Abstract

Twenty-eight clinical isolates of Escherichia coli, composed of thirteen norfloxacin resistant isolates (MIC of >16 ug/ml), one intermediately resistant isolate (MIC of 8 ug/ml), and fourteen susceptible isolates (MIC of <4 ug/ml), were randomly selected to study the norfloxacin resistance mechanism and phylogeny in clinical isolates in Korea. Eleven norfloxacin resistant isolates and one susceptible isolate were multi-drug resistant (MDR). Every norfloxacin resistant isolate with MIC higher than 32 ug/ml had the same three mutations: Ser83->Leu and Asp87->Asn or Tyr in GyrA and Ser80->Ile in ParC. Whereas a resistant isolate with MIC of 16 ug/ml had three mutations but Asp87 in GyrA was replaced with Gly instead of Asn. The intermediately resistant isolate had the same two mutations in GyrA but a different mutation in ParC, Glu84->Lys. Among the susceptible isolates, two isolates with MIC of 4 ug/ml had one mutation: Ser83->Leu in GyrA, and no mutation was found in the susceptible isolates. Resistant isolates showed higher efflux activity than the susceptible ones, with random amplification of polymorphic DNA (RAPD), six susceptible isolates form a separate group from the rest of the isolates.

• Keywords: clinical isolate; Escherichia coli; norfloxacin; QRDR; RAPD; resistance