Genetic analyses of mutations contributing to fluoroquinolone resistance in clinical isolates of Streptococcus pneumoniae

Twenty-one clinical isolates of Streptococcus pneumoniae showing reduced su sceptibility or resistance to fluoroquinolones were characterized by seroty pe, antimicrobial susceptibility, and genetic analyses of the quinolone res istance-determining regions (QRDRs) of gyrA, gyrB, parC, and parE. Five str ains were resistant to three or more classes of antimicrobial agents. In su sceptibility profiles for gatifloxacin, gemifloxacin, levofloxacin, moxiflo xacin, ofloxacin, sparfloxacin, and trovafloxacin, 14 isolates had intermed iate- or high-level resistance to all fluoroquinolones tested except gemifl oxacin (no breakpoints assigned). Fluoroquinolone resistance was not associ ated with serotype or with resistance to other antimicrobial agents. Mutati ons in the QRDRs of these isolates were more heterogeneous than those previ ously reported for mutants selected in vitro. Eight isolates had amino acid changes at sites other than ParC/S79 and GyrA/S81; several strains contain ed mutations in gyrB, parE, or both loci. Contributions to fluoroquinolone resistance by individual amino acid changes, including GyrB/E474K, ParE/E47 4K, and ParC/A63T, were confirmed by genetic transformation of S. pneumonia e R6. Mutations in gyrB were important for resistance to gatifloxacin but n ot moxifloxacin, and mutation of gyrA was associated with resistance to mox ifloxacin but not gatifloxacin, suggesting differences in the drug-target i nteractions of the two 8-methoxyquinolones. The positions of amino acid cha nges within the four genes affected resistance more than did the total numb er of QRDR mutations. However, the effect of a specific mutation varied sig nificantly depending on the agent tested. These data suggest that the heter ogeneity of mutations will likely increase as pneumococci are exposed to no vel fluoroquinolone structures, complicating the prediction of cross-resist ance within this class of antimicrobial agents.