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  • Title: In vitro development of resistance to DX-619 and other quinolones in enterococci.
    Author: Wickman PA, Black JA, Smith Moland E, Thomson KS, Hanson ND.
    Journal: J Antimicrob Chemother; 2006 Dec; 58(6):1268-73. PubMed ID: 17062613.
    Abstract:
    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. CONCLUSION: 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.
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