Association between early inhibition of DNA synthesis and the MICs and MBCs of carboxyquinolone antimicrobial agents for wild-type and mutant [gyrA nfxB(ompF) acrA] Escherichia coli K-12.

Quinolone antimicrobial agents are known to interact with DNA gyrase, but the mechanism by which bacterial cell death occurs is not fully understood. In order to determine whether there is a correlation between quinolone-induced inhibition of early (i.e., 10 to 15 min) DNA synthesis and potency (MICs and MBCs), we measured the rate of DNA synthesis in log-phase Escherichia coli K-12 by using [3H]thymidine incorporation. Three quinolones (ciprofloxacin, norfloxacin, and difloxacin) were selected based on their decreasing activity against reference strain KL16. All three quinolones caused an early 50% inhibition of DNA synthesis which was proportional to MICs and MBCs (r greater than 0.99). Furthermore, 50% inhibition of DNA synthesis and MICs were nearly identical for mutant strains with an altered quinolone target (gyrA) or with decreased [nfxB(ompF)] or increased (acrA) permeability. There were significant differences (P less than 0.001) between individual quinolones in the degree of DNA synthesis inhibition in nalidixic acid-resistant gyrA and nfxB(ompF) mutant strains. The comparison of the three mutants with the wild-type strain permitted an in vivo examination of the effects of alterations of the drug target or entry on the activity determined by DNA synthesis inhibition and MICs.