Over the 10-year study period (2000–2009), HUS caused by infection with non-O157 STEC was detected in 38 (40%) of the 95 patients investigated. This frequency of HUS cases in our study was higher than that in other studies (10,11). In contrast to other studies, in which all diarrhea samples were screened for STEC, the set of non-O157 strains in our study was isolated from patients with a reasonable clinical suspicion of infection with STEC. This fact could be the reason for a higher proportion of HUS and bloody diarrhea cases in our study group. Fourteen (36.8%) HUS cases were caused by STEC O26:H11; the remaining cases were caused by other serotypes. Of the patients with HUS, 20 (52.6%) were female and 18 (47.4%) were male. Furthermore, 30 (78.9%) were <5 years of age and only 4 (10.5%) were >60 years of age.

In a study in Minnesota, USA, over a 7-year period (2000–2006) in which 108 non-O157 STEC isolates were obtained, HUS did not develop in any of the patients; 57% were female (10). In Germany, Austria, and Australia, O111 strains were most frequently associated with HUS (11). In our study, 32 (84.2%) of the HUS isolates had stx2 as the only STEC gene (75%) or in combination with stx1 (25%), and 6 (15.8%) isolates had only stx1. This frequency of stx2 in HUS isolates is similar to that in other countries, such as the United States (12).

Among the 30 detected STEC serogroups, O26 was most common (28 strains), followed by O145 (10 strains), O103, and O121 (6 strains each). These frequently found serogroups and others (O20, O113, O128, O146, O148, and O174) identified in our strain collection have also been found in sheep and cattle in Switzerland (13,14).

Our finding that O26 isolates (28.9%) were the most common non-O157 STEC serogroup found in Switzerland is similar to findings reported from Belgium, Germany, Japan, Spain, and the United States (15–19). Similar to other countries (16), in Switzerland, STEC O26:H11/H^– also caused several HUS cases.

Of the remaining major O groups, O145 (6 isolates of O145:[H28] and 4 isolates of O145:H25/H^–) was the second most common non-O157 serogroup isolated in this study (10.3% of all isolates). Eight O145 isolates had positive results for stx2, eae, and ehxA; 1 isolate that did not ferment sorbitol had positive results for stx1, eae, and ehxA; and 1 isolate that had positive results for stx1 and stx2 also had positive results for eae and ehxA. These strain characteristics are similar to those of isolates from Finland and Germany (20,21). Four of the O145 strains were associated with bloody diarrhea, and 5 were associated with HUS, which is similar to symptom distribution associated with STEC O145 reported by Karch et al. (18).

The third most common non-O157 STEC serotypes isolated were O103:H2/H^– (6 strains) and O121:H19 (6 strains). In contrast to findings from Germany (19), none of the O103 strains from Switzerland were associated with HUS.

Among the STEC O121:H19 group, all but 1 of the strains were motile, fermented sorbitol, and were stx2, eae, and ehxA positive; the 1 exception was ehxA negative. Four patients had bloody diarrhea and HUS developed in 2 patients. These findings confirm the results of Johnson et al. (11), which showed that strains of serogroup O26, O103, O121, and O145 are more likely to be associated with cases of HUS.

Although STEC O111 is one of the most common serogroups in countries such as Germany, Austria, and Australia and is often associated with HUS (11,22), we found only 2 O111:H^– strains that had the H8 genotype. One of these strains had stx1, eae, ehxA, and the other strain had stx1, stx2, eae and ehxA. The patients from whom these 2 strains were isolated sought treatment for bloody diarrhea and HUS, respectively.

According to FoodNet 2009 (23), the most common non-O157 STEC serogroups in the United States are O26 (28.9%), O103 (20.0%), and O111 (14.9%). The frequency of O26 (28.9% of all isolates) in the United States is the same as that in Switzerland. However, the frequencies of STEC O103 and O111 in the United States are higher than those in Switzerland (STEC O103, 6.2%; STEC O111, 2.1%).

In Germany, STEC O91 (H14/H21) is currently the fourth most common STEC serogroup isolated (24). However, we have detected only 1 O91:H10, stx2-positive, eae- and ehxA-negative, sorbitol-fermenting strain in Switzerland during the past decade. This strain was isolated from a 60-year-old woman in whom HUS developed. STEC O91 isolates that express flagellar antigen H10 have been detected in different countries, albeit at low frequencies (25–27). O91 is usually eae negative and the most common serogroup isolated from adult patients (25). Another eae-negative serogroup is O117, which is often associated with travelers, mainly to Asia, Africa, and Cuba (28). We identified 3 (3.1%) O117:H7 strains that were stx1 positive and stx2, eae, and ehxA negative. This virulence pattern was identical to that for 20 STEC strains reported by Olesen et al. (28). One of our strains was associated with a patient who traveled to India, a country that has been reported as the origin of O117 infections (29). However, no information was available regarding travel for the other 2 patients in our study from whom the O117:H7 stx1-positive, stx2-, eae-, and ehxA-negative strains were isolated. Two of these patients had bloody diarrhea, but no clinical data were available for the third patient.

