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Double mutations in the quinolone resistance determining region of the gyrase A gene (gyrA) have recently been reported to be associated with high-level resistance to fluoroquinolones in clinical isolates of Escherichia coli. We examined the type and frequency of such mutations in a large number of clinical isolates that were obtained from ten different geographical locations and had been genotypically characterized by pulsed field gel electrophoresis (PFGE) of chromosomal DNA digests. Of 36 isolates with ofloxacin MICs > or = 4 mg/L that represented at least 24 distinct genotypes, 35 had double mutations at amino acid codons 83 and 87 of gyrA, while two isolates with ofloxacin MICs of 0.5 and 4 mg/L, respectively, each had a single mutation at codon 83. Mutations at codon Ser-83 were uniform, resulting in substitution by Leu. The additional mutations at amino acid codon 87 in the 35 double-mutants were diverse, resulting in Asp-87 substitutions by residues Asn (23 isolates), Gly (7 isolates), Tyr (4 isolates), or His (1 isolate) without a discernable correlation with fluoroquinolone MICs or with phenotypic resistance to chemically unrelated antibacterial agents. Maximal differences between MICs of double-mutants with the same amino acid substitution were eight-fold. The changes of amino acid residues at codon Asp-87 differed between individual patient isolates with the same genotype (and similar MICs), suggesting that the amino acid codon 87 mutations (and possibly the development of high-level fluoroquinolone resistance) might have occurred after the transmission and sharing of a precursor strain carrying the Ser-83-->Leu mutation.

Previous reports have suggested an increasing incidence of highly fluoroquinolone-resistant Escherichia coli causing bacteremia among cancer patients on prophylactic therapy. We used genotyping by pulsed-field gel electrophoresis of chromosomal DNA digests and random amplified polymorphic DNA fingerprinting to study clonal relationships among such isolates obtained at 10 cancer centers located across Europe and the Middle East. Analysis by both methods indicated that isolates from different centers were genotypically unrelated to each other. There were five centers from which more than one individual patient isolate was available, and most demonstrated significant within-center genetic diversity of strains. Strains shared among patients could be identified at two centers. At the center with the largest number of bloodstream isolates from cancer patients available, fluoroquinolone-resistant control isolates from surgical patients and fluoroquinolone-susceptible control isolates from patients admitted to medical services during the same time period were unrelated to resistant cancer patient isolates and to each other as well. A substantial number of fluoroquinolone-resistant isolates (19 of 58) were nontypeable by pulsed-field gel electrophoresis. Fluoroquinolone resistance was commonly associated with multiple antibiotic resistance to chemically unrelated antibacterial agents irrespective of the origin of the isolates.