VerotoxigenicEscherichia colifrom animals, humans and foods: who's who?

Abstract

Verocytotoxigenic (shigatoxigenic) and enterohaemorrhagic Escherichia coli, VTEC (STEC) and EHEC, produce a toxin active on Vero cells in vitro. VTEC and EHEC have been isolated from humans and different animal species, mainly ruminants and pigs. The verocytotoxins, also named shiga toxins (Stx), are active in vivo on the endothelial cells of the blood vessels of the gastro-intestinal mucosa, the kidneys, the brain, and other tissues of humans and piglets, leading to fluid leakage or haemorrhages. Conversely, their role in diseases of young ruminants remains unclear. Adult ruminants can also act as asymptomatic carriers of VTEC and EHEC strains similar to those causing diseases in humans. And they are incriminated as an important source of direct or indirect contamination of humans by the most famous EHEC strain belonging to the O157:H7 serotype, through faecal contamination of either foods of animal origin, or other foodstuffs (fruit, vegetables, etc.), or the environment. But dozens of non-O157 human and ruminant VTEC and EHEC strains with similar general and virulence-associated properties, have been described, whose epidemiology is much less well understood. The purpose of this review manuscript is to describe and compare the properties of human, ruminant and food VTEC and EHEC strains. Among the numerous classes of pathogenic Escherichia coli the verocytotoxigenic (or shigatoxigenic) strains have certainly the widest notoriety. Their names originate from the production of a toxin active on Vero cells in culture, which is also related to the shiga toxin (Stx) of Shigella dysenteriae type 1. Such verocytotoxigenic E. coli (VTEC) have been known for half a century in the pig industry, as the cause of oedema disease (ED) in recently weaned piglets. But until the early 1980s they were of little significance, if any, in human medicine. Why they have become so notorious in a few years in comparison with the other pathogenic strains of E. coli is certainly related to their high pathogenicity in humans, with potentially serious clinical outcomes and to their possible transmission to humans via foods of animal origin, especially ruminants. These particular E. coli strains have therefore rapidly become a major concern and study subject in human medicine, molecular biology and veterinary medicine. The purpose of this review is to (i) present the history and definitions of VTEC (ii) describe the current knowledge on their virulence properties and (iii) compare them based on their origin. This review manuscript is based on several review articles and books recently published in the medical, microbiological and veterinary fields (Ludwig and Goebel 1997; Scotland and Smith 1997; Hancock et al. 1998; Kaper et al. 1998a,b; Melton-Celsa and O'Brien 1998; Meng and Doyle 1998; Nataro and Kaper 1998; O'Brien and Kaper 1998; Paton and Paton 1998; Mainil 1999; Duffy et al. 2000; Blanco et al. 2001; Brown et al. 2001; De Boer and Heuvelink 2001; Gyles 2001; Schmidt et al. 2001; Tozzi et al. 2001; Koronakis and Hughes 2002; Thorpe et al. 2002) and on national and international scientific reports (Anonymous 2003; Vernozy-Rozand and Roze 2003). Other important past or recent research papers are also cited in the text. The history of VTEC follows two convergent paths. The first pathway began in 1977, when the production of a cytotoxin causing the death of Vero cells in culture (hence the name verocytotoxin) was reported for strains of E. coli isolated from humans suffering from diarrhoea and from piglets suffering of postweaning ED (Konowalchuk et al. 1977). A few years later, several human E. coli strains, including one strain previously studied by the group of Konowalchuk (strain H30) were reported to produce a similar cytotoxic effect on HeLa cells in culture, which could be neutralized by an immune serum produced against the Stx of S. dysenteriae type 1 (Stx), hence the name shiga-like toxin (O'Brien et al. 1982). During the following years, the identity of these two types of toxins was recognized, as was their heterogeneity. The nomenclature ‘verocytotoxin and VTEC’ has been used since then by British and Canadian teams, while the nomenclature ‘shiga-like