Enterohemorrhagic Escherichia coli O26: H11/H-: A New Virulent Clone Emerges in Europe

Background. Enterohemorrhagic Escherichia coli (EHEC) O26 causes diarrhea and hemolytic uremic syndrome (HUS). Strains harboring the stx(1a) gene prevail, but strains with stx(2a) as the sole Shiga toxin-encoding gene are now emerging. The traits and virulence of the latter set of strains are unknown. We correlated stx genotypes of 272 EHEC O26 strains isolated in 7 European countries between 1996 and 2012 with disease phenotypes. We determined phylogeny, clonal structure, and plasmid gene profiles of the isolates and portray geographic and temporal distribution of the different subgroups. Methods. The stx genotypes and plasmid genes were identified using polymerase chain reaction, phylogeny was assigned using multilocus sequence typing, and clonal relatedness was established using pulsed-field gel electrophoresis. Results. Of the 272 EHEC O26 isolates, 107 (39.3%), 139 (51.1%), and 26 (9.6%) possessed stx(1a), stx(2a), or both genes, respectively. Strains harboring stx(2a) only were significantly associated with HUS (odds ratio, 14.2; 95% confidence interval, 7.9-25.6; P < .001) compared to other stx genotypes. The stx(2a)-harboring strains consist of 2 phylogenetically distinct groups defined by sequence type (ST) 21 and ST29. The ST29 strains are highly conserved and correspond by plasmid genes to the new virulent clone of EHEC O26 that emerged in Germany in the 1990s. This new clone occurred in 6 of the 7 countries and represented approximately 50% of all stx(2a)-harboring EHEC O26 strains isolated between 1996 and 2012. Conclusions. A new highly virulent clone of EHEC O26 has emerged in Europe. Its reservoirs and sources warrant identification.