Molecular Characterization and Typing of Enrofloxacin-Resistant Clinical Isolates of Mycoplasma gallisepticum

Emergence of resistance to fluoroquinolones is mainly due to chromosomal mutations in genes encoding the subunits of the drug's target enzymes, DNA gyrase and topoisomerase IV, which are essential for DNA replication. The quinolone resistance-determining regions (QRDRs) of these genes were characterized in 25 Mycoplasma gallisepticum strains isolated from commercial poultry flocks during 1997-2007, which exhibited different levels of susceptibility to fluoroquinolones. All enrofloxacin-resistant isolates harbored amino acid substitutions in the QRDRs of each of three proteins (GyrA, GyrB, and ParC). Molecular typing of those strains by random amplification of polymorphic DNA and gene-targeted sequencing supports ongoing, stepwise selection of resistant strains from the existing reservoir of susceptible M. gallisepticum strains.