Assessment of Chromosomal DNA Fragmentation by Quinolones in an Isogenic Collection of Escherichia coli with Defined Resistance Mechanisms.

The aim of this study was to investigate the potential usefulness of DNA fragmentation as a quick and simple procedure for detecting resistance to fluoroquinolones (FQ) in isogenic Escherichia coli strains harboring defined and multiple quinolone resistance mechanisms, including low-level quinolone resistance (LLQR) phenotypes. DNA fragmentation assay (Micromax(®)) was evaluated for detecting resistance to FQ in 71 isogenic strains of E. coli harboring specific quinolone resistance mechanisms frequently found in clinical isolates. These isogenic strains represent a consistent and reliable model of increasing minimum inhibitory concentrations (MICs) of ciprofloxacin (CIP), ranging from 0.004 to 16 mg/L. According to CLSI criteria, the assay correctly identified all CIP-resistant strains (MIC ≥4 mg/L). As regards susceptible strains, 96% of bacterial strains were correctly assigned as susceptible to CIP. Moreover, the procedure enabled LLQR phenotypes to be efficiently identified; this subset may show different levels of DNA damage depending on the strain, even with similar MIC. Interestingly, despite increasing the dose according to the MIC, a lower response to quinolones occurs in strains with higher MIC values. This is a simple, rapid, and reliable test for evaluating susceptibility to FQ of E. coli, including the detection of strains harboring LLQR mechanisms.