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 0157:H7 isolates from calves on 20 farms in 16 states were determined by pulsed-field gel el ectrophoresis (PFGE). Digestion of genomic DNAs with the restriction e nzymes SfiI and XbaI yielded 14 and 18 restriction endonuclease digest ion profiles (REDP), respectively. Seventeen farms (85%) had E. coli 0 157:H7 with a unique REDP, and when more than one calf tested positive on a farm, the isolates displayed identical REDP. Isolates from diffe rent farms within the same state displayed distinct REDP, as did most isolates from farms in different states. The exceptions were three far ms in New York, Ohio, and Washington that had calves harboring E. coli 0157:H7 with the same REDP. In addition to REDP, the toxin profiles o f all 26 isolates were determined using oligonucleotide probes to Shig a-like toxins (SLT) I and II, Nineteen (73%) of the E. coli 0157:H7 is olates harbored the genes for both SLT I and II, while the remaining s even isolates (27%) had the gene for SLT II only. Also, isolates with the same REDP had the same toxin profile. The genomic relatedness amon g the E. coil 0157:H7 isolates was also determined by principal compon ent analysis of Dice similarity indices of REDP. Three clusters were i dentified, but none of these were associated with a geographic region or toxin profile.