that did not require accessory factors. Here, we report that two variants of the autotransporter plasmid-encoded toxin are secreted by a lab strain of Escherichia coli. Biophysical analysis and cell-based toxicity assays demonstrated that only one of the two variants was in a folded, active conformation. The misfolded variant was not produced by a pathogenic strain of enteroaggregative E. coli and did not result from protein overproduction in the lab strain of E. coli. Our data suggest a host-specific factor is required for efficient folding of plasmid-encoded toxin.

Pet secretion, internalization and induction of cell death during infection of epithelial cells by enteroaggregative Escherichia coli

In an in vitro model using HEp-2 cells treated with purified plasmid-encoded toxin (Pet), we have identified morphological changes characterized by cell rounding and detachment after toxin internalization; these changes progress to cell death. However, these effects have not yet been shown to occur during the infection of epithelial cells by enteroaggregative Escherichia coli (EAEC). Here, we show that the secretion of Pet by EAEC is regulated at the transcriptional level, since secretion was inhibited in eukaryotic cell culture medium, although Pet was efficiently secreted in the same medium supplemented with tryptone. Inefficient secretion of Pet by EAEC in DMEM prevented cell detachment, whereas efficient Pet secretion in DMEM/tryptone increased cell detachment in a HEp-2 cell adherence assay. Interestingly, Pet toxin was efficiently delivered to epithelial cells, since it was internalized into epithelial cells infected with EAEC at similar concentrations to those obtained by using 37 microg ml(-1) purified Pet protein. Additionally, Pet was not internalized when the epithelial cells were infected with a pet clone, HB101(pCEFN1), unlike the wild-type strain, which has a high adherence capability. There is a correlation between Pet secretion by EAEC, the internalization of Pet into epithelial cells, cell detachment and cell death in EAEC-infected cells. The ratio between live and dead cells decreased in cells treated with wild-type EAEC in comparison with cells treated with an isogenic mutant in the pet gene, whereas the effects were restored by complementing the mutant with the pet gene. All these data indicate that Pet is an important virulence factor in the pathogenesis of EAEC infection.

Structural characteristics of the plasmid-encoded toxin from enteroaggregative Escherichia coli

Intoxication by the plasmid-encoded toxin (Pet) of enteroaggregative Escherichia coli requires toxin translocation from the endoplasmic reticulum (ER) to the cytosol. This event involves the quality control system of ER-associated degradation (ERAD), but the molecular details of the process are poorly characterized. For many structurally distinct AB-type toxins, ERAD-mediated translocation is triggered by the spontaneous unfolding of a thermally unstable A chain. Here we show that Pet, a non-AB toxin, engages ERAD by a different mechanism that does not involve thermal unfolding. Circular dichroism and fluorescence spectroscopy measurements demonstrated that Pet maintains most of its secondary and tertiary structural features at 37 degrees C, with significant thermal unfolding only occurring at temperatures >or=50 degrees C. Fluorescence quenching experiments detected the partial solvent exposure of Pet aromatic amino acid residues at 37 degrees C, and a cell-based assay suggested that these changes could activate an ERAD-related event known as the unfolded protein response. We also found that HEp-2 cells were resistant to Pet intoxication when incubated with glycerol, a protein stabilizer. Altogether, our data are consistent with a model in which ERAD activity is triggered by a subtle structural destabilization of Pet and the exposure of Pet hydrophobic residues at physiological temperature. This was further supported by computer modeling analysis, which identified a surface-exposed hydrophobic loop among other accessible nonpolar residues in Pet. From our data it appears that Pet can promote its ERAD-mediated translocation into the cytosol by a distinct mechanism involving partial exposure of hydrophobic residues rather than the substantial unfolding observed for certain AB toxins.

