Genetic heterogeneity of Escherichia coli O157:H7 in Scotland and its utility in strain subtyping

From April 1994 to March 1995, seven outbreaks of Escherichia coli O157:H7 infection occurred throughout Scotland, including the largest milk-borne outbreak to date worldwide. Various vehicles of infection were identified, and there were 144 confirmed cases in total. All isolates associated with the outbreaks were subjected to detailed subtyping: phage typing, testing for carriage of verotoxin genes (VT), and pulsed-field gel electrophoresis. The outbreak strains were of three different phage types (2, 4, and 28). Those of phage type 2 and 28 were VT1-VT2 +, those of phage type 4 were VT1+/VT2+. To discriminate outbreak-associated isolates from the high sporadic background, real-time pulsed-field gel electrophoresis analyses were performed. The results demonstrated that, within each of the seven outbreak groups, the macrorestriction profiles observed were indistinguishable, whereas profiles for sporadic isolates were not. The consistent genetic heterogeneity observed within the Scottish Escherichia coli O157 population can be exploited in epidemiological investigations.