JAC Advance Access originally published online on October 24, 2006
Journal of Antimicrobial Chemotherapy 2006 58(6):1268-1273; doi:10.1093/jac/dkl421
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
In vitro development of resistance to DX-619 and other quinolones in enterococci
Department of Medical Microbiology and Immunology, Center for Research in Anti-Infectives and Biotechnology Creighton University School of Medicine, 2500 California Plaza, Omaha, NE 68178, USA
Received 26 June 2006; returned 31 July 2006; revised 25 August 2006; accepted 25 September 2006
*Corresponding author. Tel: +1-402-280-2921; Fax: +1-402-280-1875; E-mail: pwickman{at}creighton.edu
Objectives: To investigate the molecular events involved in the development of quinolone resistance in enterococci.
Methods: 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.
Results: 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.
Conclusions: 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.
Keywords: moxifloxacin , ciprofloxacin , levofloxacin , mutational frequency , gatifloxacin