Genetic relatedness of Escherichia coli O157 isolates from cattle feces and preintervention beef carcasses.

Our objective was to define and compare pulsed-field gel electrophoresis (PFGE) profiles of Escherichia coli O157 isolated from cattle feces and carcass samples to evaluate relationships between beef carcass contamination and fecal shedding of E. coli O157 at harvest. We used PFGE separation of Xba1-digested DNA to characterize E. coli O157 isolates (n = 174) from preevisceration carcasses (n = 39) and feces (n = 135) that were recovered from 37 E. coli O157-positive truckloads sampled at a commercial abattoir. Semiquantitative fecal culture techniques differentiated high-shedding, low-shedding, and negative cattle. Among all isolates, there were 17 PFGE types (95% homology) and 37 subtypes (100% homology). Specific subtypes were detected on multiple occasions and from different sample types within loads, among loads, and among days. Seventeen subtypes were recovered from carcasses; most were also recovered from feces of high-shedding cattle (13) and low-shedding cattle (14). Within truckload, the percentages of carcass isolates that were identical to high-shedder or low-shedder fecal isolates, as determined by PFGE, were 69.2% and 46.0%, respectively, whereas among different truckloads within the same study day, the percentages of carcass isolates that were the same subtype as high-shedder or low-shedder fecal isolates were 35.3% and 58.8%, respectively. Our results suggest that cattle feces from both low- and high-shedders pose a potential risk for E. coli O157 contamination of carcasses. Truckload may be an important factor in the potential transmission of E. coli O157, but isolates from carcasses also may be similar to those from feces of cattle on different truckloads and harvest days.