The isoleucyl-tRNA synthetase mutation V588F conferring mupirocin resistance in glycopeptide-intermediate Staphylococcus aureus is not associated with a significant fitness burden.

OBJECTIVES AND METHODS Failure to eradicate nasal carriage of a glycopeptide-intermediate Staphylococcus aureus (strain GISA-2) with mupirocin was recently attributed to a mutation that confers low-level mupirocin resistance. To identify this mutation the ileS genes of GISA-2 and its mupirocin-susceptible progenitor GISA-1 were sequenced. For comparison, the ileS genes of 10 laboratory-derived mupirocin-resistant mutants of the GISA strain Mu50 were also examined. The fitness of GISA-2 and mupirocin-susceptible GISA-1, as well as Mu50 and its mupirocin-resistant derivatives, were compared by evaluation of growth rates and performance in mixed-culture competition assays. RESULTS The point mutation V588F in the isoleucyl-tRNA synthetase was identified from the ileS sequences of GISA-2 and mupirocin-resistant mutants of Mu50. The V588F mutation was not associated with a significant fitness burden. CONCLUSIONS The low fitness cost of the V588F substitution in isoleucyl-tRNA synthetase is consistent with the frequent appearance and maintenance of this mutation in mupirocin-resistant clinical isolates, including GISA-2.