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

Shiga toxin-producing Escherichia coli (STEC) is a food-borne pathogen that may be responsible for severe human infections. Only a limited number of serotypes, including O26:H11, are involved in the majority of serious cases and outbreaks. The main virulence factors, Shiga toxins (Stx), are encoded by bacteriophages. Seventy-four STEC O26:H11 strains of various origins (including human, dairy, and cattle) were characterized for their stx subtypes and Stx phage chromosomal insertion sites. The majority of food and cattle strains possessed the stx_1a subtype, while human strains carried mainly stx_1a or stx_2a. The wrbA and yehV genes were the main Stx phage insertion sites in STEC O26:H11, followed distantly by yecE and sbcB. Interestingly, the occurrence of Stx phages inserted in the yecE gene was low in dairy strains. In most of the 29 stx-negative E. coli O26:H11 strains also studied here, these bacterial insertion sites were vacant. Multilocus sequence typing of 20 stx-positive or stx-negative E. coli O26:H11 strains showed that they were distributed into two phylogenetic groups defined by sequence type 21 (ST21) and ST29. Finally, an EspK-carrying phage was found inserted in the ssrA gene in the majority of the STEC O26:H11 strains but in only a minority of the stx-negative E. coli O26:H11 strains. The differences in the stx subtypes and Stx phage insertion sites observed in STEC O26:H11 according to their origin might reflect that strains circulating in cattle and foods are clonally distinct from those isolated from human patients.     

Dual-Serotype Biofilm Formation by Shiga Toxin-Producing Escherichia coli O157:H7 and O26:H11 Strains

Escherichia coli O26:H11 strains were able to outgrow O157:H7 companion strains in planktonic and biofilm phases and also to effectively compete with precolonized O157:H7 cells to establish themselves in mixed biofilms. E. coli O157:H7 strains were unable to displace preformed O26:H11 biofilms. Therefore, E. coli O26:H11 remains a potential risk in food safety.     

Virulence Profiling of Shiga Toxin-Producing Escherichia coli O111:NM Isolates from Cattle

Shiga toxin-producing Escherichia coli (STEC) O111:NM is an important serotype that has been incriminated in disease outbreaks in the United States. This study characterized cattle STEC O111:NM for virulence factors and markers by PCR. Major conclusions are that STEC O111:NM characterized in this study lacks stx₂ and the full spectrum of nle gene markers, and it has an incomplete OI-122.     

Long-Term Survival of Shiga Toxin-Producing Escherichia coli O26, O111, and O157 in Bovine Feces

Cattle are an important reservoir of Shiga toxin-producing Escherichia coli (STEC) O26, O111, and O157. The fate of these pathogens in bovine feces at 5, 15, and 25°C was examined. The feces of a cow naturally infected with STEC O26:H11 and two STEC-free cows were studied. STEC O26, O111, and O157 were inoculated into bovine feces at 10¹, 10³, and 10⁵ CFU/g. All three pathogens survived at 5 and 25°C for 1 to 4 weeks and at 15°C for 1 to 8 weeks when inoculated at the low concentration. On samples inoculated with the middle and high concentrations, O26, O111, and O157 survived at 25°C for 3 to 12 weeks, at 15°C for 1 to 18 weeks, and at 5°C for 2 to 14 weeks, respectively. Therefore, these pathogens can survive in feces for a long time, especially at 15°C. The surprising long-term survival of STEC O26, O111, and O157 in bovine feces shows that such feces are a potential vehicle for transmitting not only O157 but also O26 and O111 to cattle, food, and the environment. Appropriate handling of bovine feces is emphasized.     

Greater Diversity of Shiga Toxin-Encoding Bacteriophage Insertion Sites among Escherichia coli O157:H7 Isolates from Cattle than in Those from Humans

