Genotypic characterization to identify markers associated with putative hypervirulence in Swedish Escherichia coli O157:H7 cattle strains

Abstract

To establish whether investigated subtyping methods could identify any specific characteristics that distinguish Swedish VTEC O157:H7 strains isolated from cattle farms associated with human enterohaemorrhagic Escherichia coli (EHEC) cases from cattle strains isolated in prevalence studies. Strains (n = 32) isolated in a dairy herd prevalence study and strains isolated from farms associated with human cases (n = 13) were subjected to typing. Partial sequencing of the vtx(2) genes could not identify any unique variants of vtx(2) or vtx(2c) in strains associated with human cases. A specific variant of VTEC O157:H7, which was overrepresented among farms associated with human cases (P = 0·01), was by two different single-nucleotide-polymorphism (SNP) assays identified as clade 8, a subgroup of VTEC O157:H7 strains considered to be putatively hypervirulent. Multi-locus variable number tandem repeat analysis (MLVA) typing of all strains produced similar results as pulsed-field gel electrophoresis (PFGE) typing regarding clustering of the strains, but MLVA distinguished slightly better among strains than PFGE. In Sweden, VTEC O157:H7 strains from the putatively hypervirulent clade 8 are overrepresented among isolates from cattle farms associated with human cases compared with VTEC O157:H7 strains isolated in prevalence studies. Real-time PCR SNP typing for clade 8 can be used to identify cattle farms that are at higher risk of causing EHEC infections in humans.