The epidemiology and genetics of antibiotic resistance of Salmonella typhimurium isolated from diseases animals in New York.

Only 12% of 249 strains of Salmonella typhimurium isolated during the period 1973-1976 from diseased animals were sensitive to six commonly used antibiotics. Isolates from calves exhibited the highest frequency of resistance as well as a steadily increasing frequency of resistance to ampicillin and chloramphenicol. The majority of strains from horses, dogs, and cats were also resistant to more than one antibiotic, a finding which was interpreted as primarily an effect of therapeutic rather than of growth-promoting use of antibiotics in these species. Ninety-one percent of resistant strains possessed transferable resistance. In 31% of these strains, the transfer factors were heat-sensitive and did not function at 37 C. The determinant of resistance to ampicillin was usually associated with a non-heat-sensitive transfer factor, whereas resistance to chloramphenicol, kanamycin, and tetracycline was more commonly associated with heat-sensitive transfer factors. Strains of S. typhimurium with similar patterns of resistance often contained different plasmids. There was more genetic homogeneity among determinants of resistance to tetracycline than among other determinants.