Selection and genetic characterization of Streptococcus pneumoniae mutants resistant to the des-F(6) quinolone...

Existing quinolones are known to target the type II topoisomerases in bacteria. In order to determine which of these targets are of key importance in Streptococcus pneumoniae treated with BMS-284756 (T-3811ME), a novel des-F(6) quinolone, resistant mutants were selected in several steps of increasing resistance by plating pneumococci on a series of blood agar plates containing serial twofold-increasing concentrations of drug. After incubation, colonies that arose were selected and passaged twice on antibiotic-containing media at the selection level. Mutants generally showed increases in resistance of four- to eightfold over the prior level of susceptibility. Mutants in the next-higher level of resistance were selected from the previous round of resistant mutants. Subsequently, chromosomal DNA was prepared from parental (R6) pneumococci and from at least three clones from each of four levels of increasing antibiotic resistance. Using PCR primers, 500- to 700-bp amplicons surrounding the quinolone resistance determining regions (QRDR) of gyrA, gyrB, parC, and parE genes were prepared from each strain. Internal primers were used to sequence both DNA strands in the regions of approximately 400 bp centered on the QRDR. Mutations identified with increasing levels of resistance included changes in GyrA at Ser-81 and Glu-85 and changes in ParC at Ser-79 and Asp-83. Changes in GyrB and ParE were not observed at the levels of resistance obtained in this selection. The resistance to comparator quinolones (levofloxacin, ciprofloxacin, and moxifloxacin) also increased in four- to eightfold steps with these mutations. The intrinsically greater level of antibacterial activity and thus lower MICs of BMS-284756 observed at all resistance levels in this study may translate to coverage of these resistant pneumococcal strains in the clinic.