| Summary: | The efficacy of fluoroquinolone-based eradication therapy largely depends on the fluoroquinolone resistance of <i>H. pylori</i>. The aim of this study was to investigate the changes in the primary resistance rate of <i>H. pylori</i> to fluoroquinolone and the mechanism of resistance in Korea. A total of 153 strains and 48 strains of <i>H. pylori</i> were isolated at a tertiary hospital in 2005/2006 and 2017/2018, respectively. The minimum inhibitory concentrations (MICs) of fluoroquinolone were determined by the serial 2-fold agar dilution method. DNA sequences in the quinolone resistance-determining regions of <i>gyrA/gyrB</i> were analyzed in resistant strains. Subsequent natural transformation study was performed to determine the association between gyrase mutation and resistance. The resistance rates increased from 19.0% (29/153) to 43.8% (21/48) both for levofloxacin and moxifloxacin. The MIC values for resistant strains increased from 2–8 µg/mL to 4–16 µg/mL over time. Mutation of <i>gyrA</i> was detected in 93.1% (27/29) and 100% (21/21) among the resistant strains in both periods, respectively. A novel Gly-85 mutation of <i>gyrA</i> was found and confirmed to be associated with fluoroquinolone resistance. Fluoroquinolone resistance rate of <i>H. pylori</i> has markedly increased over time in Korea. The resistance is mostly due to the point mutation of <i>gyrA</i>. Fluoroquinolone-containing regimens should be carefully selected in Korea, considering the increasing fluoroquinolone resistance.
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