toxins and shiga-like toxin-producing E. coli’ was used by the American teams until 1996, when the names shiga toxins and shigatoxigenic E. coli (STEC) were proposed, based on the biological relation to the Stx produced by S. dysenteriae type1 (Calderwood et al. 1996). The second pathway starts in 1983, with the report that a rare E. coli serotype, O157:H7, was the cause of a distinctive clinical entity, named at the time ‘haemorrhagic colitis’ (HC) and characterized by bloody diarrhoea (Riley et al. 1983). In the following years O157:H7 E. coli strains were associated with mild, undifferentiated to severe, bloody diarrhoea, with, in some patients, sequelae such as a haemolytic uraemic syndrome (HUS), characterized by renal failure, and a thrombotic thrombocytopaenic purpura (TTP), possibly with central nervous System (CNS) involvement. As this E. coli serotype had been initially associated with HC, the strains were named ‘enterohaemorrhagic E. coli’ (EHEC) and for many years EHEC remained a clinical definition, synonym of O157:H7 E. coli causing HC. Progressively E. coli strains belonging to other serotypes (026:H11, 0103:H2, 0111:H-, 0145:H-, O157:H-, etc.) were associated with HC, HUS and TTP. Although sometimes reluctantly, the original EHEC definition was widened to include those serotypes. Meanwhile, the properties of the EHEC strains of the O157:H7 and other serotypes were progressively uncovered. Among others, production of the histological attaching/effacing (AE) lesion, very close to the AE lesion caused by enteropathogenic E. coli (EPEC) was recognized. Today the EHEC include all E. coli strains, from humans and animals, producing Stx and AE lesion, or harbouring the genetic information coding for them. However, this definition is not universally accepted as not all of these E. coli cause HC in humans. VTEC and STEC are the names for the strains producing only Stx, like the ED-associated E. coli in piglets. The EHEC and VTEC possess general (serotypes, biotypes and lysotypes) and specific properties (virulence-associated). They are subdivided into different evolutionary lineages: the EHEC-1 lineage comprises the O157:H7 and closely related strains (e.g. O145:H-) that are highly pathogenic in humans (HC, HUS and TTP) and are derived from the O55 EPEC. The EHEC-2 lineage regroups all other EHEC strains belonging to a wide variety of O serogroups (O5, O26, O103, O111, O118, etc.) and of various pathogenicity in humans (diarrhoea, HC and HUS); the VTEC-1 lineage strains belong to the H21 serogroup (O91, O113, etc.) and are pathogens in humans (HUS and TTP). The VTEC-2 lineage corresponds to all other VTEC of low or no pathogenicity (asymptomatic carriage, diarrhoea and rare HUS) in humans. This section will review the specific virulence-associated properties of EHEC and VTEC, i.e. the Stx, the AE lesion and the enterohaemolysins (Ehly). The E. coli Stx toxins consist of two groups: Stx1 are antigenically very close to the Stx toxin of S. dysenteriae type 1 while Stx2 are more distantly related. Variants of Stx1 have been described which differ only slightly in their gene sequences with no consequence on their antigenicity and cell toxicity. The prototype Stx1 is produced by the E. coli strains H19, H30 and EDL933. Several variants of Stx2 have also been described which differ much more from each other in their antigenicity, toxicity and gene sequences. The classification of the Stx2 variants is confusing. It has been proposed that five main biological variants are recognized (Melton-Celsa and O'Brien 1998; Duffy et al. 2001; Thorpe et al. 2002): Stx2 (human strains EDL933 and E32511), Stx2c (human strains E32511 and B2F1), Stx2d [used widely in the literature to describe several other (sub)variants] identical to Stx2c but activatable by components associated with the intestinal mucus (human strain B2F1), Sx2e (porcine strains 412 and S1191) and Stx2f (human strain H.I.8; previously named Stx2ev). VTEC strains can produce one Stx toxin or two: Stx1 and Stx2 (strain EDL933), Stx2 and Stx2c (strain E32511), two Stx2 subvariants (strain B2F1). 