In vitro activity of delafloxacin against enterococcal isolates from male urinary tract infections and genetic determinants of resistance.

UNLABELLED: Delafloxacin (DLX) is a novel fluoroquinolone (FQ) with enhanced activity against gram-positive bacteria and under acidic conditions. Since the pH of urine is usually acidic, DLX could be an alternative to current drugs for urinary tract infections (UTIs), especially in male patients, for whom good prostatic tissue penetration is required. The aim of this study was to evaluate in vitro the activity of DLX and FQ comparators against Enterococcus faecalis and Enterococcus faecium, to characterize the resistance pattern of E. faecalis against FQs, and to decipher the genetic basis of resistance among in vitro-selected mutants. DLX showed higher in vitro activity than both levofloxacin and moxifloxacin against E. faecalis, with an MIC50 of 0.06 mg/L, which was 16- and 4-fold lower than those of these two FQs, respectively. None of the three antibiotics was effective against E. faecium (MICs mostly >8 mg/L). DLX exhibited increased activity under acidic conditions, as the MIC50 for 12 E. faecalis isolates decreased by a 3-log2 dilution between pH 7.2 and pH 5, whereas comparator activities were impaired. A total of 19 mutants with significant increases in DLX MICs were selected in vitro, including 12/19 (63%) with at least one mutation in the quinolone resistance-determining region (QRDR) of GyrA (mainly at the position S84). Interestingly, four mutants lacked QRDR mutations but carried mutations in eno or infB genes, which have not previously been reported in FQ resistance. In conclusion, DLX showed greater in vitro activity than levofloxacin and moxifloxacin against E. faecalis isolates, especially under acidic conditions, suggesting its potential use for treating male UTIs. IMPORTANCE: Enterococci urinary tract infections (UTIs) are common but often difficult to treat in male patients, in whom good prostatic tissue penetration is required. Fluoroquinolones (FQs) remain attractive in this setting even though their efficacy is altered by the acidic pH of urine. Delafloxacin (DLX) is a novel FQ whose structure confers enhanced activity against gram-positive bacteria and under acidic conditions such as urine. Here, we demonstrate that DLX is substantially more active in vitro against Enterococcus faecalis than levofloxacin or moxifloxacin, an advantage further amplified under urine-mimicking acidic conditions. The low frequency of in vitro spontaneous resistant mutants and the predominance of GyrA substitutions among mutants suggest a resistance driven by target modification. Together, these results identify DLX as a promising candidate to address an unmet need in the management of male enterococcal UTIs, but further validation in animal models and clinical studies is needed.