High-level delafloxacin resistance through the combination of two different mechanisms in Staphylococcus aureus

Delafloxacin is a new fluoroquinolone indicated for the treatment of complicated bacterial skin infections caused by Staphylococcus aureus. Despite its recent approval by the US Food and Drug Administration, the emergence of S. aureus-resistant strains has been reported. As such, this study aimed to investigate the activity of delafloxacin against a collection of S. aureus, and to determine the mechanisms of resistance. The activity of delafloxacin was measured in 59 S. aureus clinical isolates [40 methicillin-resistant S. aureus (MRSA) and 19 methicillin-susceptible S. aureus (MSSA)]. Whole-genome sequencing (WGS) was performed in the isolates resistant to delafloxacin. The minimum inhibitory concentrations required to inactivate 50% and 90% of the isolates (MIC₅₀ and MIC_90, respectively) were higher in MRSA (0.19 mg/L and 0.75 mg/L, respectively) than in MSSA (0.008 mg/L and 0.25 mg/L, respectively). Furthermore, 10 S. aureus clinical isolates (16.9%) were categorized as resistant to delafloxacin. Regarding the WGS data, several mutations were found in the quinolone resistance-determining region. Nevertheless, a mutation in the same position (E84K and E84V) of topoisomerase IV (ParC) was found exclusively in the four high-level delafloxacin-resistant isolates. Interestingly, a plasmid-encoded qacC gene (efflux pump) was found to be harboured by the isolate with the highest delafloxacin MIC value (32 mg/L). The use of a wide-spectrum efflux pump inhibitor revealed an important contribution of this system to the acquisition of delafloxacin resistance. In conclusion, delafloxacin has activity against S. aureus, including MRSA. However, this study showed that mutations in position 84 of ParC and the acquisition of a QacC efflux pump are key factors for the development of delafloxacin resistance in S. aureus.