Table 2.

Influence of efflux gene expression on susceptibility of P. aeruginosa to β-lactamase inhibitors

StrainEfflux system expressedaMIC (μg/ml)
CloxacillinClavulanateBRL42715
ML5087MexAB-OprM5,120b256>1,024
K11101,024128256
K11211,02464512
K1112MexAB-OprM++c>10,240b512–1,024>1,024
K1113646464
K1114MexAB-OprM>1,024256>1,024
K1111MexCD-OprJ/MexAB-OprM>1,024256>1,024
K1131MexCD-OprJ1,02464512
K11151,02464256
K1117MexEF-OprN>1,024256>1,024
K11161,02464512
K1118MexEF-OprN>1,024128–256>1,024
K767MexAB-OprM5,120b512>1,024
K11192,560b128>1,024
OCR1MexAB-OprM++>10,240b512>1,024
  • a Those efflux systems that are known to be expressed and, thus, that contribute to drug resistance in the indicated strains are indicated.

  • b These values were all >1,024 μg/ml in the initial assays. In order to assess differences between thenalB strains OCR1 and K1112 and their parent strains K767 and ML5087, respectively, a second assay was performed with higher concentrations of cloxacillin.

  • c The two plus signs denote high-level expression of MexAB-OprM.