TABLE 2.

β-Lactamase specific activities from cultures of WT and ceftazidime-resistant E. cloacae NOR-1 strains

Substratea	Strainb	Basal β-lactamase activityc	Inducerd	Induced β-lactamase activityc	Fold increase in activity
					Induced/basal	Mutant/WT
Cephalothin	NOR-1	6	FOX	170	28
			IMP	110	18
	NOR-1D3	1,400	FOX	3,930	3	230
			IMP	2,950	2
	NOR-1D1→12	10,000 ± 1,200	FOX	9,900 ± 970	1	1,670
			IMP	10,500 ± 950	1
Imipenem	NOR-1	0.85	FOX	27	31
			IMP	11	13
	NOR-1D3	55	FOX	175	3	65
			IMP	170	3
	NOR-1D1→12	475 ± 40	FOX	480 ± 70	1	560
			IMP	465 ± 70	1

^aA substrate concentration of 100 μM was used. 

^bNOR-1, the WT E. cloacae NOR-1 strain; NOR-1D₃, E. cloacae NOR-1D mutant strain 3; NOR-1D_1→12, E. cloacae NOR-1D mutant strains 1 to 12 but not including strains 3 and 7. 

^cSpecific activity (in micromoles per minute per milligram of protein). The β-lactamase activities are geometric mean determinations for three independent cultures. The standard deviations were within 10% for each individual strain. However, the standard deviations are shown whenever the values were those of cultures of several different strains. 

^dFOX, cefoxitin (50 μg/ml); IMP, imipenem (10 μg/ml).