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

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 (MIG of pefloxacin, greater than or equal to 32 mu g/ml; MIC of ciprofloxacin, 4 mu g/ml; MIC of sparfloxacin, 0.50 mu g/ml) were obtained at high frequencies (ca. 10(-2)), while high-level-resistant transformants (MIG of pefloxacin, greater than or equal to 64 mu g/ml; MIC of ciprofloxacin, 16 to 64 mu g/ml; MIC of sparfloxacin, greater than or equal to 8 mu g/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.