High-level fluoroquinolone resistance in Streptococcus pneumoniae requires mutations in parC and gyrA

Abstract

The mechanism of high-level fluoroquinolone resistance was studied in strains of Streptococcus pneumoniae, either selected in vitro or isolated from clinical samples. By using DNA from these high-level-resistant strains, low-level-resistant transformants (MIC of pefloxacin, ≤32 μg/ml; MIC of ciprofloxacin, 4 μg/ml; MIC of sparfloxacin, 0.50 μg/ml) were obtained at high frequencies (ca. 10-2), while high-level-resistant transformants (MIC of pefloxacin, ≤64 μg/ml; MIC of ciprofloxacin, 16 to 64 μg/ml; MIC of sparfloxacin, ≤8 μg/ml) were obtained only at low frequencies (ca. 10-4). This suggested that mutations in at least two unlinked genes were necessary to obtain high-level resistance. Low-level resistance was associated with Parc mutations (change from Ser to Tyr at position 79 [Ser79Tyr, Ser79Phe, or Asp83Gly). Parc mutations were associated, in high- level-resistant strains and transformants, with alterations in the quinolone resistance-determining region of GyrA (Ser84Tyr, Ser84Phe, and/or Glu88Lys). Low-level resistance was shown to be necessary for expression of the gyrA mutations. No mutation in the region corresponding to the quinolone resistance-determining region of GyrB and no alteration of drug accumulation were found.