ASSOCIATION BETWEEN DOUBLE MUTATION IN GYRA GENE OF CIPROFLOXACIN-RESISTANT CLINICAL ISOLATES OF ESCHERICHIA-COLI AND MICS

The mutations in the quinolone resistance-determining region of the gyrA and gyrB genes from 27 clinical isolates of Escherichia coli with a range of MICs of ciprofloxacin from 0.007 to 128 mu g/ml and of nalidixic acid from 2 to >2,000 mu g/ml were determined by DNA sequencing. All 15 isolates with ciprofloxacin MICs of greater than or equal to 1 mu g/ml showed a change in Ser-83 to Leu of GyrA protein, whereas in clinical isolates with a MIC of greater than or equal to 8 mu g/ml (11 strains), a double change in Ser-83 and Asp-87 was found. All isolates with a MIC of nalidixic acid of greater than or equal to 128 mu g/ml showed a mutation at amino acid codon Ser-83. Only 1 of the 27 clinical isolates of E. coli analyzed showed a change in Lys-447 of the B subunit of DNA gyrase. A change in Ser-83 is sufficient to generate a high level of resistance to nalidixic acid, whereas a second mutation at Asp-87 in the A subunit of DNA gyrase may play a complementary role in developing the strain's high levels of ciprofloxacin resistance.