Antibiotic susceptibility profiles of Escherichia coli strains lacking multidrug efflux pump genes

The contribution of seven known and nine predicted genes or operons associa ted with multidrug resistance to the susceptibility of Escherichia coli W31 10 was assessed for 20 different classes of antimicrobial compounds that in clude antibiotics, antiseptics, detergents, and dyes. Strains were construc ted with deletions for genes in the major facilitator superfamily, the resi stance nodulation-cell division family, the small multidrug resistance fami ly, the ATP-binding cassette family, and outer membrane factors. The agar d ilution MICs of 35 compounds were determined for strains with deletions for multidrug resistance (MDR) pumps. Deletions in acrAB or tolC resulted in i ncreased susceptibilities to the majority of compounds tested, The remainin g MDR pump gene deletions resulted in increased susceptibilities to far few er compounds. The results identify which MDR pumps contribute to intrinsic resistance under the conditions tested and supply practical information use ful for designing sensitive assay strains for cell-based screening of antib acterial compounds.