Inhibition of R-plasmid transfer in Escherichia coli by 4-quinolones

Inhibition of transfer of four conjugative R plasmids by ciprofloxacin, enoxacin, norfloxacin, ofloxacin, and pipemidic acid was investigated in an Escherichia coli mating system. The absolute concentrations needed for inhibition of conjugation varied from 0.12 microgram/ml for ciprofloxacin to 16 micrograms/ml for pipemidic acid, but the relationship to the MICs for the parent strains was identical for all substrates. Concentrations for a 90% reduction of transconjugants were in the range of one to six times the MIC for the parent strains, which also had lethal effects on donors and recipients. A similar effect on conjugation was found with chloramphenicol. These observations question the specificity of transfer inhibition by quinolones and cast doubt on the clinical importance of such an effect.