Table 3.

β-Lactamase activity

HostPlasmidStatusbβ-Lactamase activity (U/mg of protein)c
Not inducedInduceda
ML4947 (AmpD wild type)pKU403Wild type0.04 ± 0.011.94 ± 0.84
pKU404H-mut18.86 ± 3.1025.94 ± 5.53
pKU405L-mut2.99 ± 1.098.26 ± 0.68
pKU406I-mut6.34 ± 1.0125.95 ± 4.01
pKU407I-mut7.10 ± 0.6030.95 ± 7.66
pKU408pKU403 ΔampR 0.14 ± 0.020.12 ± 0.01
pKU409pKU404 ΔampR 0.13 ± 0.010.12 ± 0.01
pKU410pKU405 ΔampR 0.13 ± 0.010.10 ± 0.01
pKU414pKU406 ΔampR 0.13 ± 0.010.12 ± 0.01
pKU411pKU403 ΔampC 0.010.01
pKU412pKU404 ΔampC 0.010.01
pKU413pKU405 ΔampC 0.010.01
pKU415pKU406 ΔampC 0.010.01
pMW218Vector0.010.01
0.010.01
ML4953 (AmpD mutant)pKU403Wild type4.89 ± 1.2837.68 ± 9.53
pKU404H-mut43.77 ± 5.8841.54 ± 13.82
pKU405L-mut27.72 ± 5.1148.48 ± 7.52
pKU406I-mut35.06 ± 4.4641.39 ± 5.21
pKU407I-mut31.99 ± 2.9738.28 ± 9.74
pKU408pKU403 ΔampR 0.11 ± 0.000.09 ± 0.01
pKU409pKU404 ΔampR 0.09 ± 0.010.06 ± 0.01
pKU410pKU405 ΔampR 0.11 ± 0.020.08 ± 0.01
pKU414pKU406 ΔampR 0.11 ± 0.010.07 ± 0.01
pKU411pKU403 ΔampC 0.010.01
pKU412pKU404 ΔampC 0.020.02
pKU413pKU405 ΔampC 0.020.02
pKU415pKU406 ΔampC 0.010.01
pMW218Vector0.010.01
0.020.01
  • a One to 4× the MIC of imipenem was used for induction of β-lactamase.

  • b L-, I-, and H-mut, mutant plasmids with low, intermediate, and high levels of β-lactamase activity, respectively.

  • c Values are means ± standard deviations of three independent experiments. Standard deviations for pKU411, pKU412, pKU413, pKU415, pMW218, and the host were <±0.001. One unit of β-lactamase activity was defined as the amount of β-lactamase that hydrolyzed 1 μmol of cephalothin in 1 min at 30°C.