Antimicrobial resistance and R-factor transfer among isolates of Salmonella in the northeastern United States: a comparison of human and animal isolates.

The antimicrobial susceptibility of 718 isolates of Salmonella from humans and of 688 isolates from animals was examined. Of the 46 different serotypes among the isolates from humans, Salmonella typhimurium accounted for 34%. Thirty percent of isolates were resistant to one or more antibiotic(s). Resistance to streptomycin was most common; resistance to tetracycline was next most common. Over 50% of isolates of S. typhimurium and Salmonella newport were resistant to four antibiotics. Resistance to tetracycline, kanamycin, and ampicillin has increased steadily during the past decade. Most strains possessed R-factors, and resistance to ampicillin, streptomycin, sulfisoxazole, tetracycline, chloramphenicol, and kanamycin (but not that to cephalothin) was transferable. Among the salmonellae commonly isolated from humans, only Salmonella enteritidis showed limited resistance to antibiotics (5.8%). Antibiotic resistance of isolates of S. typhimurium, Salmonella saint-paul, and Salmonella heidelberg from animals was similar to the resistance of isolates from humans. Resistance to kanamycin increased markedly over the level noted in previous studies. R-factor prevalence was high. Antibiograms of the isolates from animals and humans were similar, although some patterns were seen only in isolates from one source. Ampicillin resistance was more common in human isolates, and resistance to tetracycline, sulfonamide, and streptomycin was more common in animal isolates. Salmonellae of serotypes other than S. typhimurium that came from humans were less resistant to all antibiotics than were isolates from animals.