To detect genetic similarities and epidemiologic relationships among STEC strains, we performed PFGE on representatives of serogroups that occurred at high frequencies. O26 PFGE patterns were heterogeneous except for 2 strain sets, which had 2 and 3 strains without any obvious epidemiologic relationship to each other. Set 1 contained 2 stx1-positive, eae-positive strains isolated in 2001 (KO 2163–01 and KO 2164–01) from an 80-year-old woman and a 2-year-old-boy, respectively, each of whom had bloody diarrhea. Set 2 contained 3 stx2-positive, eae-positive strains isolated in 2002 and 2003 (KO 1103–02, KO 1995–02, and KO 964–03), from 3 female patients (1, 2, and 41 years of age), all of whom had HUS and bloody diarrhea. However, no information for risk factors, such as contact with animals, traveling, or eating animal derived-food, was available for these 5 patients.

PFGE patterns for O145 strains were also heterogeneous, except for 2 similar strains without any epidemiologic relationships to each other that were isolated in 2008 (K 145:25–1550–08 and K 145:25–2208–08; both stx2 and eae positive, 1 from a 2-year-old girl who was treated for HUS, and 1 from a 1-year-old boy who had bloody diarrhea). However, no information for risk factors, such as contact with animals, traveling, or eating animal-derived food, was available for these 2 patients. PFGE patterns of O103 and O121 strains were the most heterogeneous; none of the strains had the same patterns.

The stx gene distribution among all 97 non-O157 STEC isolates showed that 45 (46.4%) strains had only stx2, 36 (37.1%) had only stx1, and 16 (16.5%) had stx1 and stx2. Studies from Spain (17) and Finland (20) showed a similar distribution of stx genes among non-O157 STEC isolates. These findings are in contrast to those from the United States (Minnesota), in which non-O157 STEC strains generally showed a higher frequency of isolates that had stx1 (10).

The presence of the eae gene has been reported to have some predictive value for STEC seropathotypes that are associated with epidemic disease and consequently associated with severe disease such as bloody diarrhea and HUS (12,30,31). Therefore, we performed statistical analysis with multinomial regression and binary logistic regression for our data.

Strains associated with HUS, compared to those associated with bloody diarrhea, were more likely to harbor stx2 and eae, but the presence of only 1 of these virulence factors was not significantly associated. In our study, of 29 patients who provided eae-negative isolates, 17 had bloody diarrhea, 4 had bloody diarrhea and HUS, 4 had HUS, 1 had nonbloody diarrhea, and 3 had no clinical data available. Strains from the 8 patients with HUS comprised a variety of serogroups (O20, O82, O91, O148, O153, O181, and ONT); only 5 had the stx2 gene. In our study, 21.1% of isolates from HUS patients were eae negative. The 3 eae-negative, stx2-negative strains had only stx1 or stx1 and ehxA. Strains with such a pattern of virulence factors are notable because they are less likely to cause HUS than strains harboring stx2. However, these findings are consistent with epidemiologic data from other countries (19,27), which indicate that certain eae-negative STEC strains cause hemorrhagic diseases in humans. A report from the United States (Minnesota) indicated that non-O157 isolates that had only stx1 can cause severe illness (bloody diarrhea or HUS) (32).

Resistance to >1 of the 12 antimicrobial drugs tested was identified in 25 (25.8%) non-O157 STEC strains. This finding is consistent with results for a study in Spain, in which 238 (41%) of 581 non-O157 STEC strains were resistant to >1 of 26 antimicrobial drugs (33). In our study, 13 (13.4%) of 97 strains were resistant to ampicillin, 3 (3.1%) to amoxicillin/clavulanic acid, 12 (12.4%) to cefalothin, 1 (1%) to cefpodoxine, 1 (1%) to cefuroxime, 2 (2.1%) to gentamicin, and 21 (21.6%) to tetracycline. In STEC strains, the most frequent drug-resistance combination was resistance to ampicillin, cefalothin, and tetracycline, which was detected in 8 isolates, once with resistance to amoxicillin/clavulanic acid and once with resistance to gentamicin. Comparably, Schroeder et al. (34) tested 137 non-O157 E. coli human isolates (including 37 STEC strains) from the United States, Saudi Arabia, Argentina, Canada, Mexico, Zambia, and Singapore and reported STEC drug-resistance frequencies of 14% for ampicillin, 5% for amoxicillin/clavulanic acid, 11% for cefalothin, and 32% for tetracycline.

In conclusion, high genetic diversity within strains indicates that non-O157 Shiga toxin–producing E. coli infections in Switzerland most often occurred as single cases. Because little data are available for clinical non-O157 STEC infections in humans, our results may provide useful information for analysis of these strains.