Molecular analysis of low-level fluoroquinolone resistance in clinical isolates of Moraxella catarrhalis

We investigated antimicrobial susceptibility and the molecular mechanism underlying low-level resistance to fluoroquinolones in 70 non-duplicate clinical isolates of Moraxella catarrhalis. The isolates were collected in a general hospital in Tokyo, Japan, between January and October 2013 from 38 men and 32 women; most of the isolates (48 out of 70, 68.5 %) were obtained from postnasal drips of children. The antimicrobial susceptibility of M. catarrhalis isolates was determined with an Etest, and low-level fluoroquinolone-resistant isolates were subtyped by PFGE. Mutations in the gyrA and parC genes were determined by PCR and sequencing. PCR products of the gyrA and parC genes from the low-level fluoroquinolone-resistant isolates were transformed into a fluoroquinolone-susceptible strain. Among the 70 isolates, five (7.1 %) exhibited elevated fluoroquinolone MICs (levofloxacin, 1.0 mg I-1; ciprofloxacin, 0.5 mg l(-1)) and different PFGE patterns. The patients from whom these five isolates were isolated had not undergone treatment with fluoroquinolones for the past 6 months. Each of the five low-level fluoroquinolone-resistant isolates had a gyrA gene mutation resulting in a Thr-to-lle substitution at aa 80 (T80I) in the GyrA protein, while no changes were detected in the parC gene. A transformant carrying the gyrA gene containing the T80I substitution, which corresponded to Ser83 in Escherichia coli, displayed an elevated fluoroquinolone MIC and contained the T80I alteration in GyrA. Thus, our findings reveal that the low-level resistance to fluoroquinolones in M. catarrhalis is due to an amino acid substitution of Thr80 to Ile in GyrA. This is the first evidence of low-level fluoroquinolone resistance in M. catarrhalis.