In vitro development of resistance to DX-619 and other quinolones in enterococci

To investigate the molecular events involved in the development of quinolone resistance in enterococci.

Clinical isolates of Enterococcus faecium and Enterococcus faecalis were exposed to inhibitory and subinhibitory concentrations of DX-619, ciprofloxacin, levofloxacin, gatifloxacin and moxifloxacin. Mutational frequencies were calculated and susceptibility changes were determined. The quinolone resistance determining regions (QRDRs) of gyrA and parC in less-susceptible mutants were amplified by PCR and sequenced.

Single-step mutants of E. faecalis and E. faecium were selected with all drugs. There were no differences in the frequencies of mutant selection among drugs, with frequencies ranging from 10(-5) to 10(-8). All single-step mutants were inhibited by 0.03-1 mg/L DX-619, 0.25-8 mg/L moxifloxacin, 0.5-8 mg/L gatifloxacin, 1-16 mg/L levofloxacin and 1-32 mg/L ciprofloxacin. No QRDR changes were observed in single-step mutants. Less-susceptible mutants selected after five passages on agar containing subinhibitory quinolone concentrations were inhibited by 0.12-8 mg/L DX-619, 1-64 mg/L moxifloxacin, 2-64 mg/L gatifloxacin and 2-128 mg/L levofloxacin and ciprofloxacin. QRDR changes were detected in only 9 of the 20 fifth-passage mutants, suggesting that mutations outside the purported QRDRs and/or other resistance mechanisms were also involved.

The relatively high frequencies at which single-step mutants were selected with all drugs indicate that caution is necessary if quinolones are to be considered for monotherapy of serious enterococcal infections. DX-619, the most potent quinolone, may have potential as an anti-enterococcal agent if sufficient concentrations can be safely attained in vivo.