SEQUENTIAL MUTATIONS OF GYRA IN ESCHERICHIA-COLI ASSOCIATED WITH QUINOLONE THERAPY

A clinical isolate of Escherichia coli HM73 (MIG norfloxacin 2 mg/L) w as isolated during norfloxacin therapy from an urinary tract infection in a patient who had been previously treated with pipemidic acid and infected by E. coli HM72 (norfloxacin 0.25), known to harbour a substi tution Ser 83 --> Leu in the gyr A gene. No difference in accumulation of norfloxacin was found between the two strains. DNA gyrases were is olated by affinity chromatography and assayed for supercoiling activit y in the presence of norfloxacin. The minimal effective doses (MEDs) w ere 20 mg/L, for HM72 and 80 for HM73. DNA sequencing identified in HM 73, two mutations leading to substitutions Ser 83 to Leu and Asp 87 to Gly.