TABLE 3.

Cross-resistance, increased β-lactamase activity, and expression of efflux pumps in resistant coloniesa

Antibiotic used for selectionb and strainMIC (μg/ml)cFold changed in:
β-Lactamase activityeEfflux pump expression
CAZCIPTOBCHLTETBasalInducedmexFmexDmexBmexY
None, PAO120.191.5324
CAZ
    PAO1200.1251.75NDND4.41.07NDNDNDND
    mutT mutant180.172NDND5.60.78NDNDNDND
    mutY mutant260.132NDND4.90.88NDNDNDND
    mutM mutant180.162NDND5.40.87NDNDNDND
TOB
    PAO10.50.32628 (+)10NDND1.181.881.292.4
    mutT mutant0.690.163.7586 (+)5.5NDND1.040.860.890.76
    mutY mutant0.630.113.556 (+)18NDND2.051.570.942
    mutM mutant0.380.080.920 (+)4.25NDNDNDNDNDND
CIP
    PAO10.440.751.06>256>256NDND1.25618.735.51.77
    mutT mutant0.470.751.37>256>256NDND0.97607.771.071.04
    mutY mutant0.440.881.19>256>256NDND0.75437.720.590.89
    mutM mutant0.440.751.25>256>256NDND11.3342.160.735.7
  • a From the population analysis of the GO gene (mutT, mutY, and mutM) mutants isolated after selection on ceftazidime, tobramycin, or ciprofloxacin.

  • b CAZ, ceftazidime; TOB, tobramycin; CIP, ciprofloxacin.

  • c Each MIC is the mean for four colonies (two successive determinations per colony), except for PAO1 after selection on tobramycin, where the MIC is the mean for two colonies. CHL, chloramphenicol; TET, tetracycline; ND, not detected. Plus signs in parentheses indicate that the colonies isolated from LB agar plates containing tobramycin showed a small subpopulation resistant to chloramphenicol.

  • d Relative to the respective nonresistant strain.

  • e The induced β-lactamase activity of PAO1 was 26.5-fold higher than its basal β-lactamase activity.