Escherichia coli O157:H7, a zoonotic human pathogen for which domestic cattle are a reservoir host, produces a Shiga toxin(s) (Stx) encoded by bacteriophages. Chromosomal insertion sites of these bacteriophages define three principal genotypes (clusters 1 to 3) among clinical isolates of E. coli O157:H7. Stx-encoding bacteriophage insertion site genotypes of 282 clinical and 80 bovine isolates were evaluated. A total of 268 (95.0%) of the clinical isolates, but only 41 (51.3%) of the bovine isolates, belonged to cluster 1, 2, or 3 (P < 0.001). Thirteen additional genotypes were identified in isolates from both cattle and humans (four genotypes), from only cattle (seven genotypes), or from only humans (two genotypes). Two other markers previously associated with isolates from cattle or with clinical isolates showed similar associations with genotype groups within bovine isolates; the tir allele sp-1 and the Q_933W allele were under- and overrepresented, respectively, among cluster 1 to 3 genotypes. Stx-encoding bacteriophage insertion site typing demonstrated that there is broad genetic diversity of E. coli O157:H7 in the bovine reservoir and that numerous genotypes are significantly underrepresented among clinical isolates, consistent with the possibility that there is reduced virulence or transmissibility to humans of some bovine E. coli O157:H7 genotypes.     

Outbreak of Shiga Toxin-Producing Escherichia coli (STEC) O157:H7 Associated with Romaine Lettuce Consumption, 2011

Background

Shiga toxin-producing Escherichia coli (STEC) O157:H7 is the causal agent for more than 96,000 cases of diarrheal illness and 3,200 infection-attributable hospitalizations annually in the United States.

Materials and Methods

We defined a confirmed case as a compatible illness in a person with the outbreak strain during 10/07/2011-11/30/2011. Investigation included hypothesis generation, a case-control study utilizing geographically-matched controls, and a case series investigation. Environmental inspections and tracebacks were conducted.

Results

We identified 58 cases in 10 states; 67% were hospitalized and 6.4% developed hemolytic uremic syndrome. Any romaine consumption was significantly associated with illness (matched Odds Ratio (mOR) = 10.0, 95% Confidence Interval (CI) = 2.1–97.0). Grocery Store Chain A salad bar was significantly associated with illness (mOR = 18.9, 95% CI = 4.5–176.8). Two separate traceback investigations for romaine lettuce converged on Farm A. Case series results indicate that cases (64.9%) were more likely than the FoodNet population (47%) to eat romaine lettuce (p-value = 0.013); 61.3% of cases reported consuming romaine lettuce from the Grocery Store Chain A salad bar.

Conclusions

This multistate outbreak of STEC O157:H7 infections was associated with consumption of romaine lettuce. Traceback analysis determined that a single common lot of romaine lettuce harvested from Farm A was used to supply Grocery Store Chain A and a university campus linked to a case with the outbreak strain. An investigation at Farm A did not identify the source of contamination. Improved ability to trace produce from the growing fields to the point of consumption will allow more timely prevention and control measures to be implemented.     

Emerging Shiga Toxin-Producing Escherichia coli Serotypes in Europe: O100:H- and O127:H40

Novel and as yet rare non-O157 Shiga toxin (Stx)-producing Escherichia coli (STEC) serotypes are emerging in Europe. Two different sorbitol-fermenting STECs, O100:H- carrying the virulence gene stx2 and O127:H40 carrying stx1 and eae genes (found in two related subjects), were isolated from patients’ stool samples. Non-O157 STEC infections in humans are currently under-diagnosed. This report highlights the need for, and importance of, screening for Shiga toxins or serotypes other than just O157.     

Shiga Toxin and Shiga Toxin-Encoding Phage Do Not Facilitate Escherichia coli O157:H7 Colonization in Sheep

Isogenic strains of Escherichia coli O157:H7, missing either stx₂ or the entire Stx2-encoding phage, were compared with the parent strain for their abilities to colonize sheep. The absence of the phage or of the Shiga toxin did not significantly impact the magnitude or duration of shedding of E. coli O157:H7.     

Prevalence and Genetic Characterization of Shiga Toxin-Producing Escherichia coli Isolates from Slaughtered Animals in Bangladesh

