TABLE 2.

Susceptibilities and β-lactamase activities of different strains

Strain	MIC (μg/ml)
	Ampicillin	Piperacillin-tazobactam	Cefoxitin	β-Lactamase activity (U/mg of protein)
K. pneumoniae 15571	16,384	1024/128	16	1,280
K. pneumoniae 44NR	256	8/1	16–32	6
E. coli DH10B(pCWR366)	32,000	512/64	4	12,800
E. coli DH10B(pCWR366) (A→C mutant)	2,048	128/16	NDa	762
E. coli DH10B	4	<8/0	4	None

^aND, not determined.