Drug efflux and parC mutations are involved in fluoroquinolone resistance in viridans group streptococci

Nine ciprofloxacin-resistant viridans group streptococci isolated from asym ptomatic carriers were analyzed. Identification to the species level by usi ng three different commercial systems and a PCR-based approach was inconsis tent. The nucleotide sequences of fragments of the parC, parE, gyrA, and gy rB genes showed considerable intra- and interspecies variations, and these variations mainly involved silent mutations. Three isolates had changes in Ser-79 of ParC (to Phe or Tyr). Phenotypic characterization indicated that eight of the nine isolates had a putative efflux mechanism that would confe r low-level resistance to ciprofloxacin.