To determine the prevalence of Shiga toxin (Stx)-producing Escherichia coli (STEC) in slaughter animals in Dhaka, Bangladesh, we collected rectal contents immediately after animals were slaughtered. Of the samples collected from buffalo (n = 174), cows (n = 139), and goats (n = 110), 82.2%, 72.7%, and 11.8% tested positive for stx₁ and/or stx₂, respectively. STEC could be isolated from 37.9%, 20.1%, and 10.0% of the buffalo, cows, and goats, respectively. STEC O157 samples were isolated from 14.4% of the buffalo, 7.2% of the cows, and 9.1% of the goats. More than 93% (n = 42) of the STEC O157 isolates were positive for the stx₂, eae, katP, etpD, and enterohemorrhagic E. coli hly (hly_EHEC) virulence genes. STEC O157 isolates were characterized by seven recognized phage types, of which types 14 (24.4%) and 31 (24.4%) were predominant. Subtyping of the 45 STEC O157 isolates by pulsed-field gel electrophoresis showed 37 distinct restriction patterns, suggesting a heterogeneous clonal diversity. In addition to STEC O157, 71 STEC non-O157 strains were isolated from 60 stx-positive samples from 23.6% of the buffalo, 12.9% of the cows, and 0.9% of the goats. The STEC non-O157 isolates belonged to 36 different O groups and 52 O:H serotypes. Unlike STEC O157, most of the STEC non-O157 isolates (78.9%) were positive for stx₁. Only 7.0% (n = 5) of the isolates were positive for hly_EHEC, and none was positive for eae, katP, and etpD. None of the isolates was positive for the iha, toxB, and efa1 putative adhesion genes. However, 35.2% (n = 25), 11.3% (n = 8), 12.7% (n = 9), and 12.7% (n = 9) of the isolates were positive for the lpf_O113, saa, lpfA_O157/01-141, and lpfA_O157/OI-154 genes, respectively. The results of this study provide the first evidence that slaughtered animals like buffalo, cows, and goats in Bangladesh are reservoirs for STEC, including the potentially virulent STEC strain O157.     

Molecular Serotyping of Escherichia coli O26:H11

Serotyping is the foundation of pathogenic Escherichia coli diagnostics; however, few laboratories have this capacity. We developed a molecular serotyping protocol that targets, genetically, the same somatic and flagellar antigens of E. coli O26:H11 used in traditional serotyping. It correctly serotypes strains untypeable by traditional methods, affording primary laboratories serotyping capabilities.     

Prevalence of Carriage of Shiga Toxin-Producing Escherichia coli Serotypes O157:H7, O26:H11, O103:H2, O111:H8, and O145:H28 among Slaughtered Adult Cattle in France

The main pathogenic enterohemorrhagic Escherichia coli (EHEC) strains are defined as Shiga toxin (Stx)-producing E. coli (STEC) belonging to one of the following serotypes: O157:H7, O26:H11, O103:H2, O111:H8, and O145:H28. Each of these five serotypes is known to be associated with a specific subtype of the intimin-encoding gene (eae). The objective of this study was to evaluate the prevalence of bovine carriers of these “top five” STEC in the four adult cattle categories slaughtered in France. Fecal samples were collected from 1,318 cattle, including 291 young dairy bulls, 296 young beef bulls, 337 dairy cows, and 394 beef cows. A total of 96 E. coli isolates, including 33 top five STEC and 63 atypical enteropathogenic E. coli (aEPEC) isolates, with the same genetic characteristics as the top five STEC strains except that they lacked an stx gene, were recovered from these samples. O157:H7 was the most frequently isolated STEC serotype. The prevalence of top five STEC (all serotypes included) was 4.5% in young dairy bulls, 2.4% in young beef bulls, 1.8% in dairy cows, and 1.0% in beef cows. It was significantly higher in young dairy bulls (P < 0.05) than in the other 3 categories. The basis for these differences between categories remains to be elucidated. Moreover, simultaneous carriage of STEC O26:H11 and STEC O103:H2 was detected in one young dairy bull. Lastly, the prevalence of bovine carriers of the top five STEC, evaluated through a weighted arithmetic mean of the prevalence by categories, was estimated to 1.8% in slaughtered adult cattle in France.     

Analysis of the Clonal Relationship of Serotype O26:H11 Enterohemorrhagic Escherichia coli Isolates from Cattle

