TABLE 1.

Dose effect of DK4 β-lactamase on resistance of bacteria to typical β-lactams

E. coli strain	β-Lactamase activity (U/mg of bacterial protein)	MIC (μg/ml)a
		AMP	CET	CXM	CFX	ATM	IMP
AS226-51 (host cell)	<10−5	0.8	0.8	1.6	1.6	<0.2	<0.2
AS226-51/RDK4	0.098	25	100	100	200	<0.2	1.6
AS226-51/RDK4-1	0.14	100	400	400	800	<0.2	3.1
AS226-51/RDK4-2	0.62	200	400	800	800	<0.2	6.3
AS226-51/RDK4-3	0.67	200	400	800	1,600	<0.2	12.5
AS226-51/RDK4-4	0.90	200	400	800	1,600	<0.2	12.5
AS226-51/RDK4-5	1.30	200	400	800	1,600	<0.2	12.5

^aThe following β-lactams were used: ampicillin (AMP), cephalothin (CET), cefuroxime (CXM), cefoxitin (CFX), aztreonam (ATM), and imipenen (IMP).