None of the 62 strains was positive for the other above-mentioned

genes. It was previously shown that the RDF+ subgroup of O26:NM strains can be discerned from RDF− O26:H11/O26:NM strains by the sequence of the arcA allele (Leomil et al., 2005). Accordingly, we compared the arcA sequences obtained from all 62 O26 strains with corresponding sequences that were deposited to GenBank. The 18 RDF+ O26:NM strains check details from this study showed the ‘arcA allele 1’, identical to the sequence of strain DG11/2 (GenBank AJ875430), a prototype for the RDF+ O26:NM cluster (Leomil et al., 2005). The 30 RDF− O26:H11 and eight RDF− O26:NM strains showed the ‘arcA allele 2’, identical to the sequence of strain CB1025 (GenBank AJ875429), a prototype of the RDF− O26:[H11] cluster (Leomil et al., 2005). The 513-bp partial arcA sequences AJ875429 and AJ875430 differ from each other in one nucleotide (A/T) at position 90. Five of the six O26:H32 strains showed the ‘arcA allele 1’ and one O26:H32 strain (CB294) had another allelic type for arcA sequence. The results indicate that the arcA allele 1 is associated specifically with the α-hemolytic, RDF+ group of O26:NM strains,

whereas the arcA allele 2 is characteristic for the group of RDF− O26:[H11] and is also found in buy CAL-101 most of the O26:H32 strains (Table 1). A dendrogram based on similarities of XbaI PFGE patterns was created as described in Materials and methods (Fig. 1). Based on data obtained from repeated experiments, a cut-off level of 95% similarity was established for the definition

learn more of a PFGE pattern (data not shown). PFGE of XbaI macrorestriction fragments differentiated the 62 E. coli O26 strains from this study into 54 distinct patterns (Table 1, X1 to X54). Patterns X22, X24, X27, X29, X37, X40 and X50 were found in more than one strain and strains revealing patterns X24 (CB9853 and CB9857) and X40 (DG11/2, DG113/5 and DG70/2), respectively, were known to be epidemiologically related. All other strains showed individual Xba patterns (Table 1 and Fig. 1). PFGE patterns were classified into three main clusters designated A, B and C (Fig. 1). PFGE clusters A and B (>74% similarity) gather all 56 O26:[H11] strains. Similarity between strains was >77% in cluster A and >78% in cluster B. Cluster A exclusively comprises RDF− O26:H11 and O26:NM strains showing ‘arcA allele 2’. Cluster B encompasses all RDF+ O26:NM strains with ‘arcA allele 1’. The 38 strains from cluster A divided into 22 EHEC and 16 EPEC that were isolated between 1953 and 2007 in six countries on three continents. The strains were from human patients (n=20), animals (n=13) and food (n=5) (Table 1). The 18 strains grouped in cluster B divided into 17 EPEC and one EHEC strain (CB5805, Stx2). Cluster B strains were isolated between 1947 and 2003 in six countries on three continents. Fourteen of these were from human patients and four from animals (Table 1).