Contributions of the AmpC beta-lactamase and the AcrAB multidrug efflux system in intrinsic resistance of Escherichia coli K-12 to beta-lactams.

The roles of the AmpC chromosomal beta-lactamase and the AcrAB efflux system in levels of intrinsic resistance and susceptibility of Escherichia coli to beta-lactams were studied with a set of isogenic strains. MICs of ureidopenicillins, carbenicillin, oxacillin, and cloxacillin were drastically reduced by the inactivation of AcrAB, whereas those of the earlier cephalosporins were affected mostly by the loss of AmpC beta-lactamase.