The serine protease motif of Pic mediates a dose-dependent mucolytic activity after binding to sugar constituents of the mucin substrate

The pic gene is harbored on the chromosomes of three important pathogens: enteroaggregative Escherichia coli (EAEC), uropathogenic E. coli (UPEC), and Shigella flexneri. Since Pic is secreted into the intestinal lumen during EAEC infection, we sought to identify intestinal-mucosal substrates for Pic. Pic did not damage epithelial cells, cleave fodrin, or degrade host defense proteins embedded in the mucus layer (sIgA, lactoferrin and lysozyme). However, by using a solid-phase assay to evaluate the mucinolytic activity of EAEC Pic, we documented a specific, dose-dependent mucinolytic activity. A serine protease inhibitor and an enzymatically inactive variant of Pic were used to show that the Pic serine protease motif is required for mucinolytic activity. Pic binds mucin, and this binding was blocked in competition assays using monosaccharide constituents of the oligosaccharide side chains of mucin. Moreover, Pic mucinolytic activity decreased when sialic acid was removed from mucin. Thus, Pic is a mucinase with lectin-like activity that can be related to its reported hemagglutinin activity. Our results suggest that EAEC may secrete Pic into the intestinal lumen as a strategy for penetrating the gel-like mucus layer during EAEC colonization.

Intoxication of epithelial cells by plasmid-encoded toxin requires clathrin-mediated endocytosis

It has been shown that the autotransporter plasmid-encoded toxin (Pet) of enteroaggregative Escherichia coli (EAEC) produces cytotoxic and enterotoxic effects. Both effects can be explained by the proteolytic activity of Pet on its intracellular target alpha-fodrin (alphaII spectrin). In addition, Pet cytotoxicity and enterotoxicity depend on Pet serine protease activity, and on its internalization into epithelial cells. However, the mechanisms of Pet uptake by epithelial cells are unknown. Here, we show that Pet interacts with the plasma membrane of epithelial cells, and afterwards is detected inside the cells. Furthermore, Pet was internalized via clathrin-mediated endocytosis, since its internalization was inhibited by monodansylcadaverine and sucrose, but not by filipin or methyl-beta-cyclodextrin, which are drugs that interfere with protein entry via a clathrin-independent pathway. Additionally, Pet was immunoprecipitated by anti-clathrin antibodies, but not by anti-caveolin antibodies. Moreover, small interfering RNA (siRNA), designed to knock out clathrin gene expression in HEp-2 cells, prevented Pet internalization, and thereby the Pet-induced cytotoxic effect. However, the use of siRNA to knock out caveolin expression had no effect on Pet internalization, and the cytotoxic effect was clearly observed. Together, these data indicate that Pet secreted by EAEC binds to the cell surface via an unknown receptor, to be taken up by clathrin-mediated endocytosis, and exert its toxic effect in the cytoplasm.

Pet, a non-AB toxin, is transported and translocated into epithelial cells by a retrograde trafficking pathway

The plasmid-encoded toxin (Pet) of enteroaggregative Escherichia coli is a 104-kDa autotransporter protein that exhibits proteolytic activity against the actin-binding protein alpha-fodrin. Intracellular cleavage of epithelial fodrin by Pet disrupts the actin cytoskeleton, causing both cytotoxic and enterotoxic effects. Intoxication requires the serine protease activity of Pet and toxin endocytosis from clathrin-coated pits. The additional events in the intracellular trafficking of Pet are largely uncharacterized. Here, we determined by confocal microscopy that internalized Pet is transferred from the early endosomes to the Golgi apparatus and then travels to the endoplasmic reticulum (ER). Pet associates with the Sec61p translocon before it moves into the cytosol as an intact, 104-kDa protein. This translocation process contrasts with the export of other ER-translocating toxins, in which only the catalytic A subunit of the AB toxin enters the cytosol. However, like intoxication with these AB toxins, Pet intoxication was inhibited in a subset of mutant CHO cell lines with aberrant activity in the ER-associated degradation pathway of ER-to-cytosol translocation. This is the first report which documents the cell surface-to-ER and ER-to-cytosol trafficking of a bacterial non-AB toxin.

Prevention and self-treatment of traveler's diarrhea

Of the millions who travel from the industrialized world to developing countries every year, between 20% and 50% will develop at least one episode of diarrhea, making it the most common medical ailment afflicting travelers. Although usually a mild illness, traveler's diarrhea can result in significant morbidity and hardship overseas. Precautions can be taken to minimize the risk of developing traveler's diarrhea, either through avoidance of potentially contaminated food or drink or through various prophylactic measures, including both nonpharmacological and antimicrobial strategies. If diarrhea does develop despite the precautions taken, effective treatment-usually a combination of an antibiotic and an antimotility agent-can be brought by the traveler and initiated as soon as symptoms develop. In the future, vaccines-several of which are in the advanced stages of clinical testing-may be added to the list of prophylactic measures.

