Efficacy of enrofloxacin against transmissible resistance gene (qnrS and aac (6’)-Ib-cr) - containing Escherichia coli isolates and clones

Aim of this study was to investigate the effects of qnrS1 and aac (6’)-Ib-cr on susceptibility, some pharmacodynamics of enrofloxacin against molecularly constructed and natural isolates of E. coli from animals. The MPCs and mutation frequencies of the drug were determined using the agar dilution method. Time-kill assays were used to determine the antimicrobial activity of the drug against E. coli strains. MPCs of enrofloxacin for qnrS1- and aac(6′)-Ib-cr-containing transformants increased from 0.128 μg/ml to 2 and 4 μg/ml. MPC:MIC ratio of enrofloxacin for transformants and E. coli isolates were 4 and 8, respectively. Mutation frequencies were markedly higher for transformants and isolates compared to control strain. Mutation frequencies diminished up to 6.2x10-18 by increasing the concentrations used in the assay. The results of this study showed that enrofloxacin is less active against E. coli strains in case of additional mutations in QRDR. The concentration-dependent bactericidal effect of enrofloxacin is only observable by the eight times the MIC and increasing concentrations did not alter the bactericidal activity. The combination therapy can be considered to fight with resistant E. coli instead of monotherapy and artificially constructed E. coli strains, which exhibit similar reactions against ENR with natural E. coli isolates, can be used to test the pharmacological efficacy of the combinations