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Small size antibiotic resistance plasmids having molecular weights less than 10 Mdal were isolated and characterized from ten clinically isolated multiple resistant Staphylococcus aureus. Agarose gel electrophoresis profiles and antibiotic resistance patterns divided these strains into four groups.
Strain 2-23-6, the representative strain of a group of five strains conferred two plasmids of molecular weights 1.6x106 dal and 2.0x106 dal. The small plasmid (pSBK 112) specified macrolides, lincosamides_and streptogramin type B (MLS) resistance gene which are expressed constitutively. Lage plasmid (pSBK 125) specified chloramphenicol resistance gene which is inducible. Strain 10-5 conferred a 3.0x106 dal plasmid (pSBK 141) which carry an inducible ampicillin resistance gene and strain P-H-2 conferred an 1.6x106 dal plasmid (pSBK 190) which carry a constitutive MLS resistance gene. Strain D-H-1 conferred four plasmids of molecular weights 0.23x106 dal (pSBK 201), 1.6x106 dal (pSBK 202), 2.5x106 dal (pSBK 203), and 1.2x107 dal (pSBK 204), respectively. Among those four plasmids, only pSBK 203 specified chloramphenicol resistance gene.
Curing of constitutive MLS resistance using acriding orange or ethidium bromide in 2-23-6 and P-H-2 strains produced ´inducible´ MLS resistance strains which are less resistant to MLS than the wild type strains, suggesting that there are two resistance genes in both strains; one is constitutive and the other is inducible.
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