Genomic analysis using pulsed-field gel electrophoresis of Escherichia coli O157:H7 isolated from dairy calves during the United States national dairy heifer evaluation project (1991-1992)

The genomic fingerprints of 26 Escherichia coli O157:H7 isolates from calves on 20 farms in 16 states were determined by pulsed-field gel electrophoresis (PFGE). Digestion of genomic DNAs with the restriction enzymes SfiI and XbaI yielded 14 and 18 restriction endonuclease digestion profiles (REDP), respectively. Seventeen farms (85%) had E. coli O157:H7 with a unique REDP, and when more than one calf tested positive on a farm, the isolates displayed identical REDP. Isolates from different farms within the same state displayed distinct REDP, as did most isolates from farms in different states. The exceptions were three farms in New York, Ohio, and Washington that had calves harboring E. coli O157:H7 with the same REDP. In addition to REDP, the toxin profiles of all 26 isolates were determined using oligonucleotide probes to Shiga-like toxins (SLT) I and II, Nineteen (73%) of the E. coli O157:H7 isolates harbored the genes for both SLT I and II, while the remaining seven isolates (27%) had the gene for SLT II only. Also, isolates with the same REDP had the same toxin profile. The genomic relatedness among the E. coli O157:H7 isolates was also determined by principal component analysis of Dice similarity indices of REDP. Three clusters were identified, but none of these were associated with a geographic region or toxin profile.