| Summary: | Staphylococcus aureus is a Gram positive potentially pathogenic bacterium which has a propensity to gather resistance determinants. Mupirocin is a novel topical antibiotic active against many Gram positive species, including staphylococci and effective in the treatment and prevention of staphylococcal infections. Mupirocin acts by competitively inhibiting the charging of isoleucyl tRNA-synthetase (IRS) with isoleucine. Resistance has been observed in Staphylococcus aureus and coagulase negative staphylococci. Intermediate-level resistance (MIC >8μg ml<sup>-1</sup> >512μg ml<sup>-1</sup>) is thought to be due to spontaneous mutations in the native IRS. High-level resistance (>512μg ml<sup>-1</sup>) is conferred by a second IRS protein, encoded by mupA which has a much lower affinity for mupirocin than isoleucine. The mupirocin resistance gene (mupA) is usually found on a 4.05kb EcoRI fragment of plasmids of otherwise varied EcoRI restriction pattern which are easily transferred between strains by filter mating. Prior to the onset of these studies, mupirocin resistance had not been found linked to another resistance determinant. Initial investigations intended to identify mechanisms of gene flux resident on mupA plasmids revealed a family of related mupA plasmids, the p3356 family which includes three plasmid types: p3356, p3356D and p3358. p3356 contains a single copy mupA flanked by direct repeats of the staphylococcal insertion sequence IS257. p3356D is identical to p3356 except for the duplication of a "mupA-IS257" cassette in tandem repeat. p3358 is related to p3356D by the insertion of a pT181-like plasmid (tetracycline resistant) accompanied by the duplication of an IS257 in direct repeat to flank the inserted pT181; thus p3358 is the first documented example of linked resistance between mupirocin and another resistance determinant, namely tetracycline. IS257 has been implicated as the recombinogenic site in the gene duplication event involved in the evolution of p3356D from p3356. IS257 co-integrative transposition has been demonstrated to allow the co-integration of pOX7 with p3356 to generate a p3358-type plasmid in which pT181 is replaced by pOX7. Therefore, it is concluded that IS257 transposition and recombination is a mechanism by which staphylococcal replicons can evolve to form multiply resistant replicons.
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