4.1.1 Structure/activity. The Stx are two subunit toxins: the A subunit of c. 33 kDa is the biologically active part and the B subunit, of c. 7·5 kDa is present in five copies and binds to the specific-cell receptor. Following binding, the Stx toxins are internalized by receptor-mediated endocytosis and retrospectively transported into the endoplasmic reticulum after migration through the Golgi apparatus. The A subunits then translocate into the cytoplasm using the Sec61 transmembrane protein complex and are activated after cleavage of a 4 kDa C-terminal A2 peptide. The resulting active A1 peptides have N-glycosidase activity and cleave a purine residue from the 28S rRNA, altering the function of the ribosomes, which are no longer able to interact with elongation factors EF1 and EF2. The protein synthesis is therefore inhibited within the target cells that will finally die. Not all Stx toxins are equally active on different cell lines. If all Stx are highly toxic for Vero cells, the Stx1 and Stx2 only are fully toxic for HeLa cells, whereas the Stx2c and Stx2d are partially (100 times less) and the Stx2e and Stx2f are not (10 000-fold less). Conversely, only Stx2e and Stx2f are fully toxic for Madin-Darby bovine kidney (MDBK) cells, whereas the Stx2c and Stx2d are partially and Stx1 and Stx2 are not. The cell target specificity of a Stx depends on the amount of specific receptor(s) present on the cell membrane: Gb3 (Galα1-4Gal) for Stx1 and Stx2 is present on Vero and HeLa cells; Gb4 (GalNAcβ1-3Galα1-4Gal) for Stx2e and Stx2f is present on Vero and MDBK cells. 4.1.2 In vivo activity. In vivo the Stx are produced in the intestines after colonization by the EHEC or VTEC, then cross the intestinal wall (Stx2 more efficiently than Stx1) and enter the blood stream (toxaemia). The main target cells are the endothelial cells of small arteries, in the kidneys (humans), brain (piglets and to a lesser extent humans), gastro-intestinal mucosa (humans and piglets) and other tissues (piglets). The consequences are fluid leakage and/or haemorrhages leading to tissue lesions and clinical syndromes (Table 1). Nephrotoxicity is observed in humans as a sequela after intestinal infection by EHEC or VTEC strains in 10% of patients. The Stx cause chronic kidney damage leading to dialysis and possible need for transplantation (HUS). HUS is also characterized by microthrombus formation, thrombocytopaenia and haemolytic anaemia (Monnens et al. 1998; Rose and Chant 1998). Although calves are frequently infected by EHEC, HUS never develops as they lack receptors on the endothelial cells of their blood vessels (Pruimboom-Brees et al. 2000). Neurotoxicity is observed first of all in piglets suffering ED. Fluid leakage causes compression of the brain with loss of coordination, lameness, paralysis and sudden death, as main clinical signs (Gyles 2001). Neurological disorders can also be observed in a few humans after intestinal infection by EHEC or VTEC as part of a sequela known as TTP. TTP is also characterized by haemolysis, thrombocytopaenia, renal failure and a fluctuating fever (Hutchison et al. 1998; Rose and Chant 1998; Duffy et al. 2000). A number of workers have reported that the Stx1 and Stx2 are enterotoxic, i.e. cause of fluid accumulation in ligated intestinal loops in rabbits and therefore diarrhoea, but this is still controversial and can vary according to the animal species (Pruimboom-Brees et al. 2000). Today, diarrhoea observed during infection by EHEC in humans and calves is considered to be the consequence of formation of the AE lesion (see below) and not of the production of any Stx. This is supported by the absence of diarrhoea in piglets killed by ED and in humans infected by VTEC strains. On the contrary, the haemorrhagic aspect of the diarrhoea in humans, when present, would be the consequence of the action of the Stx on the endothelial cells of the vessels present in the intestinal mucosa. This is not the case in calves as the endothelial cells of their blood vessels lack Stx receptors (Pruimboom-Brees et al. 2000) and the role of Stx in calves is still unknown. In piglets fluid extravasation also causes oedema in the eyelids, in the larynx leading to the development of a characteristic hoarse squeal and, during the terminal stages of a subacute ED, in the lungs leading to respiratory distress. 