Slot blot immunoassay as a tool for plasmid-encoded toxin detection in enteroaggregative Escherichia coli culture supernatants

Plasmid-encoded toxin (Pet) is a heat-labile enterotoxin encoded in the enteroaggregative Escherichia coli (EAEC) virulence plasmid. Several evidence support the role of this 108-kDa secreted protein in the pathogenesis of EAEC diarrhea. In this study, we standardized a slot blot immunoassay for Pet detection. EAEC culture supernatants were applied onto a polyvinylidene difluoride membrane, and, using rabbit polyclonal Pet antisera, the expression of the toxin by slot blot immunoassay was observed in 9.5% of the isolates studied. In addition, no negative control reacted with Pet antiserum in this assay. This assay is a rapid, specific, reproducible, and low-cost methodology, therefore demonstrating its potential in diagnosing Pet expression. Moreover, we describe for the first time that expression of Pet can be directly detected from EAEC culture supernatants and may be used in clinical laboratorial routine instead of polymerase chain reaction detection of the pet gene, especially in developing countries where the EAEC pathotype has been considered an emerging pathogen.

Autotransporter Proteins

This review focuses on the function of the Escherichia coli and Salmonella autotransporters for which a considerable amount of literature is available. Members of the serine protease autotransporters of the Enterobacteriaceae (SPATEs) family are proteins from E. coli and Shigella spp., which, like the Neisseria and Haemophilus influenzae IgA1 proteases and Hap, possess a consensus serine protease motif. The largest subfamily of autotransporters is defined by the AidA conserved domain COG3468 and consists of members from a diverse range of animal and plant pathogens including E. coli, S. enterica, Yersinia pestis. This subfamily, which is composed of more than 55 proteins, possesses some of the best-characterized autotransporter proteins including the S. flexneri mediator of motility IcsA, the major phase-variable E. coli outer membrane protein antigen 43 (Ag43) and the diffuse adhering E. coli (DAEC) adhesin AIDA-I, from which this subfamily derives its name. Another member of the AIDA-I family, and one of the most studied autotransporter proteins, is IcsA. The autotransporter pathway is emerging as the most common mechanism of protein translocation across the gram-negative outer membrane.

Antibody response against plasmid-encoded toxin (Pet) and the protein involved in intestinal colonization (Pic) in children with diarrhea produced by enteroaggregative Escherichia coli

Enteroaggregative Escherichia coli (EAEC) is an emerging cause of pediatric and adult travellers diarrhea. The mechanism by which EAEC induce diarrhea is not completely known. Two serine protease autotransporter proteins, named Pet and Pic have been identified in EAEC strains. Pet has enterotoxic and cytotoxic activities, while the role of Pic in pathogenesis may lie on its mucinolytic activity. Little is known about Pet and Pic biological activities in vivo. In this study the antibody responses against these autotransporter proteins in convalescent children is investigated. Fifteen (83%) children showed specific antibodies against Pet or Pic in their sera. IgG and IgM antibodies were the main isotype found. Specific antibodies against Pic, but not against Pet, were detected in sera from age-matched control group. These data show that specific anti-Pet and anti-Pic antibodies are produced during the course of a natural EAEC infection in children.

Pathophysiology and impact of enteric bacterial and protozoal infections: new approaches to therapy

Despite numerous scientific advances in the past few years regarding the pathogenesis, diagnostic tools and treatment of infectious enteritis, enteric infections remain a serious threat to health worldwide. With globalization of the food supply, the increase in travel, mass food processing and antibiotic resistance, infectious diarrhea has become a critical concern for both developing and developed countries. Oral rehydration therapy has been cited as the most important medical discovery of the century due to the millions of lives that have been saved. However, statistics concerning diarrhea-induced mortality and the highly underestimated morbidity continue to demonstrate the severity of the problem. A more complete understanding of the pathogenesis of infectious diarrhea and potential new vaccines and effective treatments are badly needed. In addition, public health preventive actions, such as early detection of outbreaks, care with food, water and sanitation and, where relevant, immunization, should be considered a priority. This article provides an overview of the epidemiological impact, pathogenesis and new approaches to the management of enteric infections.

