Mutation of L4 Ribosomal Protein Conferring Unusual Macrolide Resistance in Two Independent Clinical Isolates of Staphylococcus aureus

The aim of this study was to characterize the resistance mechanisms of two strains of Staphylococcus aureus UCN42 and UCN43 isolated from cystic fibrosis patients that displayed an unusual phenotype of resistance to macrolides. The strains were resistant to erythromycin and the 16-membered macrolide spiramycin but susceptible to clindamycin. None of the strains contained erm or msr(A) genes by PCR. Sequencing of genes encoding ribosomal targets of macrolides revealed mutations in conserved regions of the L4 ribosomal protein, a R168S mutation for S. aureus UCN43 and mutations at positions G69A and T70P for both strains. This observation extends previous reports of similar mutations in streptococci and mycoplasma to S. aureus.