Genotypic analyses of Escherichia coli O157 : H7 and O157 nonmotile isolates recovered from beef cattle and carcasses at processing plants in the midwestern states of the United States

Escherichia coli O157:H7 and O157 nonmotile isolates (E. coli O157) previou sly were recovered from feces, hides, and carcasses at four large Midwester n beef processing plants (R. O. Elder, J. E. Keen, G. R. Siragusa, G. A. Ba rkocy-Gallagher, M. Koohmaraie, and W. W. Laegreid, Proc. Natl. Acad. Sci. USA 97:2999-3003, 2000). The study implied relationships between cattle inf ection and carcass contamination within single-source lots as well as betwe en preevisceration and postprocessing carcass contamination, based on preva lence. These relationships now have been verified based on identification o f isolates by genomic fingerprinting. E. coli O157 isolates from all positi ve samples were analyzed by pulsed-field gel electrophoresis of genomic DNA after digestion with XbaI. Seventy-seven individual subtypes (fingerprint patterns) grouping into 47 types were discerned among 343 isolates. Compari son of the fingerprint patterns revealed three clusters of isolates, two of which were closely related to each other. Remarkably, isolates carrying bo th Shiga toxin genes and nonmotile isolates largely fell into specific clus ters. Within lots analyzed, 68.2% of the postharvest (carcass) isolates mat ched preharvest (animal) isolates. For individual carcasses, 65.3 and 66.7% of the isolates recovered post-evisceration and in the cooler, respectivel y, matched those recovered preevisceration. Multiple isolates were analyzed from some carcass samples and were found to include strains with different genotypes. This study suggests that most E. coli O157 carcass contaminatio n originates from animals within the same lot and not from cross-contaminat ion between lots. In addition, the data demonstrate that most carcass conta mination occurs very early during processing.