Fluoroquinolone-resistant mutants of Mycoplasma hominis were selected in vitro from the PG21 susceptible reference strain either by multistep selection on increasing concentrations of various fluoroquinolones or by one-step selection on agar medium with ofloxacin. The quinolone resistance-determining regions (QRDR) of the structural genes encoding the A and b subunits of DNA gyrase were amplified by PCR, and the nucleotide sequences of eight multistep-selected resistant strains were compared to those of susceptible strain PG21. Four high-level resistant mutants that were selected on norfloxacin or ofloxacin contained a C-to-T transition in the gyrA QRDR, leading to substitution of Ser-83 by Leu in the GyrA protein. Analysis of the sequence of the gyrB QRDR of the eight multistep-selected mutants did not reveal any difference compared to that of the gyrB QRDR of the reference strain M. hominis PG21. Similar analyses of eight one-step-selected mutants did not reveal any base change in the gyrA and gyrB QRDRs. These results suggest that in M. hominis, like in other bacterial species, a gyrA mutation at Ser-83 is associated with fluoroquinolone resistance.
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