Enumeration of Escherichia coli O157 in cattle faeces using most probable number technique and automated immunomagnetic separation

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 2.4 x 10(4) MPN g(-1). There was no significant difference (P = 0.06) 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.