4.1.3 Genetic basis. The A and B subunits of the Stx toxins are coded by two different open reading frames organized in one single transcriptional unit. Sequence identity between the stx1 and stx2 prototype genes are 57% in stxA and 60% in stxB genes at the nucleotide level (Scotland and Smith 1997; Bastian et al. 1998; Piérard et al. 1998; Mainil 1999) (Table 2a). The stx2 genes coding for the different variants are more closely related to the stx2 (74–96% overall identity) than to the stx1 genes (55–60% overall identity). The more distantly related to the stx2 genes are the stx2e and stx2f gene variants (Table 2b). Within each group the subvariants are highly homologous (>95% sequence identity). The stx1, stx2, stx2c and stx2d operons are present on chromosomally located lambdoid phages. The stx2e, stx2f and stx genes of S. dysenteriae are also located on the bacterial chromosome, but not on phages. The transfer of the stx1 and stx2 operons to recipient strains by phage transduction has been performed in vitro and in vivo (Mainil 1999; Toth et al. 2003). After synthesis the Stx toxins are exported to the periplasm by a type II signal sequence secretion system. The Stx1 toxins are mainly present in the periplasm, while the Stx2 toxins are more easily excreted into the environment at least in vitro. The synthesis of Stx and Stx1, but not of the different Stx2, is repressed at the transcriptional level by high iron concentration via the Fur system and by high temperature. The AE lesion is a specific-histological lesion (Fig. 1) of the gut caused by a class of pathogenical strains, named EPEC. Most of these strains have been known for decades, but only in the early 1980s was the production of this lesion fully recognized and systematically studied (Moon et al. 1983; Clarke et al. 2003). Subsequently, it was observed that O157:H7 EHEC could produce a very similar lesion and the definition of EHEC was extended to all E. coli strains producing Stx and AE lesions. Intact intestinal microvilli and attaching/effacing lesions at the electron microscope (15 680×). (a) Effacement of the microvilli of the enterocyte; (b) intimate attachment of the bacteria to the enterocyte; (c) pedestal structure; (d) internalization of a few bacteria (author's collection: Ann Méd Vét, 2000, 144, 121–136; with permission of the publisher) 4.2.1 Description. The production of AE lesions is the result of a very specific and tightly regulated interaction between the bacteria and the eucaryotic cell, the enterocytes in vivo. This multistep event occurs in three stages. The first step is the initial adherence of the bacteria to the surface and microvilli of the host cells. In human EPEC this initial adherence triggers the expression of several genes located on a pathogenicity island (Pai) on the chromosome, the locus of enterocyte effacement (LEE), via a plasmid located regulator. The identity of the primary adherence factor remains unknown for animal EPEC and all EHEC strains. The second step consists in sending signals into the eucaryotic cell via a type III secretion system. The type III secretion system and the translocated proteins are coded by genes located on the LEE. Through phosphorylation of several eucaryotic cell proteins the type III secreted proteins cause polymerization of actin, cytoskeleton rearrangements and effacement of the enterocyte microvilli (Fig. 1). Actual modifications of biochemical pathways can differ between EPEC and EHEC strains belonging to different serotypes and to the target cells. The third and final stage is an intimate adherence of the bacteria to the nude cytoplasmic membrane of the enterocyte (Fig. 1). The bacterial adhesin is a LEE-encoded type II secreted outer membrane protein, the intimin. Surprisingly the cell receptor is also a bacterial protein (Tir for ‘translocated intimin receptor’), coded by the LEE, type III secreted and phosphorylated inside the enterocyte, before integrating the eucaryotic cell membrane. At this stage the rearrangements of the cytoskeleton are amplified, a pedestal forms under the adhering bacterial cells (Fig. 1) and bundles of actin filaments can be detected in and under the pedestal. With some EPEC and EHEC, strains invasion of the eucaryotic cells have been observed in vitro and in vivo (Fig. 1). Although the actual significance of this observation is unknown it may be related to the fact that the intimin can also use β1 integrin as a receptor, very similar to the invasin of Yersinia enterocolica. 