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Fluoroquinolone-resistant cultures of Streptococcus pneumoniae were isolated from 2 patients who were treated for pneumonia with levofloxacin. Nucleotide sequence analysis of bacterial DNA showed that the isolates contained mutations in both parC (DNA topoisomerase IV) and gyrA (DNA gyrase), which were shown previously to confer fluoroquinolone resistance. With the resistant isolates, the MICs for ciprofloxacin, gatifloxacin, grepafloxacin, levofloxacin, and trovafloxacin were above the maximal serum drug concentrations reported for standard dosage regimens. In contrast, the MICs for gemifloxacin and moxifloxacin were below the maximal serum concentrations. Increased effectiveness at blocking the growth of resistant mutants should make gemifloxacin and moxifloxacin less likely to allow the enrichment of mutants within susceptible populations. Additional resistance mutations in the isolates were …