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 isola ted from clinical samples, By using DNA from these high-level-resistan t strains, low-level-resistant transformants (MIG of pefloxacin, great er than or equal to 32 mu g/ml; MIC of ciprofloxacin, 4 mu g/ml; MIC o f sparfloxacin, 0.50 mu g/ml) were obtained at high frequencies (ca. 1 0(-2)), while high-level-resistant transformants (MIG of pefloxacin, g reater 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 ob tained only at low frequencies (ca. 10(-4)), This suggested that mutat ions in at least two unlinked genes were necessary to obtain high-leve l resistance, Low-level resistance was associated with ParC mutations (change from Ser to Tyr at position 79 [Ser79Tyr], Ser79Phe, or Asp83G ly). ParC mutations were associated, in high-level-resistant strains a nd transformants, with alterations in the quinolone resistance determi ning region of GyrA (Ser84Tyr, Ser84Phe; and/or Glu88Lys), Low-level r esistance was shown to be necessary for expression of the gyrA mutatio ns. No mutation in the region corresponding to the quinolone resistanc e-determining region of GyrB and no alteration of drug accumulation we re found.