4.2.2 The locus of enterocyte effacement. The genes necessary and sufficient for the production of the AE lesion are grouped on a 35–45 kb chromosomal DNA fragment, which represents a type III Pai, originally named LEE. The LEE comprises up to 50 genes and open reading frames, five transcriptional units and three functional regions (Fig. 2). Schematic description the locus of enterocyte effacement (LEE) pathogenicity island of the human enteropathogenic Escherichia coli strain E2348/69 after Nataro and Kaper (1998) and Duffy et al. (2000) The genes coding for the type III secretion system (esc and sep genes) are located in what is arbitrarily called the left part of the LEE. The genes coding for the type III secreted proteins (esp genes) are located in the right part of the LEE. In the middle part of the LEE are present the eae gene coding for the bacterial intimin adhesin and the tir gene coding for its translocated receptor. The LEE of human and animal EHEC and EPEC can differ in size, numbers of genes and ORF, and insertion sites on the chromosome (selC locus, pheU locus, pheV locus, etc.). However, the internal organization of the genes on the LEE is conserved (Kaper et al. 1998b; Goffaux et al. 2001a). Variation has also been described within some LEE-located genes. The number of variants of the eae gene has dramatically increased since the original descriptions of the prototype EPEC and EHEC LEE (Adu-Bobie et al. 1998; Oswald et al. 2000; Zhang et al. 2002; Blanco et al. 2004b). Currently, 14 variants, of relative importance and frequency, have been identified by PCR: α, β, γ, δ, ɛ, ζ, η, ι, κ, λ, μ, ν, θ, ξ and other variants are still unnamed. In addition, restriction variants of the amplified fragments exist for some PCR variants: α1, α2, β1, β2, γ1 and γ2. Most variants differ highly (<60–70% homology) in the 3′-end third of the gene while the 5′-end two-thirds of the gene are highly homologous (>90% homology). Moreover, the variants β2 and δ are identical and the variants γ2 and θ are very closely related. If the α and γ1 variants are typical, but not exclusive, of human EPEC and of O157:H7 EHEC, respectively, the other variants are more or less widely distributed in human and animal EHEC and EPEC. Besides the eae gene, variants have also been described in the tir and esp genes (α, β and γ), while the esc and sep genes are more conserved. The following associations between eae (α, β, γ1, γ2 and ɛ), tir and esp genes observed so far are: α/α/α, β/β/β,γ1/γ/γ, γ2/α/α,ɛ/β/β (China et al. 1999; Goffaux et al. 2001b). Associations between tir, esp and the other eae gene variants have not been studied so far to our knowledge. 4.2.3 The diarrhoea. Although spectacular, the AE lesion is not directly responsible for the diarrhoea observed during infections with EPEC or EHEC. The actual reason for the diarrhoea is still unknown, but several hypothesis have been advanced according to in vitro and in vivo studies. These include inhibition of sodium and chloride absorption, activation of the major chloride channel in the microvilli, opening of the tight junctions, increased paracellular permeability, production of another toxin, inflammatory response in the mucosa and production of cytokines. The cause of diarrhoea may differ between strains of different serotypes, according to the biochemical modifications initiated within the eucaryotic cells. Several types of haemolysins (α, β, etc.) are produced by invasive, uropathogenical and enterotoxigenic E. coli. More recently, haemolysins active only on washed red blood cells have been associated with EHEC strains and called enterohaemolysins (Ehly). Different Ehly have been described in E. coli, but one is specific of EHEC strains, the so-called EHEC-Ehly or Ehx (Karch et al. 1998; Beutin 2000). 4.3.1 Structure/activity. The Ehx belongs to the RTX (Repeats in ToXin) family of toxins, like the α-haemolysin (α-Hly). The Ehx, like the α-Hly, is a monomeric pore-forming toxin that inserts into the cytoplasmic membrane of the eucaryotic target cells and creates pores. It is exported by a type I secretion system, but remains attached to the bacteria in contrast to α-Hly, probably as a consequence of a mutation in the genes coding for the transport mechanism. 