High-level resistance to moxifloxacin and gatifloxacin associated with a novel mutation in gyrB in toxin-A-negative, toxin-B-positive Clostridium difficile

Objectives: To determine the mechanism of high-level resistance to fluoroquinolone antimicrobials in toxin-A-negative, toxin-B-positive (A^-B⁺) Clostridium difficile isolates. Methods: Following culture 16-23S PCR ribotyping was used to determine genomic relationships between A^-B⁺ C. difficile isolates. Antimicrobial susceptibilities were determined using Etests in the presence and absence of the efflux pump inhibitors reserpine (20 μg/mL), L-phenylalanine-L-arginine-β-naphthylamide (PAβN; 20 μg/mL) and verapamil (100 μg/mL). Genomic regions including the quinolone-resistance-determining-region (QRDR) of gyrA and gyrB were amplified and characterized. Results: PCR ribotyping profilesidentified one major cluster of A^-B⁺ C. difficile, universally resistant to the fluoroquinolones tested (ofloxacin, ciprofloxacin, levofloxacin, moxifloxacin and gatifloxacin; MICs > 32 mg/L). All isolates with high-level resistance had a transversion mutation (A→T) resulting in the amino acid substitution Asp-426→Val in gyrB. Non-clonal isolates were susceptible to moxifloxacin and gatifloxacin (MICs 0.3 and 0.4 mg/L, respectively) with reduced susceptibility to levofloxacin (MIC 3 mg/L) consistent with the wild-type genotype. The MICs for resistant isolates were not significantly affected by the addition of any of the efflux pump inhibitors. No amino acid substitutions were identified in the QRDR of gyrA. Conclusions: High-level resistance to fluoroquinolones in A^- B⁺ C. difficile is associated with a novel transversion mutation in gyrB. The emergence of universal resistance in different C. difficile strain types may be a factor promoting outbreaks in hospitals.