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The mechanisms responsible for macrolide resistance in Streptococcus pneumoniae mutants, selected from susceptible strains by serial passage in azithromycin, were investigated. These mutants were resistant to 14- and 15-membered macrolides, but resistance could not be explained by any clinically relevant resistance determinant [mef(A), erm(A), erm(B), erm(C), erm(TR), msr(A), mph(A), mph(B), mph(C), ere(A), ere(B)]. An investigation into the sequences of 23S rRNAs in the mutant and parental strains revealed individual changes of C2611A, C2611G, A2058G, and A2059G (Escherichia coli numbering) in four mutants. Mutations at these residues in domain V of 23S rRNA have been noted to confer erythromycin resistance in other species. Not all four 23S rRNA alleles have to contain the mutation to confer resistance. Some of the mutations also confer coresistance to streptogramin B (C2611A, C2611G, and A2058G), 16-membered macrolides (all changes), and clindamycin (A2058G and A2059G). Interestingly, none of these mutations confer high-level resistance to telithromycin (HMR-3647). Further, two of the mutants which had no changes in their 23S rRNA sequences had changes in a highly conserved stretch of amino acids ((63)KPWRQKGTGRAR(74)) in ribosomal protein L4. One mutant contained a single amino acid change (G69C), while the other mutant had a 6-base insert, resulting in two amino acids (S and Q) being inserted between amino acids Q67 and K68. To our knowledge, this is the first description of mutations in 23S rRNA genes or ribosomal proteins in macrolide-resistant S. pneumoniae strains.

Mutations in 23S rRNA and Ribosomal Protein L4 Account for Resistance in Pneumococcal Strains Selected In Vitro by Macrolide Passage