Twelve cluster groups of Escherichia coli O26 isolates found in three cattle farms were monitored in space and time. Cluster analysis suggests that only some O26:H11 strains had the potential for long-term persistence in hosts and farms. As judged by their virulence markers, bovine enterohemorrhagic O26:H11 isolates may represent a considerable risk for human infection.Shiga toxin (Stx)-producing Escherichia coli (STEC) strains comprise a group of zoonotic enteric pathogens (42). In humans, infections with some STEC serotypes result in hemorrhagic or nonhemorrhagic diarrhea, which can be complicated by hemolytic-uremic syndrome (HUS) (49). These STEC strains are also designated “enterohemorrhagic E. coli” (EHEC). Consequently, EHEC strains represent a subgroup of STEC with a high pathogenic potential for humans. Strains of the E. coli serogroup O26 were originally classified as enteropathogenic E. coli due to their association with outbreaks of infantile diarrhea in the 1940s. In 1977, Konowalchuk et al. (37) recognized that these bacteria produced Stx, and 10 years later, the Stx-producing E. coli O26:H11/H− strains were classified as EHEC. EHEC O26 strains constitute the most common non-O157 EHEC group associated with diarrhea and HUS in Europe (12, 21, 23, 24, 26, 27, 55, 60). Reports on an association between EHEC O26 and HUS or diarrhea from North America including the United States (15, 30, 33), South America (51, 57), Australia (22), and Asia (31, 32) provide further evidence for the worldwide spread of these organisms. Studies in Germany and Austria (26, 27) on sporadic HUS cases between 1996 and 2003 found that EHEC O26 accounted for 14% of all EHEC strains and for ∼40% of non-O157 EHEC strains obtained from these patients. A proportion of 11% EHEC O26 strains was detected in a case-control study in Germany (59) between 2001 and 2003. In the age group <3 years, the number of EHEC O26 cases was nearly equal to that of EHEC O157 cases, although the incidence of EHEC O26-associated disease is probably underestimated because of diagnostic limitations in comparison to the diagnosis of O157:H7/H− (18, 34). Moreover, EHEC O26 has spread globally (35). Beutin (6) described EHEC O26:H11/H−, among O103:H2, O111:H, O145:H28/H−, and O157:H7/H−, as the well-known pathogenic “gang of five,” and Bettelheim (5) warned that we ignore the non-O157 STEC strains at our peril.EHEC O26 strains produce Stx1, Stx2, or both (15, 63). Moreover, these strains contain the intimin-encoding eae gene (11, 63), a characteristic feature of EHEC (44). In addition, EHEC strains possess other markers associated with virulence, such as a large plasmid that carries further potential virulence genes, e.g., genes coding for EHEC hemolysin (EHEC-hlyA), a catalase-peroxidase (katP), and an extracellular serine protease (espP) (17, 52). The efa1 (E. coli factor for adherence 1) gene was identified as an intestinal colonization factor in EHEC (43). EHEC O26 represents a highly dynamic group of organisms that rapidly generate new pathogenic clones (7, 8, 63).Ruminants, especially cattle, are considered the primary reservoir for human infections with EHEC. Therefore, the aim of this study was the molecular characterization of bovine E. coli field isolates of serogroup O26 using a panel of typical virulence markers. The epidemiological situation in the beef herds from which the isolates were obtained and the spatial and temporal behavior of the clonal distribution of E. coli serogroup O26 were analyzed during the observation period. The potential risk of the isolates inducing disease in humans was assessed.In our study, 56 bovine E. coli O26:H11 isolates and one bovine O26:H32 isolate were analyzed for EHEC virulence-associated factors. The isolates had been obtained from three different beef farms during a long-term study. They were detected in eight different cattle in farm A over a period of 15 months (detected on 10 sampling days), in 3 different animals in farm C over a period of 8 months (detected on 3 sampling days), and in one cow on one sampling day in farm D (Table ​(Table1)1) (28).

TABLE 1.

Typing of E. coli O26 isolates

Intimin Gene (eae) Subtype-Based Real-Time PCR Strategy for Specific Detection of Shiga Toxin-Producing Escherichia coli Serotypes O157:H7, O26:H11, O103:H2, O111:H8, and O145:H28 in Cattle Feces

