Sequential mutations of gyrA in Escherichia coli associated with quinolone therapy.

A clinical isolate of Escherichia coli HM73 (MIC norfloxacin 2 mg/L) was 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 substitution Ser 83-->Leu in the gyrA gene. No difference in accumulation of norfloxacin was found between the two strains. DNA gyrases were isolated by affinity chromatography and assayed for supercoiling activity in the presence of norfloxacin. The minimal effective doses (MEDs) were 20 mg/L, for HM72 and 80 for HM73. DNA sequencing identified in HM73, two mutations leading to substitutions Ser 83 to Leu and Asp 87 to Gly.