Table 5.

MIC and susceptibility data: species producing a chromosomal AmpC β-lactamasea

Inoculum (CFU/ml) (n) and antibioticMICd (μg/ml)% Susceptibleb,c
Range50%90%
105(23)
 Meropenem0.03–40.030.06100
 Cefoteten0.12–>644>6479
 Cefotaxime0.12–25686454
 Ceftazidime1–1,0243251229
 Ceftriaxone0.5–256812850
 Cefepime0.06–322896
 Aztreonam0.25–1,0241625638
 Pip-Tazo0.5–256412875
107(23)
 Meropenem0.06–>6411679 (17/23)
 Cefoteten0.25–>6432>6425 (11/19)
 Cefotaxime1–>1,024>1,024>1,0244 (22/23)
 Ceftazidime16–>1,024>1,024>1,0240 (17/20)
 Ceftriaxone4–>1,024>1,024>1,0244 (21/22)
 Cefepime0.25–>128>128>1288 (22/23)
 Aztreonam8–>1,024512>1,0244 (10/21)
 Pip-Tazo4–>1,02464>1,02442 (13/22)
  • a The strains comprised C. freundii (n = 2) , E. cloacae(n = 4) , E. aerogenes (n = 10) , M. morganii (n = 1) , andSerratia marcescens (n = 6) . All produced a TEM- or SHV-derived ESBL. Three isolates produced an additional β-lactamase. An E. cloacae and an S. marcescensstrain each also produced TEM-1, and one S. marcescensstrain also produced TEM-2.

  • b Susceptibility based on the percentage of strains inhibited at the NCCLS susceptible breakpoint concentration of each agent. (The breakpoints are only validated for the tests with the 105 CFU/ml inoculum.)

  • c For the inoculum of 107 CFU/ml, in addition to percent susceptibility, the following is shown parenthetically: number of strains showing inoculum effect/number of strains evaluable. (Not all tests were evaluable. For some MICs out of the test range it was impossible to determine if there was an eightfold increase in MIC.)

  • d 50 and 90%, MICs at which 50 and 90% of isolates are inhibited, respectively.