4.3.2 In vivo activity. Despite the observation of a preferential association with the development of HUS in humans, the actual contribution of the Ehx to the pathogenesis of EHEC and VTEC remains unclear. 4.3.3 Genetic basis. The Ehx is encoded by four genes (ehxC, ehxA, ehxB and ehxD) located on a plasmid of c. 60 Mdal (pEHEC plasmid). The structural gene ehxA shows c. 60% homology with the hlyA gene coding for the α-Hly. The α-Hly (HlyA), as all RTX toxins, is secreted by a type I secretion system involving the participation of the HlyB and HlyD proteins. HlyB is a membrane-bound ATPase providing energy for the secretion of the HlyA. HlyD forms a channel through the bacterial membranes with the help of the TolC outer membrane protein through which HlyA is secreted. The ehxB and ehxD genes appears to be deficient in the secretion of EhxA and can be complemented by the hlyB and hlyD genes. In that case Ehx is excreted into the supernatant and gives an α-Hly like haemolysis. Other genes are located on the pEHEC plasmid coding for a type II secretion system (etp genes), for a catalase-peroxidase (katP gene) and for a secreted serine protease (espP gene in O157:H7 EHEC or pssA gene in O26:H11 EHEC). The pEHEC plasmid carrying the ehx and etp genes is present in all O157:H7 EHEC strains, in a majority of O26:H11 EHEC strains and in variable proportion of EHEC strains belonging to other serotypes. The katP, espP and pssA genes are, however, present on a majority, but not all, pEHEC plasmids (Karch et al. 1998; Beutin 2000). Human EHEC and VTEC strains belong to three classes on a clinical basis: (i) those highly infectious in humans, but only exceptionally in animals (essentially EHEC-1 and VTEC-1) (ii) those frequently associated with diseases in humans and animals (essentially EHEC-2) and (iii) those more rarely associated with disease in humans or animals (essentially VTEC-2). The most famous EHEC strains belong to the EHEC-1 lineage and to the O157:H7 serotype. National surveillance programmes tend to concentrate on the O157:H7 EHEC. Infections of humans with the O157:H7 serotype occur most frequently as food-borne outbreaks in communities, such as families, schools, elderly homes and day care centres. These outbreaks are for reasons unknown primarily restricted to North America and UK. They are rare in continental Europe, South America, Asia (Japan), Oceania and Africa. Mainly individual cases are reported in these other locations. The food vehicles are mainly of ruminant origin such as raw, inadequately cooked meat products or unpasteurized milk. O157:H7 EHEC are carried by healthy cattle and, to a lesser degree, other ruminants in the gastro-intestinal tract. Contamination occurs during milking or slaughter and processing of the carcass. The primary colonization site appears to be the recto-anal junction (Rice et al. 2003). The faecal shedding lasts for <2 months in most animals, but it is believed that some cattle are effectively colonized for longer periods of time. Food vehicles can also originate from other ruminants (unpasteurized goats’ cheese, venison, etc.). Increasingly, infections originate from nonruminant sources, which have been contaminated directly or indirectly by ruminant faeces or by infected humans. Thus transmission occurs through consumption of nonruminant meat and meat products (poultry meat sandwiches, cold pork meat, salami, etc.) and of fruit and vegetables (potatoes, cabbages, sprouts, cole slaw and unpasteurized apple cider) or via nonfood-borne routes (water in swimming pools or lakes, mud, direct animal contacts including sporadically nonruminants and person-to-person contacts). Although the sources are numerous, they have one common feature – the foods and food products have been inadequately prepared, stored or cooked, allowing survival and growth of the O157:H7 EHEC. Infections, outbreaks and individual cases, with other serotypes occur to a lesser extent, although they represent the majority of cases in some areas (continental Europe for instance). The serogroups O26, O103, O111 and O145 are the most frequent. The sources of infection for humans, the rate of carriage in