In Vitro Activity of Delafloxacin Against Corynebacterium spp

Background/Objectives: The susceptibility of Corynebacterium spp. to antimicrobial agents is species-related, with increasing levels of resistance to fluoroquinolones in several species related to their continued use in clinical practice. The objectives were to determine the in vitro activity of delafloxacin in comparison with other fluoroquinolones against clinical isolates of Corynebacterium spp., to compare MICs of delafloxacin obtained with gradient strips and with reference microdilution, and to investigate the mechanisms related to fluoroquinolone resistance in the tested strains. Methods: Fifty-three clinical isolates, assigned to five species of Corynebacterium spp., were evaluated using reference microdilution for delafloxacin, ciprofloxacin, levofloxacin, and moxifloxacin (with and without reserpine or phenylalanine-arginine β-naphthylamide), and gradient strips for delafloxacin. The QRDR of the gyrA gene was amplified using primers specific to the different species, and mutations were defined after aligning against the corresponding reference sequences. Results: Delafloxacin was the most active compound with MIC₅₀/MIC₉₀ values of 0.5/8 mg/L. Single mutations at the QRDR were observed in isolates, with MICs of delafloxacin ranging from 0.016 to 4 mg/L, while double mutations occurred in isolates, with MICs ranging from 0.125 to 16 mg/L. The delafloxacin gradient strips showed an essential agreement of 88.7%, bias of -5%, and a Kappa index of 0.848. Conclusions: Increased MICs of delafloxacin against Corynebacterium spp. are related to the presence of non-conservative mutations in the QRDR of gyrA. Delafloxacin gradient strips could be a reasonable alternative for use in the clinical routine of the microbiology laboratory. Delafloxacin could represent an alternative for treating infections due to some species of Corynebacterium.