Abstract

Pulsed-field gel electrophoresis of genomic DNA was carried out on Escherichia coli O157:H7 strains from different geographic locations to determine its value in an epidemiological survey of O157 infections. Pulsed-field gel electrophoresis of XbaI-digested DNA fragments clearly separated E. coli O157:H7 strains from nontoxigenic E. coli O157:H19, O157:H43, and O157:H45 strains and from Shiga-like-toxin-producing E. coli strains of other serogroups. However, among the E. coli O157:H7 strains, the restriction patterns either were identical or differed only by a few fragment bands. In some cases, it was therefore impossible to distinguish among epidemiologically unrelated strains. Hybridization experiments with a DNA probe complementary to Shiga-like toxin II sequences revealed that the Shiga-like toxin II genes were located on DNA fragments of different lengths. Our data show that for a single highly conserved clone, such as E. coli O157:H7, other typing techniques may need to be performed in addition to DNA fingerprinting in epidemiological surveys.