Type V protein secretion pathway: the autotransporter story

Gram-negative bacteria possess an outer membrane layer which constrains uptake and secretion of solutes and polypeptides. To overcome this barrier, bacteria have developed several systems for protein secretion. The type V secretion pathway encompasses the autotransporter proteins, the two-partner secretion system, and the recently described type Vc or AT-2 family of proteins. Since its discovery in the late 1980s, this family of secreted proteins has expanded continuously, due largely to the advent of the genomic age, to become the largest group of secreted proteins in gram-negative bacteria. Several of these proteins play essential roles in the pathogenesis of bacterial infections and have been characterized in detail, demonstrating a diverse array of function including the ability to condense host cell actin and to modulate apoptosis. However, most of the autotransporter proteins remain to be characterized. In light of new discoveries and controversies in this research field, this review considers the autotransporter secretion process in the context of the more general field of bacterial protein translocation and exoprotein function.

A biologically active lectin of enteroaggregative Escherichia coli

The pathogenesis of enteroaggregative Escherichia coli, a major contributor to paediatric diarrhoea, is still not clearly understood. A complex carbohydrate specific lectin was identified from the culture supernatant of an enteroaggregative E. coli strain. The lectin was purified to 660-fold by a combination of sequential saturated ammonium sulphate precipitation and gel filtration chromatography in the FPLC system. The homogeneity of the purified lectin was established by analytical isoelectrofocusing [pI 6.75]. Hemagglutination of rabbit erythrocytes by the purified lectin was best inhibited by fetuin. The N-terminal sequence of the 41.7 kDa subunit showed homology to the outermembrane porins and the 23.4 kDa subunit showed homology to a hypothetical protein of Yersinia pestis and secreted Hcp protein. This protein could induce extensive morphological changes in HEp-2 cells and significant amount of fluid accumulation in rabbit ileal loop. GM1 showed maximum binding to the lectin among all other gangliosides. This purified protein showed cross-reactivity to the binding subunit of cholera toxin in western immunoblot. The presence of this toxin in some of the clinical isolates of enteroaggregative E. coli was also observed. The structural and functional characteristics of the toxin revealed that it is a novel virulence determinant of aggregative E. coli.

The serine protease motif of EspC from enteropathogenic Escherichia coli produces epithelial damage by a mechanism different from that of Pet toxin from enteroaggregative E. coli

EspC (Escherichia coli secreted protein C) of enteropathogenic E. coli (EPEC) shows the three classical domains of the autotransporter proteins and has a conserved serine protease motif belonging to the SPATE (serine protease autotransporters of Enterobacteriaceae) subfamily. EspC and its homolog Pet in enteroaggregative E. coli (EAEC) bear the same sequence within the serine protease motif, and both proteins produce enterotoxic effects, suggesting that like Pet, EspC could be internalized to reach and cleave the calmodulin-binding domain of fodrin, causing actin cytoskeleton disruption. Even though both proteins cause cytoskeleton damage by virtue of their serine protease motifs, the following evidence supports the hypothesis that the mechanisms are different. (i) To obtain similar cytotoxic and cytoskeletal effects, a threefold-higher EspC concentration and a twofold-higher exposure time are needed. (ii) EspC internalization into epithelial cells takes more time (6 h) than Pet internalization (30 min), and the distributions of the two proteins inside the cells are also different. (iii) Both proteins have affinity for fodrin and cleave it, but the cleavage sites are different; EspC produces two cleavages, while Pet produces just one. (iv) EspC does not cause fodrin redistribution within epithelial cells. (v) An EspC serine protease motif mutant, but not a Pet serine protease mutant, competes with EspC by blocking cytoskeletal damage. All these data suggest that the protein conformational structure is very important for the activity of the catalytic site, influencing its interaction with the target protein and its internalization. The differences between these proteins may explain the reduced ability of EspC to cause cytopathic effects. However, these differences may confer a specialized role on EspC in the pathogenesis of EPEC, which is different from that of Pet in EAEC pathogenesis.

