[Mutations of gyrA in ciprofloxacin resistant Escherichia coli strains]

In this study, the relationship between gyrA mutations and ciprofloxacin minimum inhibitory concentration (MIC) values was investigated in Escherichia coli strains. For this purpose, ciprofloxacin MIC values of 46 E. coli strains, isolated from out-patients and hospitalized patients, were determined by the agar dilution method. The "Quinolone Resistance Determining Region" (QRDR) of gyrA gene was amplified and restricted by Hinf-I enzyme. Ser-83 mutation was observed in all strains that have ciprofloxacin MIC values of 0.062 mg/L and higher. Afterwards, eight strains, that were found susceptible (MIC < 1 mg/L, n: 1), intermediate (MIC: 1-4 mg/L, n: 1) and high level resistant (MIC > 4 mg/L, n: 6) to ciprofloxacin, were chosen and mutations in QRDRs of these strains investigated by DNA sequence analysis. Ser 83 Leu mutation was found in all the chosen strains and Asp 87 Tyr or Asp 87 Asn mutations were also observed except the ciprofloxacin susceptible (MIC: 0.062 mg/L) one. In addition, base substitutions that don't lead to aminoacid changes were detected. The strain in which only Ser 83 Leu mutation was observed, showed high level nalidixic acid resistance (MIC > 256 mg/L). This fact was in favour of that, one mutation is enough to develop high level resistance to nalidixic acid. It was concluded that high level ciprofloxacin resistance requires at least two mutations in the QRDR of gyrA gene.