Mutations in 23S rRNA and ribosomal protein L4 account for resistance in Chlamydia trachomatis strains selected in vitro by macrolide passage

Thirteen strains of Chlamydia trachomatis were exposed to subinhibitory concentrations of erythromycin (0.5 microg ml(-1)), azithromycin (0.5 microg ml(-1)) and josamycin (0.04 microg ml(-1)) to select macrolide-resistant mutants with serial passages. The C. trachomatis mutants presented with low-level resistance to erythromycin, azithromycin and josamycin for which a 16-fold increase, a 16-fold increase and an 8-fold increase respectively in the minimal inhibitory concentration (MICs) for the mutant strains compared with the MIC for the susceptible strains were found. The results of chemosensitivity showed that josamycin had the highest susceptibility rate compared with erythromycin and azithromycin in the treatment of C. trachomatis. The ribosomal protein L4 and 23S rRNA genes of the susceptible and resistant strains of C. trachomatis were partially sequenced. A double mutation was found in ribosomal protein L4 of the mutants, leading to Pro109(CCG)-->Leu(CTG), and Pro151(CCG)-->Ala(GCC) (Escherichia coli numbering) in the corresponding protein, but these mutations were also found in parent strains. An investigation into the sequences of 23S rRNAs in the mutants revealed point mutations of A2057G, A2059G and T2611C (E. coli numbering). These results suggest that point mutations located in 23S rRNA were associated with macrolide resistance in C. trachomatis.