Pathogenic Escherichia coli

Few microorganisms are as versatile as Escherichia coli. An important member of the normal intestinal microflora of humans and other mammals, E. coli has also been widely exploited as a cloning host in recombinant DNA technology. But E. coli is more than just a laboratory workhorse or harmless intestinal inhabitant; it can also be a highly versatile, and frequently deadly, pathogen. Several different E. coli strains cause diverse intestinal and extraintestinal diseases by means of virulence factors that affect a wide range of cellular processes.

Pathogenicity islands in bacterial pathogenesis

In this review, we focus on a group of mobile genetic elements designated pathogenicity islands (PAI). These elements play a pivotal role in the virulence of bacterial pathogens of humans and are also essential for virulence in pathogens of animals and plants. Characteristic molecular features of PAI of important human pathogens and their role in pathogenesis are described. The availability of a large number of genome sequences of pathogenic bacteria and their benign relatives currently offers a unique opportunity for the identification of novel pathogen-specific genomic islands. However, this knowledge has to be complemented by improved model systems for the analysis of virulence functions of bacterial pathogens. PAI apparently have been acquired during the speciation of pathogens from their nonpathogenic or environmental ancestors. The acquisition of PAI not only is an ancient evolutionary event that led to the appearance of bacterial pathogens on a timescale of millions of years but also may represent a mechanism that contributes to the appearance of new pathogens within a human life span. The acquisition of knowledge about PAI, their structure, their mobility, and the pathogenicity factors they encode not only is helpful in gaining a better understanding of bacterial evolution and interactions of pathogens with eukaryotic host cells but also may have important practical implications such as providing delivery systems for vaccination, tools for cell biology, and tools for the development of new strategies for therapy of bacterial infections.

Expression, purification, and biochemical characterization of enteroaggregative Escherichia coli heat-stable enterotoxin 1

Enteroaggregative Escherichia coli (EAEC) heat-stable enterotoxin 1 (EAST1) is a 4.1k Da protein originally discovered in EAEC but known to be scattered in other diarrheagenic E. coli as well, possibly causing diarrhea in humans and animals. We report for the first time a method to express and purify EAST1 using the Glutathione S-transferase (GST) fusion system. The gst and astA genes were fused together on a pGEX-2T plasmid vector to produce a GST-EAST1 fusion protein. Using Glutathione Sepharose affinity chromatography and C(8) reverse phase high pressure liquid chromatography, EAST1 was purified to homogeneity with a yield of 0.29 mg/L of culture. The protein purified by this method was confirmed to be EAST1 by NH(2)-terminal sequencing and mass spectrometry. The molecular weight of EAST1 is 4104.0 Da, confirming a 38 amino acid peptide as predicted by the astA gene sequence. Mass spectrometry analysis of EAST1 and of two generated peptides established the presence and suggested the position of two disulfide bridges of EAST1 in the conformations C1-C2 and C3-C4. Polyclonal antibodies were raised against EAST1 in New Zealand white rabbits to a titer of 1:8000 using affinity-purified GST-EAST1 fusion protein and to a titer of 1:100 using HPLC-purified EAST1. The biological activity of various EAST1 preparations was tested using the suckling mouse assay with CD-1 and CFW mice strains, but failed to produce fluid accumulation in the intestine.

Fodrin CaM-binding domain cleavage by Pet from enteroaggregative Escherichia coli leads to actin cytoskeletal disruption

We have previously shown that the plasmid-encoded toxin (Pet) of enteroaggregative Escherichia coli produces cytotoxic and enterotoxic effects. Pet-intoxicated epithelial cells reveal contraction of the cytoskeleton and loss of actin stress fibres. Pet effects require its internalization into epithelial cells. We have also shown that Pet degrades erythroid spectrin. Pet delivery within the intestine suggests that Pet may degrade epithelial fodrin (non-erythroid spectrin). Here we demonstrate that Pet has affinity for alpha-fodrin (formally named alphaII spectrin) in vitro and in vivo and cleaves epithelial fodrin, causing its redistribution within the cells. When Pet has produced its cytoskeletal effects, fodrin is found in intracellular aggregates as membrane blebs. Pet cleaves recombinant GST-fodrin, generating two breakdown products of 37 and 72 kDa. Sequencing of the 37 kDa fragment demonstrated that the cleavage site occurred within fodrin's 11th repetitive unit between M1198 and V1199, in the calmodulin binding domain. Site-directed mutagenesis of these amino acids prevented fodrin degradation by Pet. Pet also cleaves epithelial fodrin from cultured Pet-treated cells. A mutant in the Pet serine protease motif was unable to cause fodrin redistribution or to cleave GST-fodrin. This is the first report showing cleavage of alpha-fodrin by a bacterial protease. Cleavage occurs in the middle of the calmodulin binding domain, which leads to cytoskeleton disruption.

