Antimicrobial-resistant Salmonella spp isolated from healthy broiler chickens after slaughter

Of 105 Salmonella organisms of any serotype selected from a sample of 1,824 serotyped salmonellae isolated during a nationwide bacteriologic survey of healthy broiler chickens after slaughter, 60 (57%) were resistant to 1 or more antimicrobial agents and 47 (45%) were resistant to 2 or more agents. Highest resistance was to tetracycline (45%), streptomycin (41%), sulfisoxazole (19%), gentamicin (10%), and trimethoprim/sulfamethoxazole (8%). Additional isolates of S typhimurium, heidelberg, agona, and enteritidis were selected from the sample of 1,824 isolates for testing because of the high frequency with which these 4 serotypes are isolated from human patients. The highest frequency of resistance among 104 isolates of S heidelberg, 92 isolates of S typhimurium, and 30 isolates of S agona was to streptomycin (33 to 57%), sulfisoxazole (33 to 50%), tetracycline (26 to 50%), and gentamicin (13 to 40%); 51 to 63% of these isolates were resistant to 1 or more agents and 37 to 59% were resistant to 2 or more agents. Resistance to ampicillin among these 3 serotypes was uncommon (0 to 4%). In contrast, 15 of 19 tested isolates (79%) of S enteritidis were resistant to ampicillin and 13 of the 19 isolates (68%) were resistant only to ampicillin. This pattern of resistance was associated with a specific bacteriophage type and indicated the potential role of bacterial clones in determining the frequency and patterns of antimicrobial resistance in populations of broiler chickens. Resistance to gentamicin and trimethoprim/sulfamethoxazole was higher than that previously reported and is of public health concern because of the frequency with which these drugs are used to treat bacterial infections in human patients.