Phylogenetic relationship and virulence composition of Escherichia coli O26:H11 strains derived from cattle and human clinical cases in Scotland between 2002-2020

O26 is the commonest non-O157 Shiga toxin (stx)-producing Escherichia coli (STEC) serogroup reported in clinical cases worldwide. Ruminant animals, particularly cattle, are the primary reservoir source for human STEC infection. In this study we compared whole genomes and virulence profiles of O26:H11 isolates from cattle and human samples in Scotland. Strains were obtained from two cross-sectional cattle surveys conducted between 2002-2004 (n=60) and 2014-2015 (n=49), and from human clinical submissions (n=96) to the Scottish Escherichia coli Reference Laboratory between 2002-2020. A Maximum-Likelihood phylogeny was constructed from a core genome alignment of 205 sequences. Genomes were screened against a panel of 2701 virulence genes from the Virulence Finder Database. All stx-positive cattle strains belonged to the ST21 lineage and were grouped into three main clades, with cattle and human strains interspersed and stx subtype profiles relatively clade specific. Highly pathogenic stx2a-only strains were identified in two herds from the second cattle survey and in clinical submissions from 2010 onwards. The closest pairwise distance between cattle and human strains was 14 SNPs, and between the first and second cattle surveys was 28 SNPs. Virulence profiles of stx-positive cattle and human strains were similar within stx subtype groupings. O26:H11 stx-negative strains from four herds encoded identical core virulence profiles to stx-positive strains, including ehxA, katP and espP genes. All stx-negative strains encoded the bacteriophage insertion site genes wrbA, yecE, sbcB and yehV. This study sheds light on the diversity of O26:H11 in Scottish cattle and their relationship to human STEC infection.