Table 3.

Influence of MexAB-OprM on β-lactamase inhibitor enhancement of β-lactam activitya

β-Lactamβ-Lactamase inhibitorMIC (μg/ml) for the following strain:
K767 (wild type)K1119 (ΔmexAB-oprM)OCR1 (nalB)
AmpicillinNone1,024256–512 NDb
Cloxacillin2564ND
BRL4271564<2ND
Clavulanate1,024512ND
CephaloridineNone>10,240c2,560cND
Cloxacillin>1,024258ND
BRL42715324–8ND
Clavulanate>1,024>1,024ND
CarbenicillinNone641–2256
Cloxacillin641256
BRL42715640.125256
Clavulanate642256
PiperacillinNone40.2516
Cloxacillin40.2516
BRL4271580.12516
Clavulanate40.2516
  • a Determinations of the MICs of the various β-lactams were carried out in the presence of constant amounts (cloxacillin, 100 μg/ml; BRL42715, 20 μg/ml; and clavulanate, 10 μg/ml) of the indicated β-lactamase inhibitors.

  • b ND, not determined.

  • c These values were >1,024 μg/ml in the initial assays. In order to assess differences between these strains, a second assay was performed with higher concentrations of cephaloridine.