Mutation of gyrA in Quinolone-Resistant Clinical Isolates of Escherichia coli

ABSTRACT

To study the gyrA mutations of E. coli from clinical specimens, 410 strains were isolated from 1994 to 1997 in Kyungpook National Vniversity hospital. Antimicrobial susceptibility tests, PCR and sequencing of gyrA, and in vitro induction of quinolone resistance were done. The frequency of quinolone resistant E. coli strains increased constantly during 1994 through 1996. Quinolone-resistant strains were more often resistant to unrelated antibiotics than quinolone-susceptible strains (chi-square test, p Leu (TCG -> TIG) at codon 83. In addition, four different types of amino acid substitution affecting codon 87 (Asp) were detected, 1) type I Asn (GAC -> AAC); 2) type II Tyr (GAC -> TAC); 3) type III Oly (GAC -> GGC); 4) type IV His (GAC -> CAC). The mutation of type IV has not been reported previously in quinolone-resistant E. coli strains. It is thought that the specific amino acid substitution probably affects minimum inhibitory concentrations (MIC) of quinolones because the MICs of ciprofloxacin, norfloxacin, and ofloxacin in type II were significantly higher than those of type I. By in vitro induction, MICs to quinolone-susceptible strains resulted in the increase in the MICs of all quinolones tested by 2- to 2048-fold. The induced mutants by quinolones had amino acid substitutions at codon 83, SerLeu or Asp87Asn, Gly or Tyr. Alteration of Ser83 results in the most effective increase in the MIC of quinolone such as NAL and alterations of Asp87 result in the effective increase of MIC of fluoroquinolone. These results suggest that the continuous use of quinolones might induce the specific amino acid substitution at gyrA.