The antibiotic resistance profiles and transferable R factors of Salmonella and Escherichia coli isolates from 104 broiler carcasses taken from one processing plant were determined. Carcasses were sampled after immersion chilling. All samples were transported iced and immediately analyzed upon arrival to the laboratory. The resistance patterns of isolates to 12 antibiotics were determined (i.e., ampicillin, cephalothin, streptomycin, sulfisoxazole, trim-ethoprim-sulfamethoxazole, nalidixic acid, tetracycline, neomycin, chloramphenicol, gentamicin, colistin, and nitrofurantoin). Isolates resistant to one or more antibiotics were utilized as donors of resistance to completely antibiotic-sensitive strains, an E. coli K-12, F, J5, azide-resistant strain and a Salmonella serovar Enteritidis. Transfer of the different R plasmids was confirmed by the determination of the resistance patterns of the transconjugants. Of the 93 Salmonella and 71 E. coli strains isolated from these samples, the largest numbers were resistant to tetracycline (52.7% and 49.3%), sulfisoxazole (45.2% and 42.3%), and streptomycin (37.6% and 39.4%). Large percentages of the Salmonella (33.3%) and the E. coli (30.0%) strains transferred all or part of their resistance to E. coli K-12 in mixed cultures. Great variation was observed between different strains in the frequency at which they transferred resistance. Resistance to tetracycline, sulfisoxazole, and streptomycin was found to be conferred by 31.7%, 29.8%, and 21.6% of the 19 R factors identified. No transfer of resistance to nalidixic acid, gentamicin, cephalothin, nitrofurantoin, and chloramphenicol was detected. When 30 antibiotic-resistant E. coli strains were cultured with a sensitive strain of Salmonella serovar Enteritidis,7 (23.3%) of the resistant strains were found capable of transferring R factors. Only 2 (6.7%) of the resistant strains could transfer R factors and unusual β-galactosidase activity.

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