Variable In Vitro Efficacy of Delafloxacin on Multidrug-Resistant Pseudomonas aeruginosa and the Detection of Delafloxacin Resistance Determinants

<b>Background:</b> In this study, molecular mechanisms contributing to delafloxacin resistance in <i>Pseudomonas aeruginosa</i> strains were investigated. Delafloxacin is a recently approved fluoroquinolone currently introduced to clinical applications. <b>Methods:</b> A total of 52 <i>P. aeruginosa</i> strains were collected from clinical isolates. Antimicrobial susceptibility testing was performed via broth microdilution, and the minimum inhibitory concentration (MIC) values for ciprofloxacin, levofloxacin, delafloxacin, ceftazidime and imipenem were determined. Five delafloxacin-resistant <i>P. aeruginosa</i> strains were selected for whole-genome sequencing (WGS). <b>Results:</b> MIC50 values were determined, and the following results were obtained: ciprofloxacin 0.25 mg/L, levofloxacin 0.25 mg/L and delafloxacin 1 mg/L. All five selected strains showed both extended-spectrum beta-lactamase and carbapenemase production. WGS analysis of these strains determined the sequence types (STs), namely, ST235 (two strains), ST316 (two strains) and ST395. Several mutations in quinolone-resistance-determining regions (QRDRs) were detected in all five delafloxacin-resistant <i>P. aeruginosa</i> strains as follows: <i>gyrA</i> Thr83Ile and <i>parC</i> Ser87Leu mutations were present in all five strains, while <i>parE</i> Thr223Ala in ST235, Glu459Val in ST316 and Val200Met in ST395 were detected. MexAB-OprM and MexCD-OprJ efflux pumps were uniformly present in all delafloxacin-resistant <i>P. aeruginosa</i> strains. All strains of ST235 and ST316 carried <i>bla</i>_NDM-1 in combination with other beta-lactamases. In our study, the <i>in vitro</i> efficacy of delafloxacin is inferior compared to previous fluoroquinolones based on MIC50 values; however, MIC values of delafloxacin ranged between 0.125 and 128 mg/L in our <i>P. aeruginosa</i> collection, and 21 out of 52 strains showed susceptibility to delafloxacin. <b>Conclusions</b>: Multiple QRDR mutations combined with several efflux pumps confer delafloxacin resistance in <i>P. aeruginosa</i>. Among the different detected multidrug-resistant <i>P. aeruginosa</i> strains in this study, we also report on an NDM-1 producing <i>P. aeruginosa</i> ST316 in Hungary.