Molecular characterization of R-plasmids pST1 and pST2 from Salmonella typhimurium S24

Salmonella typhimurium S24 was isolated in September 1986 at Kaohsiung Medical College Hospital, Kaohsiung, Taiwan, from a patient suffering from gastroenteritis during an outbreak of salmonellosis. Two conjugative R-plasmids have been isolated from Escherichia coli K-12 14R525, which was mated with S. typhimurium S24. The two R-plasmids found in S. typhimurium S24 belong to two different incompatibility (Inc) groups: the 130-kilobase IncFI plasmid pST1 and the 56-kb IncN plasmid pST2. These two R-plasmids of pST1 and pST2 together mediate resistance to multiple antibiotics in S. typhimurium S24. By DNA probes hybridization, plasmid pST1 was shown to carry an enteric type II chloramphenicol acetyltransferase (CAT) gene, a class C tetracycline resistance (TetR) gene and a type III dihydrofolate reductase (DHFR) gene, all of which confer resistance to chloramphenicol, tetracycline, and trimethoprim respectively. A Richmond's type III beta-lactamase gene was located on each plasmid of pST1 and pST2. beta-lactamases specified by both plasmids pST1 and pST2 conferred high level resistance to amoxicillin, carbenicillin, piperacillin, sulbenicillin, ticarcillin in addition to ampicillin. A novel aminoglycoside 6'-N-acetyltransferase [AAC(6')] was demonstrated on plasmid pST2. This AAC(6') enzyme modified kanamycin, amikacin, dibekacin, tobramycin, gentamicin, netilmicin, sisomicin, butirosin and ribostamycin.