Submitted to: Applied and Environmental Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/8/2001
Publication Date: N/A
Interpretive Summary: Human disease can be caused by consumption of undercooked beef contaminated with E. coli O157. The bacteria are commonly carried by cattle in their feces and on their hides, and get transferred to carcasses during processing. Previously, samples taken from four large, Midwestern beef processing plants were examined for E. coli O157. Within each lot or group of cattle, samples were taken from the feces and hides of animals, as well as from carcasses tracked and sampled before evisceration, after evisceration, and after processing. The identity of an E. coli O157 isolate from each positive sample now has been determined by a method called fingerprinting, which associates a specific DNA pattern with each isolate. It is the same method used by members of PulseNet to track disease outbreaks. Fingerprints that are the same or very similar indicate that the bacteria are from the same source. Forty-seven types of E. coli O157 were identified by fingerprinting in this study. Those E. coli O157 that could produce both kinds of Shiga toxin were more closely related to each other than to isolates able to make only one type of Shiga toxin. The fingerprints also were used to track carcass contamination. Within lots, just over two-thirds of the carcass isolates matched animal isolates. For individual carcasses, approximately two-thirds of the isolates recovered after evisceration and after processing matched those recovered before evisceration. These data suggest that most E. coli O157 carcass contamination originates from animals within the same lot and not from cross-contamination between lots. In addition, the data demonstrate that most carcass contamination occurs very early during processing.
Technical Abstract: Escherichia coli O157:H7 and O157 nonmotile isolates (E. coli O157) were recovered at four large Midwestern beef processing plants (Elder, R. O., J. E. Keen, G. R. Siragusa, G. A. Barkocy-Gallagher, M. Koohmaraie, and W. W. Laegreid, 2000, Proc. Natl. Acad. Sci. USA, 97:2999-3003). Samples were taken from feces and hides of animals in the same lot as carcasses tracked and sampled preevisceration, postevisceration, and post-processing in the cooler. Genomic DNA from one isolate out of each E. coli O157-positive sample now has been analyzed by pulsed-field gel electrophoresis after digestion with XbaI. Seventy-seven individual subtypes (fingerprint patterns) grouping into 47 types were discerned among 343 isolates. Comparison of the fingerprint patterns revealed three clusters of isolates, two of which were closely related to each other. Remarkably, isolates carrying both Shiga toxin genes and nonmotile isolates largely fell into specific clusters. Within lots, 68.2% of the postharvest (carcass) isolates matched preharvest (animal) isolates. For individual carcasses, 65.3% and 66.7% of the isolates recovered postevisceration and in the cooler, respectively, matched those recovered preevisceration. Multiple isolates from some carcass samples included strains with different genotypes. This study suggests that most E. coli O157 carcass contamination originates from animals within the same lot and not from cross-contamination between lots. In addition, the data demonstrate that most carcass contamination occurs very early during processing.