Delafloxacin activity against Staphylococcus aureus with reduced susceptibility or resistance to methicillin, vancomycin, daptomycin or linezolid

Background Delafloxacin is a recently approved anionic fluoroquinolone antibiotic with broad-spectrum activity against Gram-positive and Gram-negative organisms. The drug has been approved for patients with acute bacterial skin and skin structure infections including those caused by MRSA. There are limited data available against MRSA blood isolates (MRSABIs), vancomycin-intermediate strains (VISA), vancomycin-resistant strains (VRSA), daptomycin-non-susceptible strains (DNSSA) and linezolid-resistant Staphylococcus aureus (LRSA). Methods Antimicrobial activity of delafloxacin, levofloxacin, vancomycin, daptomycin and linezolid was determined against 110 MRSABIs, 15 VRSA, 35 VISA, 40 DNSSA and 6 LRSA. Microdilution testing using CAMHB was used to determine MIC according to CLSI guidelines. FDA breakpoints were used to determine delafloxacin susceptibility, and CLSI breakpoints were used for all other antibiotics. PCR testing for molecular markers was performed. Results Delafloxacin demonstrated activity against MRSABIs with an MIC90 of 1 mg/L and 68% susceptibility. Against the other groups the MIC90 and susceptibility were 1 mg/L and 40%, respectively, for VISA, 4 mg/L and 7% for VRSA and 1 mg/L and 38% for DNSSA. None of the LRSA isolates was susceptible to delafloxacin. Delafloxacin was active against 94% of MRSA blood isolates that were genotype SCC IVa. For MRSABIs with a levofloxacin MIC ≥8 mg/L (55/110), suggesting multiple mutations in the QRDR, delafloxacin MIC90 was 1 mg/L with a 36.4% susceptibility rate. Conclusions Delafloxacin demonstrates superior activity to levofloxacin against recent MRSA blood isolates, VISA, VRSA and DNSSA, and demonstrates good activity against blood isolates most commonly found in the community.