Escherichia coli O157:H7 strain 3704 was used throughout
this study. Strain 3704 is a nontoxigenic environmental
isolate obtained from the E. coli Reference Laboratory,
Aberdeen. The strain was originally isolated from a farm
drain and has been proven to be nontoxigenic due to
the absence of toxin activity (by Verocell assay) and
toxin genes (by PCR) (F. Thomson-Carter, unpublished
observation).