Farm-to-fork characterization of Escherichia coli associated with feedlot cattle with a known history of antimicrobial use.

This study investigated antimicrobial-resistant (AR) Escherichia coli isolated from "farm-to-fork" production of cattle fed diets containing the antimicrobial growth promoter (AGP) chlortetracycline plus sulfamethazine (44ppm each, AS700) or no AGP (control). For each treatment, samples included: feces just prior to euthanization; hides after euthanization; intestinal digesta from the lower digestive tract; carcasses immediately after evisceration and after 24h in the chiller; and ground beef stored at 5 degrees C for 1 and 8days. Samples were also collected from the abattoir environment and from air during hide removal. Total, ampicillin (Amp(r))-, and tetracycline (Tet(r))-resistant E. coli were isolated on MacConkey agar or MacConkey agar containing ampicillin or tetracycline, respectively. Amp(r) and Tet(r)E. coli were isolated from the feces and hides of all cattle. Compared to the control, the prevalence of Amp(r) (26.5% vs. 7.9%) and Tet(r) (50.9% vs. 12.6%) E. coli was greater in feces from AS700 treated animals (P<0.05), but was similar between treatments for hide samples (P>0.05). The prevalence of carcass or ground beef contamination with AR E. coli was not different between treatments. Resistant E. coli were isolated from the abattoir environment after processing of both groups of cattle. Susceptibilities to 11 antimicrobials and pulsed-field gel electrophoresis (PFGE) analyses were conducted on 360 Amp(r) and Tet(r)E. coli isolates. Twenty-five antibiogram profiles were detected, with isolates exhibiting resistance to up to 9 antimicrobials. Most (28.2%) Amp(r)E. coli were also resistant to streptomycin and tetracycline, whereas Tet(r)E. coli (53.5%) were mainly resistant to only tetracycline. Thirty one genotypes were detected by PFGE with most isolates from meat and environmental samples having similar genetic profiles to isolates from hides or digesta. These data demonstrate that antimicrobial-resistant E. coli can contaminate meat products during slaughter and enter the food chain regardless of whether or not cattle are administered AGP. The abundance of AR E. coli on the hides of animals is likely a key element for controlling end-product contamination.