Methicillin-resistant staphylococci are characterised by: (a) chromosomal resistance to streptomycin, (b) tetracycline resistance determined by a stable plasmid, (c) production of orange pigment, (d) a similar degree of survival on glass. We consider that all or almost all isolates of methicillin-resistant staphylococci have evolved from a single clone. If so, plasmids determining a variety of antibiotic resistances (penicillinase, penicillinase/fusidic acid, erythromycin, erythromycin/lincomycin, neomycin and chloramphenicol) have probably been acquired in recent years by their transfer from other strains. One isolate (no. 11164) has probably acquired novobiocin resistance by mutation and lost the genes determining pigment and survival.

Strain no. 13136 contains a “penicillinase plasmid” with a molecular weight of about 20 million daltons and a plasmid specifying high-level resistance to tetracycline (MIC 200 μg per ml) of molecular weight about 2.9 million daltons. The latter plasmid was also present in other methicillin-resistant strains. After storage of strain no. 13136, some colonies were resistant to only low levels of tetracycline (MIC 2.5 μg per ml). In these derivatives, the “tetracycline plasmid” of molecular weight 2.9 million daltons was replaced by a plasmid of 2.2 million daltons.

Although methicillin resistance can be lost irreversibly from strains nos. 13136, 2273 and 9463, no discrete CCC-DNA component corresponding to a plasmid determining this resistance was resolved in any of them.


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