Comparison of the molecular genotypes of Escherichia coli O157:H7 from the hides of beef cattle in different regions of North America.

Cattle hides become contaminated with Escherichia coli O157:H7 via pathogen transmission in the feedlot, during transport, and while in the lairage environment at the processing facility, and the bacteria can be transferred to beef carcasses during processing. Several studies have shown that E. coli O157:H7 strains possessing indistinguishable restriction digest patterns (RDPs) can be isolated from distant locations. Most of these studies, however, examined RDPs from strains isolated within a single region of the United States or Canada. The experiment described in the present study was designed to identify the molecular genotypes of E. coli O157:H7 isolates from beef cattle hides in nine major cattle-producing regions of North America. Prevalence for E. coli O157:H7 in beef cattle hide samples ranged from 9 to 85%. Pulsed-field gel electrophoresis (PFGE) analysis of XbaI-digested genomic DNA from 1,193 E. coli O157:H7 isolates resulted in 277 unique RDPs. Of the 277 unique XbaI RDPs, 54 contained isolates collected from multiple regions. After two subsequent rounds of PFGE analysis (BlnI and SpeI), there were still many isolates (n = 154) that could not be distinguished from others, even though they were collected from different regions separated by large geographical distances. On multiple occasions, strains isolated from cattle hides in Canada had RDPs that were indistinguishable after three enzyme digestions from cattle hide isolates collected in Kansas and Nebraska. This information clearly shows that strains with indistinguishable RDPs originate from multiple sources that can be separated by large distances and that this should be taken into account when the source tracking of isolates is based on PFGE.