The mechanisms of intrinsic resistance of Mycoplasma hominis to 14-and 15-membered macrolides were investigated in comparison with those of M. pneumoniae, which is naturally susceptible to macrolides. Radiolabeled erythromycin was not accumulated by M. hominis PG21, but addition of an ABC transporter inhibitor increased the level of erythromycin uptake more than two times, suggesting the existence of an active efflux process. The affinity of [ 14 C]erythromycin to ribosomes isolated from M. hominis was dramatically reduced relative to that to ribosomes isolated from M. pneumoniae. The nucleotide sequences of 23S rRNA of both ribosomal operons rrnA and rrnB and ribosomal proteins L4 and L22 of M. hominis were obtained. Compared to the sequence of M. pneumoniae, M. hominis harbored a G2057A transition in its 23S rRNA sequence, as did M. fermentans, another mycoplasma that is erythromycin resistant. An additional C2610U change was also found in the sequence of M. hominis. Moreover, two M. hominis clinical isolates with acquired resistance to 16-membered macrolides were examined for mutations in domain II and domain V of 23S rRNA and in ribosomal proteins L4 and L22. Compared to the sequence of reference strain PG21, one isolate harbored a A2059G transition and a C2611U transition in one of the two rrn operons, while the other one was mutated only at position 2059, also on the same operon. No mutation was found in the two ribosomal protein sequences. Overall, the present study is an exhaustive characterization of the intrinsic resistance of M. hominis to 14-and 15-membered macrolides and the first description of mycoplasma clinical isolates resistant to macrolide, lincosamide, and streptogramin antibiotics harboring a mutation at position 2611 in the 23S rRNA.Human mycoplasmas are responsible for urogenital and respiratory tract infections. Macrolide, lincosamide, and streptogramin antibiotics (MLSs) are a class of antimicrobials commonly used for the treatment of these infections. The MICs of ketolides, a new class of antimicrobials derived from erythromycin, are low for these microorganisms (3). The MLSs and ketolide antibiotics (MLSKs) inhibit protein synthesis by binding to domain V and domain II of 23S rRNA (13, 21). Three main mechanisms of resistance have been reported: drug inactivation, active efflux, and modification of the target sites by methylation or mutation (46, 49). Resistance by ribosomal mutations in domain II and domain V of 23S rRNA has been described, and recently, mutations in ribosomal proteins L4 and L22 were also associated with resistance to MLSKs (7,30,33,42,43).Mycoplasmas present different phenotypes of intrinsic resistance to macrolides. Mycoplasma pneumoniae, a respiratory mycoplasma, is susceptible to all MLSKs. In contrast, M. hominis, a genital species, is naturally resistant to 14-and 15-membered macrolides and ketolides but is susceptible to josamycin, a 16-membered macrolide, and lincosamides. Intrinsic resistance to 14-membered macrolides has been observed in other mycoplasma s...
