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

Abstract

Aims: 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. Methods and Results: 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 vtx2 genes could not identify any unique variants of vtx2 or vtx2c 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. Conclusion: 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. Significance and Impact of the Study: 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.

Molecular Typing of Escherichia coli O157:H7 Isolates from Swedish Cattle and Human Cases: Population Dynamics and Virulence

“…It is conceivable that the low copy number contributes to the low mutation rate in this VNTR. We have previously shown that this subtype generally belongs to phage type 14 and that it frequently carries vtx 2c genes and cytolethal distending toxin (12,21). The few cattle isolates included in this study that do not cluster with the main group of 211111 isolates have been shown to belong to other phage types (23) that appear to be present only transiently or at very low frequencies.…”

“…Thus, the prevalence of high-virulence subgroups might be more relevant than the overall E. coli O157:H7 prevalence, partially explaining the marked differences between countries and regions regarding E. coli O157:H7 morbidity. We previously showed a strong connection between one such group, clade 8, and Swedish farms linked to cases of infection in humans versus E. coli O157:H7-positive dairy farms in general (12). The representatives of this clade caused the biggest domestic EHEC outbreak in Sweden, with 135 cases in 2005 being associated with the consumption of lettuce (1).…”

“…Multilocus variable-number tandem-repeat analysis (MLVA) is fast and cost-effective and has been applied to E. coli O157 (12,13), as well as many other bacteria. This method can be used, e.g., to trace spread of the bacteria between animals and from animals to humans.…”

“…While MLVA profiles are expected to change over time, a system including six variable-number tandem repeats (VNTRs) and insertions/deletions forming a lineage-specific polymorphism assay (LSPA-6) uses more stable genetic variation to divide E. coli O157:H7 into lineages I, II, and I/II (14,15). Based on a set of informative single nucleotide polymorphisms, Manning et al (9) defined a number of clades, among which the previously mentioned clade 8 is of particular interest for the Swedish situation and can be identified using a single nucleotide polymorphism (SNP) in a gene encoding a hypothetical protein (ECs2357) (12,16). Whole-genome sequencing-based SNP typing has largely confirmed the division of E. coli O157:H7 into lineages and clades and identified further markers for these subgroups (17)(18)(19)(20).…”

Use of multilocus variable-number tandem repeat analysis (MLVA) in eight European countries, 2012

“…At SVA, this method has recently been shown to offer comparable performance to PFGE typing for cattle isolates [50], while being substantially faster and less laborious. An ongoing research project is comparing clinical isolate profiles generated at SMI to those from isolates from periodical nationwide slaughterhouse prevalence studies on cattle and from sheep isolates.…”