Additive effect of qnr genes, Ser83Leu substitution in DNA gyrase and Ser80Arg in topoisomeraseIV on the fluoroquinolone resistance in E.coli

Abstract number: P756

Objectives: The aim of this study was to evaluate the impact of qnrA, qnrB and qnrS in E. coli wild type strain and E. coli harbouring the most frequent substitutions in the QRDR regions of GyrA and ParC, against four different fluoroquinolones (FQ).

Methods:E. coli ATCC 25922 was used as a background strain. E. coli (GyrA Ser83Leu), E. coli (ParC Ser80Arg) and E. coli (GyrA Ser83Leu/ParC Ser80Arg) were obtained by gene replacement, and the respective derivatives carrying qnrA1, qnrB1 or qnrS1 into pBK-CMV vector by transfromation. In total twelve isogenic strains were investigated. MICs and MBCs of 4 fluoroquinolones were determined. Time killing curves were performed for each strain at 1 and 4 times MIC values, and at ciprofloxacin 1 and 2 mg/L.

Results: The effect on FQ resistance of the qnr genes was similar for CIP, LEV, MXF and NFX in the different genetic backgrounds. MICs of CIP against E. coli wild-type, E. coli (GyrA Ser83Leu) and E. coli (GyrA Ser83Leu/ParC Ser80Arg) were 0.002, 0.125, 0.25 mg/L, respectively. In presence of qnrA, qnrB or qnrS, MICs increased from 4 to 64-fold against this FQ. E. coli (gyrA Ser83Leu/parC Ser80Arg) harboring qnr genes showed MICs for CIP, LEV, MXF and NFX that ranging from 1–2, 1–4, 1–2 and 4–8 mg/L, respectively. MBCs of the four FQ were the same or 1–2 dilutions higher than the corresponding MICs in all the cases. In the time-killing assays, viable bacteria recovered for E. coli wild-type harbouring qnr genes were at least 100-fold higher compared to E. coli wild-type at 4xMIC of CIP at 24 h. In the time-killing assay at 1–2 mg/L of CIP, no viable bacteria were recovered for E. coli wild-type at 8 h, while 102–104 CFU/ml were recovered in presence of qnr. Those values were observed to 24 h for qnrA, qnrB and qnrS. At 1 mg/L of CIP, a marked re-growth is observed in E. coli (gyrA Ser83Leu) harbouring qnrA and qnrS. This re-growth was not observed for qnrB under the same conditions.

Conclusions: The additive effect of the qnr genes, Ser83Leu in the GyrA and Ser80Arg in ParC lead to intermediated susceptibility or resistance to FQs. Expression of qnr genes in Enterobacteriaceae, at least in E. coli, may have an important role to select one-step mutants with high level of FQ resistance with additional substitutions in the QRDR region of GyrA and ParC.

Session Details

Date: 10/04/2010
Time: 00:00-00:00
Session name: Abstracts 20th European Congress of Clinical Microbiology and Infectious Diseases
Location: Vienna, Austria, 10 - 13 April 2010
Presentation type:
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