Molecular mechanism of macrolide-lincosamide resistance in Moraxella catarrhalis

We identified a Moraxella catarrhalis strain with high-level resistance to azithromycin (MIC>256 mg l(-1)), NSH1, isolated from nasopharyngeal swab samples from an inpatient with acute bronchitis in a Japanese hospital in 2011 and determined its mechanism of macrolide-lincosamide resistance. Antimicrobial susceptibility of M. catarrhalis strains was determined using the Etest and agar dilution methods. Mutations in the four 23S rRNA alleles, the ribosomal proteins L4 and L22, and methylase genes erm(B) and erm(F) were tested by PCR and/or sequencing. The efflux system was examined using appropriate inhibitors. Transformation experiments were performed using DNA amplicons of the 23S rRNA gene of M. catarrhalis strain NSH1. This strain showed high-level resistance to erythromycin, clarithromycin, azithromycin, clindamycin (MICs>256 mg l(-1)) and josamycin (MIC = 128 mg l(-1)), and contained the A2058T mutation (Escherichia coli numbering) in four of the 23S rRNA alleles. Mutation of the ribosomal proteins and overproduction of the efflux system were not observed, and methylase genes were not detected. When amplified DNA containing the single A2058T mutation was transformed into M. catarrhalis strains, transformants with three A2058T-mutated 23S rRNA alleles showed high-level resistance to macrolide-lincosamide, similar to strain NSH1. In contrast, transformants with two A2058T-mutated 23S rRNA alleles showed low-level MICs (azithromycin: 0.38-0.5 mg l(-1)). Thus, a single A2058T mutation occurring in at least three 23S rRNA alleles confers high-level resistance to 14-, 15- and 16-membered macrolides and lincosamides in M. catarrhalis possessing four 23S rRNA alleles. This study represents the first evidence, to our knowledge, of this effect in M. catarrhalis.