Set of virulence genes and genetic relatedness of O113 : H21 Escherichia coli strains isolated from the animal reservoir and human infections in Brazil.

Escherichia coli strains of serotype O113 : H21 are commonly described as belonging to a Shiga toxin (Stx)-producing E. coli (STEC) pathotype worldwide. Albeit this STEC serotype is frequently identified among cattle and other domestic animals, to the best of our knowledge no human infections associated with STEC O113:H21 have been registered in Brazil to date. Here, we report the virulence profile and genetic relatedness of a collection of O113:H21 E. coli strains mainly isolated from the animal reservoir aimed at determining their potential as human pathogens. The strains from the animal reservoir (n=34) were all classified as STEC, whereas the few isolates recovered so far from human diarrhoea (n=3) lacked stx genes. Among the STEC, the stx2d-activatable gene was identified in 85% of the strains that also carried lpfAO113, iha, saa, ehxA, subAB, astA, cdt-V, espP, espI and epeA; the human strains harboured only lpfAO113, iha and astA. All the strains except one, isolated from cattle, were genetically classified as phylogenetic group B1. High mass plasmids were observed in 25 isolates, but only in the STEC group were these plasmids confirmed as the STEC O113 megaplasmid (pO113). Many closely related subgroups (more than 80% similarity) were identified by PFGE, with human isolates clustering in a subgroup separate from most of the animal isolates. In conclusion, potentially pathogenic O113:H21 STEC isolates carrying virulence markers in common with O113:H21 clones associated with haemolytic uraemic syndrome cases in other regions were demonstrated to occur in the natural reservoir in our settings, and therefore the risk represented by them to public health should be carefully monitored.