Sequencing of Gyrase and Topoisomerase IV Quinolone-Resistance-Determining Regions of Chlamydia trachomatis and Characterization of Quinolone-Resistant Mutants Obtained In Vitro

The L2 reference strain ofChlamydia trachomatis was exposed to subinhibitory concentrations of ofloxacin (0.5 μg/ml) and sparfloxacin (0.015 μg/ml) to select fluoroquinolone-resistant mutants. In this study, two resistant strains were isolated after four rounds of selection. TheC. trachomatis mutants presented with high-level resistance to various fluoroquinolones, particularly to sparfloxacin, for which a 1,000-fold increase in the MICs for the mutant strains compared to the MIC for the susceptible strain was found. The MICs of unrelated antibiotics (doxycycline and erythromycin) for the mutant strains were identical to those for the reference strain. The gyrase (gyrA ,gyrB ) and topoisomerase IV (parC ,parE ) genes of the susceptible and resistant strains ofC. trachomatis were partially sequenced. A point mutation was found in thegyrA quinolone-resistance-determining region (QRDR) of both resistant strains, leading to a Ser83→Ile substitution (Escherichia coli numbering) in the corresponding protein. ThegyrB ,parC , andparE QRDRs of the resistant strains were identical to those of the reference strain. These results suggest that inC. trachomatis , DNA gyrase is the primary target of ofloxacin and sparfloxacin.