Variation in the mutation frequency determining quinolone resistance in Chlamydia trachomatis serovars L-2 and D

Objectives: Quinolone resistance of chlamydiae is supposed to be extremely rare. To assess the risk for the emergence of chlamydial quinolone resistance, we analysed the occurrence of resistant mutants in a quantitative perspective. Methods: Infectious elementary bodies of Chlamydia trachomatis serovar L-2 (ATCC VR-902B) and D (ATTC VR-885) clones were purified on density gradients, and mutants resistant to moxifloxacin and rifampicin were selected by a plaque assay. Plaque assays were conducted with 2 x 10(9) inclusion forming units (IFUs) of each serovar for rifampicin and 2.66 x 10(9) IFUs for moxifloxacin. Resistant clones were analysed for mutations in the gyrA, gyrB, parC and parE genes, and respective MICs were determined by titration experiments. Results: Mutation frequencies for rifampicin (MIC >= 0.2 mg/L) did not differ significantly between serovars L-2 and D (5.7 x 10(-7) versus 6.3 x 10(-7)). In contrast, the occurrence of moxifloxacin-resistant mutants (MIC >= 0.6 mg/L) was determined to be 2.0-2.2 x 10(-8) for the serovar L-2 isolate and less than 2.66 x 10(-9) for the serovar D isolate. Moxifloxacin resistance of all serovar L-2 clones depended on single-nucleotide point mutations in the quinolone resistance-determining region of the gyrA, whereas no additional mutations were found in the gyrB, parC or parE genes. Conclusions: C. trachomatis isolates have the potential to present with clinically relevant antibiotic resistance in future. Serovar-specific differences in the occurrence of spontaneous mutations should be taken into account to predict quinolone resistance in different chlamydial diseases.