ORIGINAL ARTICLE Molecular characterization of Irish E. coli O157:H7 isolates of human, bovine, ovine and porcine origin M. Lenahan 1,2 , S.B. O’Brien 1,2 , C. Byrne 2 , M. Ryan 2 , C.-A. Kennedy 2 , E.B. McNamara 3 , S. Fanning 2 , J.J. Sheridan 1 and T. Sweeney 2 1 Teagasc, Ashtown Food Research Centre, Ashtown, Dublin, Ireland 2 Centres for Food Safety & Food-borne Zoonomics, UCD Veterinary Sciences Centre, School of Agriculture, Food Science and Veterinary Medicine, University College Dublin, Belfield, Dublin, Ireland 3 Public Health Laboratory, Health Service Executive, South Western Area, Cherry Orchard Hospital, Ballyfermot, Dublin, Ireland Introduction Cattle are generally regarded as the most important ani- mal reservoir for human infection with Escherichia coli O157:H7, as a large number of outbreaks has been associ- ated with the consumption of contaminated beef (Riley et al. 1983; Vogt and Dippold 2005). Escherichia coli O157:H7 can be found in many other reservoirs through contamination with faecal material from cattle for exam- ple water, soil and vegetables (De Boer and Heuvelink 2001; Bopp et al. 2003). Furthermore, the presence of naturally occurring E. coli O157:H7 has been reported in sheep, and results from these studies indicate they may be a significant contributing source to human infection (Chapman et al. 2001; Lenahan et al. 2007). Pigs have also been shown to be carriers of the pathogen but the reported incidence in swine is generally much lower than in ruminants (Johnsen et al. 2001; Feder et al. 2003). Despite these well-established transmission routes, comparative studies have observed that only a small per- centage of E. coli O157:H7 strains isolated from humans have identical pulsed-field gel electrophoresis (PFGE) pro- files and virulence genotypes to those isolated from cattle (Noller et al. 2003; Roldgaard et al. 2004). Furthermore, Verocytotoxin production has been observed to be signifi- cantly greater in strains isolated from patients suffering clinical symptoms, such as haemolytic uraemic syndrome (HUS) and bloody diarrhoea, than those isolated from cattle (Ritchie et al. 2003). An investigation of E. coli O157 strains isolated from human sporadic infections in Keywords cattle, E. coli O157:H7, human, pulsed-field gel electrophoresis, single nucleotide polymorphism. Correspondence Mary Lenahan, Ashtown Food Research Centre, Ashtown, Dublin 15, Ireland. E-mail: mary.lenahan@teagasc.ie 2008 ⁄ 1916: received 7 November 2008, revised 3 February 2009 and accepted 1 March 2009 doi:10.1111/j.1365-2672.2009.04320.x Abstract Aims: To determine the degree of relatedness between isolates of Escherichia coli O157:H7 of human, bovine, ovine and porcine origin. Methods and Results: Escherichia coli O157:H7 isolates were compared using (i) PFGE XbaI patterns, (ii) PCR profiles of virulence genes and (iii) the DNA sequences of genes reported to play a role in pathogenicity. The 77 E. coli O157:H7 isolates demonstrated 49 different PFGE patterns of which, eight were common to multiple isolates, and the remaining 41 were distinct. Isolates of different origin did not correlate, except for one cluster consisting of two human and two beef isolates. The majority of animal isolates had the same PCR profiles of virulence genes as those isolated from clinical patients. Single nucleotide polymorphisms (SNPs) were identified in the sequence of a 255-bp region of the vtx2 subunit A gene. Conclusions: Six SNPs were detected in the vtx2A gene, defining four different haplotypes. One nonsynonymous substitution encoded for an amino acid change from glutamic to aspartic acid. Significance and Impact of the Study: Results indicate that although E. coli O157:H7 isolates of differing origin were distinct by PFGE, the DNA sequences of the main virulence genes associated with human clinical illness were conserved. Journal of Applied Microbiology ISSN 1364-5072 1340 Journal compilation ª 2009 The Society for Applied Microbiology, Journal of Applied Microbiology 107 (2009) 1340–1349 ª 2009 The Authors