Abstract:

Objective To test the in vitro susceptibility to macrolides of urogenital Chlamydia trachomatis （Ct） isolates, screen resistant Ct strains, and to explore resistance mechanism at the molecular level. Methods A total of 42 Ct strains were isolated from cervical or urethral swab samples and propagated in McCoy cells until the infection rate reached more than 90%. Then, susceptibility test was performed to evaluate the activity of three macrolides. Reverse transcription PCR and PCR were used to amplify two macrolide-resistance related genes, i.e., 23S rRNA gene and L4 gene, respectively in 2 erythromycin-resistant Ct strains and 4 erythromycin-sensitive strains followed by direct sequencing. Results The minimal inhibitory concentration （MIC） varied from 0.5 to 2 mg/L for erythromycin, 0.008 to 0.032 mg/L for clarithromycin, 0.125 to 0.5 mg/L for azithromycin. Erythromycin resistance was found in 2 isolates with the MIC value being 2 mg/L. Two mutations, C2452A and T2611A/C （Escherichia coli numbering） in the 23S rRNA gene, were detected in the resistant strains only, while the other 2 mutations, Pro113Leu and Pro156 Ala in L4 gene, were observed in all the tested strains. Conclusions Erythromycin-resistant Ct strains have emerged in clinical settings. The low-level erythromycin resistance may be associated with C2452A and T2611C mutations in the 23S rRNA gene, whereas the point mutations in L4 gene is unlikely related to erythromycin resistance.

Key words: Susceptibility test