Resistance phenotypes and genotypes of 182 ampicillin-resistant Salmonella typhimurium strains of human and animal origin

Among the Salmonellae, an increase in the frequency of antibiotic resistance is mainly observed for S. Typhimurium, one of the most common serotypes encountered in human and animal diseases. One hundred and eighty-two ampicillin-resistant strains of S. Typhimurium, including 82 of human and 100 of animal origin, have been compared. The frequency of tetracycline, sulfonamide, streptomycin and chloramphenicol resistance was high (>84%) in both groups, the most common resistance pattern including these four antibiotics. By dot-blotting and hybridization with DNA probes, the genes encoding three types of beta-lactamase were detected. The TEM-type was found in 20% and 22% of human and animal strains, the CARS-type in 73% and 77%, respectively. The TEM- and CARB-types were found associated in five strains (four from humans an one from animal), and the OXA-2-type in only one human strain. The presence of the CARB-type genes was strongly correlated with that of the integrase (TnpI), independently of the origin of the strains, while the integrase gene in animal strains was also found in ca. 50% of the strains carrying only TEM-type genes. These results suggest the acquisition and concommittant diffusion, in S. Typhimurium of human and animal origin, of integrons carrying multiple resistance genes including bla(CARB).