Investigation of antimicrobial drug resistance in bacterial isolates from commercial cattle processing systems: pre and post-harvest food safety issues
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To determine the patterns of cross-contamination and antibiotic susceptibility, a total of 60 cattle were shipped to a commercial abattoir, 20 in each of three separate trial periods. The same animals were followed through the process and bacterial isolates were collected from these animals immediately before shipping, at the abattoir after exsanguination, after hide removal and in the cooler. Samples were cultured for Salmonella, non-type specific E. coli, and Enterococcus. Salmonella was identified in 33.9% («=20) of fecal samples and on 37.3% (n=22) of hides prior to shipment. At the abattoir, the proportion of hides from which Salmonella was isolated increased (P < 0.001) to 84.2% (n=48). Generic E. coli was recovered from 40.4 % of preevisceration carcass samples, while Salmonella was recovered from 8.3% and Enterococcus from 58.3% of these carcasses. No Salmonella or generic E. coli were recovered from hotbox carcass samples. Enterococcus however, was recovered from 8.3% of the hotbox carcass samples. Isolates were also tested for antimicrobial drug susceptibility. For generic E. coli 80.3% (n=270) of isolates were resistant to at least one antimicrobial. For Salmonella, 97% (n=101) of the isolates were resistant to at least one antimicrobial however only 4.0% were resistant to two or more. The most common resistance for gramnegative bacteria was to sulfamethoxazole. Antibiotic susceptibility patterns were fairly consistent across sampling points. Enterococcus isolates showed a higher degree of resistance to the antimicrobials tested. Of 279 confirmed Enterococcus all were resistant to at least one antimicrobial. Interestingly, 179 (64.15%) of these isolates were resistant to at least six agents tested. The most common resistance was to chloramphenicol (100%) followed by flavomycin (90.32%), hncomycin (87.81%), tylosin (78.49%), erythromycin (76.34%), tetracycline (58.87%), synercid or quinupristin/dalfopristin (47.67%), bacitracin (17.92), streptomycin (8.96%), ciprofloxacin (1.43%), linezolid (0.72%) and salinomycin (0.36%). Enterococcus spp. were also analyzed for pulsed-field gel electrophoresis profiles from all sampling points. Similar or identical PFGE patterns were found from isolates recovered at the feedlot and in the commercial abattoir. This presents strong evidence that bacterial isolates are being propagated from the feedlot to the processing environment and onto the final processed carcasses.