Nontypeable Haemophilus influenzae (NTHi) is a common cause of
respiratory infections in adults, who are frequently treated with
fluoroquinolones. The aims of this study were to characterize the genotypes of
fluoroquinolone-resistant NTHi isolates and their mechanisms of resistance. Among 7,267 H. influenzae isolates collected from adult patients from 2000 to 2013, 28 (0.39%) were
ciprofloxacin resistant according to Clinical and Laboratory Standards Institute (CLSI) criteria. In addition, a
nalidixic acid screening during 2010 to 2013 detected five (0.23%) isolates that were
ciprofloxacin susceptible but
nalidixic acid resistant. Sequencing of their
quinolone resistance-determining regions and genotyping by pulse-field gel electrophoresis and multilocus sequence typing of the 25
ciprofloxacin-resistant isolates available and all 5
nalidixic acid-resistant isolates were performed. In the NTHi isolates studied, two mutations producing changes in two GyrA residues (Ser84, Asp88) and/or two ParC residues (Ser84, Glu88) were associated with increased
fluoroquinolone MICs. Strains with one or two mutations (n = 15) had
ciprofloxacin and
levofloxacin MICs of 0.12 to 2 μg/ml, while those with three or more mutations (n = 15) had MICs of 4 to 16 μg/ml. Long persistence of
fluoroquinolone-resistant strains was observed in three
chronic obstructive pulmonary disease patients. High genetic diversity was observed among
fluoroquinolone-resistant NTHi isolates. Although
fluoroquinolones are commonly used to treat
respiratory infections, the proportion of resistant NTHi isolates remains low. The
nalidixic acid disk test is useful for detecting the first changes in GyrA or in GyrA plus ParC among
fluoroquinolone-susceptible strains that are at a potential risk for the development of resistance under selective pressure by
fluoroquinolone treatment.