In vitro susceptibility of Staphylococcus aureus and Pseudomonas aeruginosa to a novel fluoroquinolone

Abstract

Purpose : Staphylococcus aureus (S. aureus) and Pseudomonas aeruginosa (P. aeruginosa) are the most common pathogens in keratitis. Increased prevalence of antibiotic-resistant strains pose a treatment challenge for physicians. Delafloxacin is a new fluoroquinolone recently approved by the FDA for treatment of methicillin-resistant S. aureus (MRSA), methicillin-sensitive S. aureus (MSSA) and P. aeruginosa infections. The purpose of this study was to evaluate the in vitro efficacy of delafloxacin compared to commonly prescribed fluoroquinolones against isolates of MRSA, MSSA, and P. aeruginosa recovered from patients with ocular surface disease.

Methods : 36 MRSA, 27 MSSA, and 51 P. aeruginosa isolates recovered from patients were included in the study. Antibiotic susceptibility testing to delafloxacin, ciprofloxacin, levofloxacin, and moxifloxacin was performed using a combination of Epsilometer tests (E-tests) and VITEK2 tests (Biomerieux). Minimum inhibitory concentration (MIC) breakpoints were determined based on CLSI standards.

Results : Percent susceptibility of MRSA, MSSA, and P. aeruginosa to delafloxacin was found to be 75%, 88%, and 100%, respectively. Percent susceptibility of MRSA, MSSA, and P. aeruginosa to commonly-prescribed fluoroquinolones was 36%, 82%, and 100%, respectively. For the MRSA strains, the MIC90 (ug/ml) and MIC50 (ug/ml) for delafloxacin were 1.0 and 0.19, while the MIC90 and MIC50 for commonly-prescribed fluoroquinolones was 8, except for levofloxacin with a MIC50 of 4. Similarly, MSSA isolates had MIC90 and MIC50 for delafloxacin of 0.2 and 0.002, while MIC90 and MIC50 of the commonly-prescribed fluoroquinolones were 4 and 0.5. Agreement between delafloxacin and commonly-prescribed fluoroquinolones for MRSA, MSSA, and P. aeruginosa was 55%, 94%, and 100%, respectively.

Conclusions : Ocular MRSA, MSSA, and P. aeruginosa strains demonstrated mixed in vitro susceptibility to delafloxacin. Delafloxacin demonstrated excellent in vitro efficacy against P. aeruginosa strains. Although more MRSA strains were inhibited by delafloxacin at lower MICs compared to current ocular fluoroquinolones, the in vitro efficacy was less than 80%. In vitro efficacy for MSSA isolates was less than 90%. Delafloxacin may hold promise in treating P. aeruginosa infections, but provide similar in vitro coverage for MRSA and MSSA as current ocular fluoroquinolones and should be embraced with caution.

This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.