In vitro development of resistance to DX-619 and other quinolones in enterococci.
OBJECTIVE
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.
Wickman
Paul A
PA
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. pwickman@creighton.edu
Black
Jennifer A
JA
Smith Moland
Ellen
E
Thomson
Kenneth S
KS
Hanson
Nancy D
ND
D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D001372:Aza Compounds / Q000494:pharmacology; D001426:Bacterial Proteins / Q000235:genetics; D002939:Ciprofloxacin / Q000494:pharmacology; D027081:DNA Gyrase / Q000235:genetics; D027101:DNA Topoisomerase IV / Q000235:genetics; D004269:DNA, Bacterial / Q000737:chemistry / Q000235:genetics; D024881:Drug Resistance, Bacterial; D013293:Enterococcus faecalis / Q000187:drug effects*; D016984:Enterococcus faecium / Q000187:drug effects*; D024841:Fluoroquinolones / Q000494:pharmacology; D000077734:Gatifloxacin; D064704:Levofloxacin; D008826:Microbial Sensitivity Tests; D000077266:Moxifloxacin; D009154:Mutation; D015242:Ofloxacin / Q000494:pharmacology; D016133:Polymerase Chain Reaction; D011759:Pyrrolidines / Q000494:pharmacology*; D011804:Quinolines / Q000494:pharmacology; D015363:Quinolones / Q000494:pharmacology*; D017422:Sequence Analysis, DNA
D000900:Anti-Bacterial Agents; D001372:Aza Compounds; D001426:Bacterial Proteins; D004269:DNA, Bacterial; C505440:DX 619; D024841:Fluoroquinolones; D011759:Pyrrolidines; D011804:Quinolines; D015363:Quinolones; D002939:Ciprofloxacin; D064704:Levofloxacin; D015242:Ofloxacin; D027101:DNA Topoisomerase IV; D027081:DNA Gyrase; D000077734:Gatifloxacin; D000077266:Moxifloxacin