TABLE 5.

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

Inoculum (CFU/ml) (n) and antibiotic	MICd (μg/ml)			% Susceptiblebc
	Range	50%	90%
105 (23)
Meropenem	0.03–4	0.03	0.06	100
Cefoteten	0.12–>64	4	>64	79
Cefotaxime	0.12–256	8	64	54
Ceftazidime	1–1,024	32	512	29
Ceftriaxone	0.5–256	8	128	50
Cefepime	0.06–32	2	8	96
Aztreonam	0.25–1,024	16	256	38
Pip-Tazo	0.5–256	4	128	75
107 (23)
Meropenem	0.06–>64	1	16	79 (17/23)
Cefoteten	0.25–>64	32	>64	25 (11/19)
Cefotaxime	1–>1,024	>1,024	>1,024	4 (22/23)
Ceftazidime	16–>1,024	>1,024	>1,024	0 (17/20)
Ceftriaxone	4–>1,024	>1,024	>1,024	4 (21/22)
Cefepime	0.25–>128	>128	>128	8 (22/23)
Aztreonam	8–>1,024	512	>1,024	4 (10/21)
Pip-Tazo	4–>1,024	64	>1,024	42 (13/22)

^aThe strains comprised C. freundii (n = 2), E. cloacae (n = 4), E. aerogenes (n = 10), M. morganii (n = 1), and Serratia marcescens (n = 6). All produced a TEM- or SHV-derived ESBL. Three isolates produced an additional β-lactamase. An E. cloacae and an S. marcescens strain each also produced TEM-1, and one S. marcescens strain also produced TEM-2. 

^bSusceptibility 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.) 

^cFor 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.) 

^d50 and 90%, MICs at which 50 and 90% of isolates are inhibited, respectively.