Class 1 integron-borne multiple-antibiotic resistance carried by IncFI andIncL/M plasmids in Salmonella enterica serotype typhimurium

The presence and genetic content of integrons were investigated for 37 epid emiologically unrelated multiple-drug-resistant strains of Salmonella enter ica serotype Typhimurium from humans. All isolates mere resistant to ampici llin, chloramphenicol, kanamycin, streptomycin, sulfonamides, and trimethop rim, as well as to tetracycline and/or nalidixic acid; 20% of them were als o resistant to gentamicin and amikacin, Three different class 1 integrons ( In-t1, In-t2 and In-t3) were identified by Southern blot hybridization, PCR , and DNA sequencing, and these integrons were found to carry the aadB, cat B3, oral, aadA1a, aacA4, and aacC1 gene cassettes. Integrons In-t1 (aadB an d catB3) and In-t2 (oxa1 and aadA1a) were both located on a conjugative Inc FI plasmid of 140 kb, In-t3 (aacA4, aacC1, and aadAIa) was located on an In cL/M plasmid of 100 kb which was present, in association with the IncFI pla smid, in gentamicin- and amikacin-resistant isolates. Despite the extensive similarity at the level of the antibiotic resistance phenotype, integrons were not found on the prototypic IncFI plasmids carried by epidemic Salmone lla strains isolated during the late 1970s. The recent appearance and the c oexistence of multiple integrons on two conjugative plasmids in the same Sa lmonella isolate are examples of how mobile gene cassettes may contribute t o the acquisition and dissemination of antibiotic resistance.