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Aims: To determine the prevalence and concentration of Escherichia coli O157 shed in faeces at slaughter, by beef cattle from different production systems. Methods and Results: Faecal samples were collected from grass-fed (pasture) and lot-fed (feedlot) cattle at slaughter and tested for the presence of E. coli O157 using automated immunomagnetic separation (AIMS). Escherichia coli O157 was enumerated in positive samples using the most probable number (MPN) technique and AIMS and total E. coli were enumerated using Petrifilm. A total of 310 faecal samples were tested (155 from each group). The geometric mean count of total E. coli was 5 · 10 5 and 2AE5 · 10 5 CFU g )1 for lot-and grass-fed cattle, respectively. Escherichia coli O157 was isolated from 13% of faeces with no significant difference between grass-fed (10%) and lot-fed cattle (15%). The numbers of E. coli O157 in cattle faeces varied from undetectable (<3 MPN g )1 ) to 1AE1 · 10 5 MPN g )1 . Twenty-six (67%) of 39 O157 positive faeces had <10 MPN g )1 and three (8%) had counts between 10 3 -10 5 MPN g )1 . There was no significant difference between concentrations of E. coli O157 in the faeces of grass-fed or lot-fed cattle. Conclusion: The prevalence and numbers of E. coli O157 in the faeces of cattle at slaughter were not affected by the production systems evaluated in this study. Significance and Impact of the Study: Information on the prevalence and numbers of E. coli O157 can be used for formulating intervention strategies and in quantitative risk assessments.

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Aims: To determine the numbers of Escherichia coli O157 present in the faeces of naturally infected cattle. Methods and Results: A combination of the most probable number (MPN) technique and automated immunomagnetic separation (AIMS) was used to enumerate E. coli O157 in cattle faeces from both pasture-fed and grain-fed animals. A total of 22 E. coli O157 positive faecal samples were enumerated for E. coli O157 (10 from pasture-fed and 12 from grain-fed animals). The numbers of E. coli O157 in cattle faeces varied from undetectable (<3 MPN g )1 of faeces) to 2AE4 · 10 4 MPN g )1 . There was no significant difference (P ¼ 0AE06) between the numbers of E. coli O157 in pasture-fed or grain-fed cattle faeces, although the geometric mean (antilog of the mean of log 10 transformed MPN values) was higher in grain-fed (130 MPN g )1 ) than in pasture-fed (13 MPN g )1 ). Conclusions:Although the number of samples tested is small, the results indicate that E. coli O157 make up a small proportion of the total E. coli population present in cattle faeces. Significance and Impact of the Study: Information on the numbers of E. coli O157 present in cattle will assist in developing more robust quantitative risk assessments and formulating intervention strategies.