Changes in antimicrobial resistance of Salmonella isolated from humans in the United States.

Changes were examined that occurred in the antimicrobial susceptibility between 1979-1980 and 1984-1985 of Salmonella organisms isolated from humans who resided in a random sample of counties in the United States. Resistance to one or more antimicrobials rose significantly from 16% (82 of 511 isolates) in 1979-1980 to 24% (117 of 485 isolates) in 1984-1985. In addition, statistically significant increases occurred in the proportion of organisms resistant to only one antimicrobial (from 4% to 9%), the rate of resistance of Salmonella typhimurium (from 14% to 26%), and the rate of resistance to tetracycline. For one commonly occurring serotype, Salmonella heidelberg, the proportion of resistant strains fell significantly from 67% in 1979-1980 to 35% in 1984-1985. When epidemiologic characteristics of patients were examined, previous use of antimicrobials for treatment of other illnesses was a significant risk factor for acquiring clinically important infections with antimicrobial-resistant Salmonella. Changes detected in the antimicrobial resistance of Salmonella during this time will ultimately affect the efficacy of antimicrobial therapy in treating human disease.