Table 5.

β-Lactamase expression phenotypes and MICs at which 90% of strains are inhibited for selected β-lactams for 16M. morganii clinical strains

PhenotypeIsolateβ-Lactamase activity (mU/mg of protein)aMIC90(μg/ml)b
BasalInducedcCeftazidimeCefotaximePiperacillin
High level, constitutive11,4301,3504432
25,03110,4872563264
38,7616,1138416
49601,99032864
Low level, inducible53410.060.120.25
6398<0.06<0.060.25
732290.06<0.060.25
83242<0.06<0.060.25
9450<0.06<0.060.25
10493<0.06<0.060.25
115160.060.060.12
1253,644<0.06<0.060.50
1310430<0.06<0.060.50
14355,532<0.06<0.060.50
15703,165<0.06<0.060.50
163652,774118
  • a One unit of β-lactamase is defined as 1 μmol of cephalothin hydrolyzed per min.

  • b MIC90, MIC at which 90% of isolates are inhibited.

  • c Imipenem (0.5 μg/ml) was used as the inducer.