Virulence markers of enteroaggregative Escherichia coli isolated from children and adults with diarrhea in Brasília, Brazil

Escherichia coli strains isolated from sporadic cases of acute diarrhea in children and adults and from children without diarrhea were investigated for the presence of the pAA plasmid. Strains harboring the pAA plasmid were isolated at similar frequencies from children with (19.6%) and without (10.8%) diarrhea and from adults with diarrhea (11.8%). The genotypic and phenotypic virulence markers of these strains were further analyzed. Most of the strains were positive for EAST1 (73%), and this toxin was detected significantly more frequently in strains from children with diarrhea than in strains from adults with diarrhea (P < 0.05). Likewise, pic sequences were detected significantly more frequently in strains from children with diarrhea than in strains from adults with diarrhea (P < 0.005) and controls (P < 0.025). Furthermore, the association of pAA positivity (pAA(+)) and pic positivity (pic(+)) was more frequently found for strains from children with diarrhea than for strains from controls, indicating that pAA(+) pic(+) strains may represent a subset of pAA(+) strains associated with disease in children. Most of the strains (82.5%) adhered to cells presenting the typical aggregative pattern. The frequency of occurrence of enteropathogenic E. coli (EPEC) serogroups in the strains from children with diarrhea was very high (56%), while none of the strains from adults with diarrhea belonged to EPEC serogroups. Extraintestinal virulence markers were very commonly found in strains from adults with diarrhea. The frequencies of occurrence of the adhesins AFA and SFA were significantly higher in strains from adults with diarrhea than in strains from children with diarrhea. More than one extraintestinal virulence marker was found in 58% of the strains from adults with diarrhea but in only 7.7% of the strains from children with diarrhea. Our results show that pAA(+) strains isolated from children and adults with diarrhea present very different profiles when enteroaggregative E. coli virulence markers, serotypes, and extraintestinal virulence markers are considered.

Functional comparison of serine protease autotransporters of enterobacteriaceae

The plasmid-encoded toxin (Pet) of enteroaggregative Escherichia coli (EAEC) belongs to a family of high-molecular-weight serine protease autotransporters of Enterobacteriaceae (SPATEs) which also includes Pic from EAEC and Shigella flexneri, EspC from enteropathogenic E. coli, EspP from enterohemorrhagic E. coli, Sat from uropathogenic E. coli, Tsh from avian pathogenic E. coli, and SepA from S. flexneri. Phylogenetic analysis shows the SPATE proteins to represent a distinct subfamily of autotransporters with amino acid identities ranging from 35 to 55%, providing a powerful resource to direct structure-function studies. In this study, we show that these related proteins are proteases with divergent substrate specificities, suggesting different functions. The cleavage profile of oligopeptides was found to be unique for each SPATE protein. The SPATEs showed proteolytic activity for several substrates, namely mucin, pepsin, human coagulation factor V, and erythroid spectrin. The cleavage of spectrin has been hypothesized as the mechanism through which Pet induces cytopathic effects. However, whereas Pet, Sat, and EspC cleaved spectrin, only Pet and Sat elicited cytopathic effects; the remaining SPATEs did not cause any morphological changes to HEp-2 cell monolayers. EspC and Pet exhibited acid-dissociable binding to HEp-2 cells. However, Pet was more efficient at entering HEp-2 cells, suggesting a basis for the different abilities of these two proteases to damage cells. Our data suggest that, despite the homologies observed among these proteins, the SPATEs have different pathogenetic functions only partly dependent on their substrate specificities.

