Phenotypic characterization of quinolone-resistant mutants of Enterobacteriaceae selected from wild type, gyrA type and multiplyresistant (marA) type strains

Abstract
The NCTC type strains of Escherichia coli, Enterobacter cloacae, Serratia marcescnes and Klebsiella pneumoniae were exposed to 3, 5 and 10 × MIC of nalidixic acid, enoxacin, ciprofloxacin, PD 117596 and PD 127391. From each strain a mutant with a high MIC of quinolones alone (gyrA) and a mutant with intermediate resistance to quinolones, some β-lactams, chloramphenicol and tetracycline (multiply resistant, m-r) were selected on agar containing antibiotics. The gyrA mutants required a higher concentration of quinolone to inhibit DNA synthesis by 50% but quinolone uptake kinetics and outer membrane profile were the same as the wild type. The m-r mutants had similar DNA synthtesis IC50, as the wild type, decreased quinolone uptake kinetics and had decreased expression of an OMP of ˜40 kD. The gyrA and m-r mutants were then exposed to 3, 5 and 10 × MIC of the same quinolones and new mutants (F2) sckcted. The F2 mutants from the gyrA mutants displayed a further increase in quinolone MIC; the F2 mutants from the m-r mutants had several phenotypes: high quinolone MICs with cross resistance to other agents, high quinolone resistance alone, or intermediate quinolone resistance alone. Most F2 mutants had MICs above the recommended breakpoint conantrations for quinolones. The F2 mutants often had altered biochemical profiles (API 20E). however, only in the case of E. cloacae did this affect speciation with the strains being identified as Rhanella aquatalis
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