TABLE 5.

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

Phenotype	Isolate	β-Lactamase activity (mU/mg of protein)a		MIC90 (μg/ml)b
		Basal	Inducedc	Ceftazidime	Cefotaxime	Piperacillin
High level, constitutive	1	1,430	1,350	4	4	32
	2	5,031	10,487	256	32	64
	3	8,761	6,113	8	4	16
	4	960	1,990	32	8	64
Low level, inducible	5	3	41	0.06	0.12	0.25
	6	3	98	<0.06	<0.06	0.25
	7	3	229	0.06	<0.06	0.25
	8	3	242	<0.06	<0.06	0.25
	9	4	50	<0.06	<0.06	0.25
	10	4	93	<0.06	<0.06	0.25
	11	5	16	0.06	0.06	0.12
	12	5	3,644	<0.06	<0.06	0.50
	13	10	430	<0.06	<0.06	0.50
	14	35	5,532	<0.06	<0.06	0.50
	15	70	3,165	<0.06	<0.06	0.50
	16	365	2,774	1	1	8

^aOne unit of β-lactamase is defined as 1 μmol of cephalothin hydrolyzed per min. 

^bMIC₉₀, MIC at which 90% of isolates are inhibited. 

^cImipenem (0.5 μg/ml) was used as the inducer.