Shiga toxin-producing Escherichia coli (STEC) strains belonging to serotypes O157:H7, O26:H11, O103:H2, O111:H8, and O145:H28 are known to be associated with particular subtypes of the intimin gene (eae), namely, γ1, β1, ε, θ, and γ1, respectively. This study aimed at evaluating the usefulness of their detection for the specific detection of these five main pathogenic STEC serotypes in cattle feces. Using real-time PCR assays, 58.7% of 150 fecal samples were found positive for at least one of the four targeted eae subtypes. The simultaneous presence of stx, eae, and one of the five O group markers was found in 58.0% of the samples, and the five targeted stx plus eae plus O genetic combinations were detected 143 times. However, taking into consideration the association between eae subtypes and O group markers, the resulting stx plus eae subtype plus O combinations were detected only 46 times. The 46 isolation assays performed allowed recovery of 22 E. coli strains belonging to one of the five targeted STEC serogroups. In contrast, only 2 of 39 isolation assays performed on samples that were positive for stx, eae and an O group marker, but that were negative for the corresponding eae subtype, were successful. Characterization of the 24 E. coli isolates showed that 6 were STEC, including 1 O157:H7, 3 O26:H11, and 2 O145:H28. The remaining 18 strains corresponded to atypical enteropathogenic E. coli (aEPEC). Finally, the more discriminating eae subtype-based PCR strategy described here may be helpful for the specific screening of the five major STEC in cattle feces.     

Shiga Toxin-Producing Escherichia coli in Yaks (Bos grunniens) from the Qinghai-Tibetan Plateau,China

Shiga toxin (Stx)-producing Escherichia coli (STEC) are recognized as important human pathogens of public health concern. Many animals are the sources of STEC. In this study we determined the occurrence and characteristics of the STEC in yaks (Bos grunniens) from the Qinghai-Tibetan plateau, China. A total of 728 yak fecal samples was collected from June to August, 2012 and was screened for the presence of the stx₁ and stx₂ genes by TaqMan real-time PCR after the sample was enriched in modified Tryptone Soya Broth. Of the 138 (18.96%) stx₁ and/or stx₂-positive samples, 85 (61.59%) were confirmed to have at least 1 STEC isolate present by culture isolation, from which 128 STEC isolates were recovered. All STEC isolates were serotyped, genotyped by pulsed-field gel electrophoresis (PFGE) and characterized for the presence of 16 known virulence factors. Fifteen different O serogroups and 36 different O:H serotypes were identified in the 128 STEC isolates with 21 and 4 untypable for the O and H antigens respectively. One stx₁ subtype (stx_1a) and 5 stx₂ subtypes (stx_2a, stx_2b, stx_2c, stx_2d and stx_2g) were present in these STEC isolates. Apart from lpfA_O157/OI-141, lpfA_O157/OI-154, lpfA_O113, katP and toxB which were all absent, other virulence factors screened (eaeA, iha, efa1, saa, paa, cnf1, cnf2, astA, subA, exhA and espP) were variably present in the 128 STEC isolates. PFGE were successful for all except 5 isolates and separated them into 67 different PFGE patterns. For the 18 serotypes with 2 or more isolates, isolates of the same serotypes had the same or closely related PFGE patterns, demonstrating clonality of these serotypes. This study was the first report on occurrence and characteristics of STEC isolated from yaks (Bos grunniens) from the Qinghai-Tibetan plateau, China, and extended the genetic diversity and reservoir host range of STEC.     

International Comparison of Clinical, Bovine, and Environmental Escherichia coli O157 Isolates on the Basis of Shiga Toxin-Encoding Bacteriophage Insertion Site Genotypes

Escherichia coli O157:H7 genotypes in the bovine reservoir may differ in virulence. The proportion of clinical genotypes among cattle isolates was weakly (P = 0.054) related to the international incidence of E. coli O157:H7-associated hemolytic-uremic syndrome, varied among clinical isolates internationally, and also differed along the putative cattle-hamburger-clinical case transmission chain.     

Genomic Subtraction To Identify and Characterize Sequences of Shiga Toxin-Producing Escherichia coli O91:H21

To identify Shiga toxin-producing Escherichia coli genes associated with severe human disease, a genomic subtraction technique was used with hemolytic-uremic syndrome-associated O91:H21 strain CH014 and O6:H10 bovine strains. The method was adapted to the Shiga toxin-producing E. coli genome: three rounds of subtraction were used to isolate DNA fragments specific to strain CH014. The fragments were characterized by genetic support analysis, sequencing, and hybridization to the genome of a collection of Shiga toxin-producing E. coli strains. A total of 42 fragments were found, 19 of which correspond to previously identified unique DNA sequences in the enterohemorrhagic E. coli EDL933 reference strain, including 7 fragments corresponding to prophage sequences and others encoding candidate virulence factors, such a SepA homolog protein and a fimbrial usher protein. In addition, the subtraction procedure yielded plasmid-related sequences from Shigella flexneri and enteropathogenic and Shiga toxin-producing E. coli virulence plasmids. We found that lateral gene transfer is extensive in strain CH014, and we discuss the role of genomic mobile elements, especially bacteriophages, in the evolution and possible transfer of virulence determinants.     