ruminants and the true incidence in humans of these non-O157:H7 cases are impossible to estimate, as surveillance programmes include only the nonsorbitol-fermenting O157:H7 EHEC. Human infections with VTEC (serogroups O48, O91, O104 and O113) have also been reported, but more recently. They do not seem to find their origin in food consumption, although similar strains can be isolated from ruminant faeces and intestinal content and from foodstuffs. It is important to emphasize that porcine VTEC causing ED in weaned piglets differ by their general and specific properties and that porcine VTEC should be considered as very unlikely zoonotic agents. As porcine VTEC responsible for ED do not represent any important public health concern and as the epidemiological significance of EHEC and VTEC described outside the ruminant animal species is still largely speculative (Mainil 1999; Gyles 2001; Wasteson 2001), this section will focus on EHEC and VTEC isolated from humans, ruminants and foods (Tables 3–5). Humans are infected by EHEC which are responsible for HC, HUS and TTP and by VTEC, which are responsible for HUS and TTP only (Table 1). Asymptomatic carriage also exists, especially of VTEC strains. Human EHEC and VTEC belong to an ever-increasing list of O serogroups and serotypes. The most well-known is, of course, the O157:H7 EHEC and among the VTEC serotypes the most famous ones are O91:H21 and O113:H21. But more than 130 other O serogroups in combination with various H serogroups have been described in the literature and new ones are described each year (Blanco et al. 2004a). More than 100 O:H serotypes have been isolated from patients with HUS. The incidence of the serotypes varies geographically. Infection with O157:H7 EHEC has been directly or indirectly associated with cattle and other ruminants. Nevertheless, direct and indirect human-to-human contamination also occurs frequently, especially during an outbreak. Conversely, the source of human contamination by other serotypes of EHEC and VTEC (O26, O91, O103, O111, O113, O118 and O145) is never identified in most instances, although many human serotypes have also been isolated from ruminants. The O157:H7 and non-O157 EHEC and VTEC produce Stx1 and/or Stx2, in relatively similar proportions, although differences have been observed, probably because of geographical and population sample variations. The isolates producing Stx2 are more often associated with HUS than those producing only Stx1. The stx2 variants are mainly stx2, stx2c and stx2d. There is some degree of association between the serotypes and the stx gene variants. In only very few cases the variants stx2e and stx2f have been described. Most human strains are EHEC and their eae genes belong most frequently to the following variants: β, γ1, γ2, ɛ. For instance, all O157 and O145 EHEC harbour the γ1 variant and all O26 EHEC, the β variant of the eae, tir and esp genes. In many others the γ2 variant of the eae gene and the α variant of the tir and esp genes are associated with the O111 serogroup while the ɛ variant of the eae gene and the β variant of the tir and esp genes are associated with the O103 serogroup (China et al. 1999; Goffaux et al. 2001b; J. Mainil, L. Janssen, F. Goffaux unpublished data). All O157:H7 EHEC are positive for the Ehx Hly and the pEHEC plasmid. Two-thirds of the EHEC and VTEC of other serotypes are positive for the Ehx (% variable according to the O serogroup) whose encoding genes are often located on a pEHEC-like plasmid. Infections in cattle with EHEC/VTEC show two distinct patterns, with some strains producing diarrhoea in young calves of 2 weeks to 2 months of age and other strains being carried asymptomatically by adult animals (Table 1). 6.2.1 From diarrhoeic calves. Strains causing diarrhoea in calves belong almost exclusively to the EHEC pathotype. Like human strains, they belong to an always extending number of serotypes, many of which are also associated with diseases in humans. On the contrary, O157:H7 EHEC have been exceptionally associated with diarrhoea in young calves (Dean-Nyström et al. 1998; Brown et al. 2001; Kang et al. 2004). The most important calf serotypes are O5:H-, O26:H-, O26:H11, O111:H- and O118:H16. Data on VTEC strains causing disease in calves are rare.