Abstract: 【Objective】To detect mutations of gyrA and parC in 72 Escherichia coli strains isolated from human feces, animal feces and the environment.【Method】MICs of ciprofloxacin or enrofloxacin for 72 isolates were determined by an agar dilution test with standard strain ATCC25922 as the control. The mutations in the quinolone resistance-determining regions (QRDRs) of the gyrA and parC genes were analyzed by PCR and sequencing. 【Result】Amino acid substitutions (number of strains) found in 63 mutants were Ser83→Leu（62）and Asp87→Asn（52），Asp87→Tyr（2），Asp87→His（2）in GyrA and Ser80→Ile（47），Ser80→Arg（2）and Glu84→Val（3），Glu84→Lys（4），Glu84→Gly（5），Glu84→Ala（1）in ParC．No amino acid changes in GyrA or ParC were detected when strains MIC of ciprofloxacin were under 0.125 μg·ml-1．A single substitution in GyrA was found in the strains whose MIC of ciprofloxacin ranged from 0.125 to 0.25μg·ml-1．Double substitutions in GyrA or one in GyrA with one in ParC were found in the strains with MICof ciprofloxacin ranging from 0.5 to 32 μg·ml-1．A double changes in GyrA and a single change in ParC were found in strains with MIC of ciprofloxacin ranging from 4 to 128 μg·ml-1．Double changes in GyrA and double changes in ParC were found in strains with MIC of ciprofloxacin ranging from 16 to 128 μg·ml-1．【Conclusion】A variety of amino acid changes in GyrA and ParC were detected in the fluoroquinolone-resistant E. coli isolates from different origin．A correlation was found between the number of changes in the QRDR of GyrA and ParC and the resistance level to fluoroquinolone in E. coli strains.

Key words: Escherichia coli, Fluoroquinolones, Resistance, gyrA, parC