Analysis of the Clonal Relationship of Shiga Toxin-Producing Escherichia coli Serogroup O165:H25 Isolated from Cattle

Variations in time and space of a clonal group of Escherichia coli O165:H25 on a cattle farm were monitored. The virulence marker pattern (stx genes, eae gene, hly_EHEC gene, katP gene, espP gene, efa gene) suggests that E. coli O165:H25 of bovine origin may represent a risk for human infection.     

Genomic Comparison of Escherichia coli O104:H4 Isolates from 2009 and 2011 Reveals Plasmid,and Prophage Heterogeneity,Including Shiga Toxin Encoding Phage stx2

In May of 2011, an enteroaggregative Escherichia coli O104:H4 strain that had acquired a Shiga toxin 2-converting phage caused a large outbreak of bloody diarrhea in Europe which was notable for its high prevalence of hemolytic uremic syndrome cases. Several studies have described the genomic inventory and phylogenies of strains associated with the outbreak and a collection of historical E. coli O104:H4 isolates using draft genome assemblies. We present the complete, closed genome sequences of an isolate from the 2011 outbreak (2011C–3493) and two isolates from cases of bloody diarrhea that occurred in the Republic of Georgia in 2009 (2009EL–2050 and 2009EL–2071). Comparative genome analysis indicates that, while the Georgian strains are the nearest neighbors to the 2011 outbreak isolates sequenced to date, structural and nucleotide-level differences are evident in the Stx2 phage genomes, the mer/tet antibiotic resistance island, and in the prophage and plasmid profiles of the strains, including a previously undescribed plasmid with homology to the pMT virulence plasmid of Yersinia pestis. In addition, multiphenotype analysis showed that 2009EL–2071 possessed higher resistance to polymyxin and membrane-disrupting agents. Finally, we show evidence by electron microscopy of the presence of a common phage morphotype among the European and Georgian strains and a second phage morphotype among the Georgian strains. The presence of at least two stx2 phage genotypes in host genetic backgrounds that may derive from a recent common ancestor of the 2011 outbreak isolates indicates that the emergence of stx2 phage-containing E. coli O104:H4 strains probably occurred more than once, or that the current outbreak isolates may be the result of a recent transfer of a new stx2 phage element into a pre-existing stx2-positive genetic background.     

Heterogeneity of Shiga Toxin-Producing Escherichia coli Strains Isolated from Hemolytic-Uremic Syndrome Patients, Cattle, and Food Samples in Central France

A detailed analysis of the molecular epidemiology of non-O157:H7 Shiga toxin-producing Escherichia coli (STEC) was performed by using isolates from sporadic cases of hemolytic-uremic syndrome (HUS), animal reservoirs, and food products. The isolates belonged to the O91 and OX3 serogroups and were collected in the same geographical area over a short period of time. Five typing methods were used; some of these were used to explore potentially mobile elements like the stx genes or the plasmids (stx₂-restriction fragment length polymorphism [RFLP], stx₂ gene variant, and plasmid analyses), and others were used to study the whole genome (ribotyping and pulsed-field gel electrophoresis [PFGE]). The techniques revealed that there was great diversity among the O91 and OX3 STEC strains isolated in central France. A close relationship between strains of the same serotype having the same virulence factor pattern was first suggested by ribotyping. However, stx₂-RFLP and stx₂ variant analyses differentiated all but 5 of 21 isolates, and plasmid analysis revealed further heterogeneity; a unique combination of characteristics was obtained for all strains except two O91:H21 isolates from beef. The latter strains were shown by PFGE to be the most closely related isolates, with >96% homology, and hence may be subtypes of the same strain. Overall, our results indicate that the combination of stx₂-RFLP, stx₂ variant, and plasmid profile analyses is as powerful as PFGE for molecular investigation of STEC diversity. Finally, the non-O157:H7 STEC strains isolated from HUS patients were related to but not identical to those isolated from cattle and food samples in the same geographical area. The possibility that there are distinct lineages of non-O157:H7 STEC, some of which are more virulent for humans, should be investigated further.