Mechanisms of Emerging Diarrheagenic Escherichia coli Infection

Diarrheagenic Escherichia coli organisms are major causes of morbidity and mortality worldwide. Although most strains of E. coli are harmless commensals, a few types have emerged that are capable of disrupting the normal physiology of the human gut, producing illness ranging from watery diarrhea to fatal hemorrhagic colitis. Diarrheagenic E. coli cause infection by a variety of complex mechanisms, some of which are incompletely understood. These include adherence, elaboration of toxigenic mediators, invasion of the intestinal mucosa, and transportation of bacterial proteins into the host cells. Specific components of the host-microbial interaction that cause damage have been identified, increasing our understanding of the mechanisms of diarrhea. This article reviews some of the recent findings about the pathogenesis and infectious processes involved in three emerging pathotypes of this fascinating gram-negative bacterium.

Enteroaggregative Escherichia coli

Enteroaggregative Escherichia coli (EAEC) are an increasingly important cause of diarrhoea. E. coli belonging to this category cause watery diarrhoea, which is often persistent and can be inflammatory. EAEC have been implicated in sporadic diarrhoea in children and adults, in both developing and developed countries, and have been identified as the cause of several outbreaks worldwide. EAEC are defined by their ability to adhere to epithelial cells in a characteristic "stacked-brick" pattern but are otherwise highly heterogeneous. Genes that could contribute to the pathogenicity of EAEC encode adhesins, toxins, and other factors, all of which are only partially conserved. Practicable tools are needed to improve diagnosis and identify risk factors. EAEC-infected individuals can be treated with fluoroquinolones but there is a need to examine alternative treatment protocols.

Diarrhoeagenic Escherichia coli--an emerging problem?

Diarrhea remains one of the main sources of morbidity and morbidity in today's world and a large proportion is caused by diarrheagenic Escherichia coli. They are a particular problem in developed countries although traveler's diarrhea and hemorrhagic colitis are also a problem in developed countries. There are seven classes of diarrheagenic E. coli, namely enteropathogenic E. coli (EPEC), enterohaemorrhagic E. coli (EHEC), enteroinvasive E. coli (EIEC), enterotoxigenic E. coli (ETEC), enteroaggregative E. coli (EAggEC), diarrhea-associated hemolytic E. coli (DHEC) and cytolethal distending toxin (CDT)- producing E. coli. Many of their virulence determinants have been determined and some classes of diarrheagenic E. coli produce toxins. The virulence factors of some diarrhogenic E. coli have yet to be full determined and in the meantime they remain a large and emerging problem without the availability of effective vaccines.

Inhibition of enteroaggregative Escherichia coli adhesion to HEp-2 cells by secretory immunoglobulin A from human colostrum

BACKGROUND

Enteroaggregative Escherichia coli (EAEC) is an important agent of the persistent diarrhea among low socioeconomic level children in developing countries that may be associated with chronic undernourishment. Breast-feeding is effective in protecting infants against diarrhea and other infectious diseases. The aim of the study is to verify the ability of human colostrum to inhibit aggregative adhesion of EAEC to HEp-2 cells and the presence of antibodies reactive to antigenic fractions of EAEC in colostrum samples.

METHODS

Enzyme-linked immunosorbent assay, immunoblotting and adhesion assays of EAEC to HEp-2 cells were done with pooled or individual colostrum samples (n = 35). Assays were performed with a well-known EAEC strain, 044:H18 E. coli (strain 042). Colostral IgA was isolated by affinity chromatography in Sepharose anti-human alpha chain column.

RESULTS

Total colostrum and isolated IgA inhibited EAEC adhesion, and this ability was associated with the presence of IgA antibodies against a 15-kDa band, compatible with the subunits of aggregative adherence fimbrial adhesin II, characteristic of the 042 strain, absent in its plasmid-cured isogenic strain, that was used as control. Individual colostrum samples also inhibited adhesion, showed variable antibody titles against EAEC antigens in enzyme-linked immunosorbent assay and recognized many antigenic fractions in immunoblotting assays, including the 15-kDa band.

CONCLUSIONS

These results confirm that IgA from human colostrum inhibits adhesion of EAEC to HEp-2 cells and suggest that colostrum IgA antibodies reactive to EAEC antigens may play a role in protection of infants against diarrhea caused by these bacteria.