Genetic and Phenotypic Identification of Fusidic Acid-Resistant Mutants with the Small-Colony-Variant Phenotype in Staphylococcus aureus

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Abstract
Small-colony variants (SCVs) of Staphylococcus aureus are a slow-growing subpopulation whose phenotypes can include resistance to aminoglycosides, defects in electron transport, and enhanced persistence in mammalian cells. Here we show that a subset of mutants selected as SCVs by reduced susceptibility to aminoglycosides are resistant to the antibiotic fusidic acid (FA) and conversely that a subset of mutants selected for resistance to FA are SCVs. Mutation analysis reveals different genetic classes of FA-resistant SCVs. One class, FusA-SCVs, have amino acid substitution mutations in the ribosomal translocase EF-G different from those found in classic FusA mutants. Most of these mutations are located in structural domain V of EF-G, but some are in domain I or III. FusA-SCVs are auxotrophic for hemin. A second class of FA-resistant SCVs carry mutations in rplF, coding for ribosomal protein L6, and are designated as FusE mutants. FusE mutants fall into two phenotypic groups: one auxotrophic for hemin and the other auxotrophic for menadione. Accordingly, we have identified new genetic and phenotypic classes of FA-resistant mutants and clarified the genetic basis of a subset of S. aureus SCV mutants. A clinical implication of these data is that FA resistance could be selected by antimicrobial agents other than FA.

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