Reduced cell permeability and target penicillin-binding protein modification were investigated as mechanisms of intrinsic resistance in strains of resistant to carbenicillin (MIC ≫ 128 mg/L) independently of β-lactamase production. The carbenicillin-resistant strains were also remarkably resistant to other β-lactams, quinolones, tetracyline and chloramphenicol, whereas carbenicillin-hypersusceptible strains (MIC ≪ 2 mg/L) were very sensitive to these antimicrobial compounds. These observations suggested a non-specific mechanism of resistance involving reduced permeability of the outer layers of the bacterial cell. However, carbenicillin-resistant and carbenicillin-sensitive strains had identical porin levels and the target penicillin-binding proteins of carbenicillin-resistant (MIC 256-2048 mg/L), carbenicillin-sensitive (MIC 64 mg/L) and carbenicillin-hypersusceptible (MIC 0.015 mg/L) strains were equally sensitive to β-lactams. Thus, subtle changes in porin function or additional outer-membrane barriers regulating permeability may be involved in intrinsic resistance.


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