A multiple-antibiotic resistance-independent active chloramphenicol efflux in an Escherichia coli clinical isolate.

The clinical isolate, Escherichia coli 1941, exhibits high resistance to chloramphenicol and tetracycline (minimum inhibitory concentrations of 512 micrograms/ml). Neither resistance is linked to the large conjugative plasmid present in the strain. The intracellular accumulation of radiolabeled chloramphenicol increased about 9-fold after the addition of the energy uncoupler carbonyl cyanide m-chlorophenol-hydrazone to an E. coli 1941 culture, indicating the presence of an active efflux mechanism. Sequence analysis and expression study suggested that the multiple-antibiotic resistance marRAB locus and the AcrAB drug-efflux pump were not involved in this active efflux of chloramphenicol.