Acetylation of fluoroquinolone antimicrobial agents by an Escherichia coli strain isolated from a municipal wastewater treatment plant

Aims:  To isolate environmental bacteria capable of transforming fluoroquinolones to inactive molecules.
Methods and Results:  Bacteria were isolated from the aerobic liquor of a wastewater treatment plant on a medium containing norfloxacin (100 mg l⁻¹). Twenty-two isolates were highly resistant (minimal inhibitory concentration: 6·25−200 μg ml⁻¹) to five fluoroquinolones and six of them were positive by PCR amplification for the aminoglycoside resistance gene aac(6')-Ib. Of these, only Escherichia coli strain LR09 had the ciprofloxacin-acetylating variant gene aac(6')-Ib-cr ; HPLC and mass spectrometry showed that this strain transformed both ciprofloxacin and norfloxacin by N -acetylation. This bacterium also had mutations in the quinolone-resistance determining regions of the gyrA and parC genes.
Conclusions:  An E. coli isolate from wastewater, which possessed at least two distinct fluoroquinolone resistance mechanisms, inactivated ciprofloxacin and norfloxacin by N -acetylation.
Significance and Impact of the Study:  This is the first report of N -acetylation of fluoroquinolones by an aac(6')-Ib-cr -containing bacterium from an environmental source.