Combination of known and unknown mechanisms confers high-level resistance to fluoroquinolones in Enterococcus faecium.
In order to elucidate the mechanisms of fluoroquinolone resistance in Enterococcus faecium, spontaneous mutants isolated from Ent. faecium ATCC 19434 by stepwise selection with sparfloxacin (SPX) or norfloxacin (NOR) and 13 clinical isolates of Ent. faecium were characterized by analysing quinolone-resistance-determining regions (QRDRs) of the gyrA, gyrB, parC and parE genes and examining changes in MICs of SPX and NOR in the presence of efflux pump inhibitors. The SPX-selected first-step mutant had a point mutation only in gyrA, and the mutants QR7-18 and QR7-39, and clinical isolates that had point mutations in parC, showed NOR resistance. These results indicate that the primary targets of SPX and NOR are DNA gyrase and topoisomerase IV, respectively, and therefore that the primary target of fluoroquinolones in Ent. faecium differs depending on the structure of the compound used. The characterization of the spontaneous mutants and the clinical isolates demonstrates that in addition to the previously reported alterations in GyrA and ParC, an alteration in GyrB, a NorA-like pump, an unknown efflux pump, which excretes both SPX and NOR from bacterial cells, and probably other unknown mechanism(s) all contribute to fluoroquinolone resistance in Ent. faecium.
Oyamada
Yoshihiro
Y
Pharmacology and Microbiology Research Laboratories, Dainippon Pharmaceutical Co. Ltd, Enoki 33-94, Suita, Osaka 564-0053, Japan.
Ito
Hideaki
H
Pharmacology and Microbiology Research Laboratories, Dainippon Pharmaceutical Co. Ltd, Enoki 33-94, Suita, Osaka 564-0053, Japan.
Fujimoto
Kouichi
K
Pharmacology and Microbiology Research Laboratories, Dainippon Pharmaceutical Co. Ltd, Enoki 33-94, Suita, Osaka 564-0053, Japan.
Asada
Reiko
R
Pharmacology and Microbiology Research Laboratories, Dainippon Pharmaceutical Co. Ltd, Enoki 33-94, Suita, Osaka 564-0053, Japan.
Niga
Toshiyuki
T
Pharmacology and Microbiology Research Laboratories, Dainippon Pharmaceutical Co. Ltd, Enoki 33-94, Suita, Osaka 564-0053, Japan.
Okamoto
Ryoichi
R
Department of Microbiology, School of Medicine and Environmental Infectious Disease, Graduate School of Medical Sciences, Kitasato University, Kanagawa, Japan.
Inoue
Matsuhisa
M
Department of Microbiology, School of Medicine and Environmental Infectious Disease, Graduate School of Medical Sciences, Kitasato University, Kanagawa, Japan.
Yamagishi
Jun-Ichi
JI
Pharmacology and Microbiology Research Laboratories, Dainippon Pharmaceutical Co. Ltd, Enoki 33-94, Suita, Osaka 564-0053, Japan.