Variation in the persistence of Escherichia coli O157:H7 in experimentally inoculated 6-week-old conventional lambs

*Department of Bacterial Diseases, Veterinary Laboratories Agency (Weybridge), Woodham Lane, New Haw, Addlestone, Surrey KT15 3NB, Department of Pathology and Microbiology and Department of Clinical Veterinary Science, University of Bristol, Langford, Bristol BS40 5DU Corresponding author: Professor M. J. Woodward (e-mail: M.J.Woodward{at}vla.defra.gsi.gov.uk ). Present address: AgResearch Ltd, Grasslands Research Centre, Tennent Drive, Private Bag 11008, Palmerston North, New Zealand. Received 6 Dec. 2001; revised version received 25 June 2002; accepted 25 June 2002. Six-week-old lambs were inoculated orally with 10 9 cfu of an antibiotic-resistance marked four-strain mixture of enterohaemorrhagic Escherichia coli (EHEC) O157:H7 to investigate faecal excretion and intestinal colonisation. In the first experiment, three E. coli O157:H7 isolates were not detected in the faeces of any lambs beyond day 8 post inoculation (pi), or from any of the tissues derived from inoculated animals. One strain, 140065 Nal r , was isolated from the caecum and colon of one lamb on day 9 pi, from the rectum of another on day 22 pi and persisted in the faeces for up to 28 days pi. All animals remained clinically normal throughout the study period and histological evidence of adhesion of E. coli O157:H7 to the intestinal mucosa was not found. In a separate experiment, four 6-week-old lambs were inoculated orally with 10 9 cfu of E. coli O157:H7 strain 140065 Nal r alone. Faecal samples were positive for this strain until the end of the experiment (day 19 pi). This strain was also recovered from the gastrointestinal tract of lambs on days 6, 18 and 19 pi, but was not isolated at day 17 pi. When sampled separately, rectum and terminal colon contents contained higher numbers of the inoculated strain than the intestinal tissue at these sites. Animals inoculated with O157:H7 strain 140065 Nal r alone produced soft faeces from day 5 pi onwards. Although attaching and effacing lesions were observed in the caecum, proximal colon and rectum in one animal on day 18 pi, the adherent bacteria did not stain with antiserum raised against the O157 antigen.