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 ou tbreak to date worldwide. Various vehicles of infection were identified, an d 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 back ground, real-time pulsed-field gel electrophoresis analyses were performed. The results demonstrated that, within each of the seven outbreak groups, t he macrorestriction profiles observed were indistinguishable, whereas profi les for sporadic isolates were not. The consistent genetic heterogeneity ob served within the Scottish Escherichia coli O157 population can be exploite d in epidemiological investigations.