Molecular characterization of fluoroquinolone resistance in invasive clinical isolates of Streptococcus pneumoniae susceptible to delafloxacin.

Background: Delafloxacin is a dual-targeting fluoroquinolone against topoisomerase IV and DNA gyrase that could decrease resistance selection by diminishing the likelihood of multiple mutational events in both enzymes.

Objectives: To determine the activity of delafloxacin against invasive Streptococcus pneumoniae isolates resistant to levofloxacin (LEV-R), compare delafloxacin MICs for LEV-R isolates with those of susceptible strains, and analyse mutations in QRDRs.

Methods: A total of 130 S. pneumoniae isolates (2014-20) were studied. The isolates were distributed according to levofloxacin MICs: high-level LEV-R (n = 46; MIC > 32 mg/L), low-level LEV-R (n = 36; MIC range 3-12 mg/L) and susceptible (LEV-S; n = 48; MIC ≤2 mg/L). We considered delafloxacin-resistant to be MIC ≥ 0.12 mg/L (EUCAST epidemiological cut-off). MICs were determined by gradient diffusion (control strain S. pneumoniae ATCC 49619). All isolates were subjected to PCR and sequencing of parC, parE, gyrA and gyrB genes.

Results: All LEV-S isolates showed delafloxacin MICs of ≤0.008 mg/L, and did not show mutations in QRDRs. Isolates with levofloxacin MICs of 3-12 mg/L showed delafloxacin MICs of <0.12 mg/L, with 3 (8.3%) presenting mutations in gyrA, and 11 (30.6%) in parC previously related to resistance. Isolates with levofloxacin MICs of >32 mg/L showed two to four mutations in QRDRs and 11 (24%) were delafloxacin resistant, presenting at least two mutations in gyrAS81F/L/V + parCS79F; four accumulated three mutations, and two showed four mutations in QRDRs.

Conclusions: Among LEV-R pneumococci, 71 (87%) were susceptible to delafloxacin, indicating that it maintains its activity despite the presence of mutations in gyrA + parC that lead to high-level resistance to levofloxacin.