Isolation and characterisation of Shigatoxigenic Escherichia coli (STEC) serotype O157:H7 from Scottish sheep

Shiga toxin-producing Escherichia coli (STEC) serotype O157:H7 is a priority zoonotic pathogen which, in humans causes gastrointestinal infections, haemorrhagic colitis and haemolytic uremic syndrome. Scotland has more human cases per head of population than any other part of the United Kingdom. Previous studies have demonstrated how strains from cattle, wild deer and fresh beef mince on retail sale in Scotland relate to Scottish human clinical cases. Here, we examined another important Scottish livestock ruminant species, sheep, to identify the diversity of STEC strains they carry. To do this, 875 faecal samples were collected from Scottish sheep during 2022/23; some were collected from the environment during the periparturient period and others collected post-mortem, at abattoirs. STEC was isolated from the samples using enrichment procedures and immunomagnetic separation followed by recovery on CT-SMAC and Chromocult coliform media. Fourteen of the samples each yielded a single presumptive STEC O157 isolate with thirteen from abattoir faecal samples and a single isolate from the field flocks. The isolates were confirmed using an agglutination test that targets the STEC O157 lipopolysaccharide and by whole genome sequencing. Genome analyses showed that all the isolates are STEC O157:H7, ST11 and encode the eae and stx genes (except one isolate which lacked the stx genes). Genotyping and phylogenetic analyses revealed that all the isolates cluster closely with human isolates. Our data supports the carriage of STEC O157:H7 among Scottish sheep and the genomic relationship of the isolates with those from humans suggest a potential for inter-host transmission.