TABLE 3.

Ability of β-lactamase inhibitors to reduce MICs of study drugs ≥8-fold in tests with E. coli and K. pneumoniae

Druga	Inhibitorb	No. of strains with reduced MIC requirement in β-lactamase group (no. of strains tested)c:
		ESBL (36)	AmpC (10)	OBL (22)
CAZ	C1	30	0	1k
	C2	31	0	3 (2)l
	C4	33	0	3 (2)l
	S8	25	1 (1)df	0
CTX	C1	33	0	1m
	C2	33	0	1n
	C4	32	1 (1)g	0
	S8	28	4 (3)h	0
CTR	C1	35e	0	0
	C2	35e	0	0
	C4	32e	2 (1)i	0
	S8	28	3 (2)j	0
AZT	C1	35	0	1k
	C2	34	0	1k
	C4	34	0	0
	S8	28	1 (1)f	0
POD	C1	35	0	2o
	C2	35	0	2o
	C4	36	0	2p
	S8	29	0	0

^aSee footnote b of Table 2. 

^bC1, C2, and C4, 1, 2, and 4 μg of clavulanate per ml, respectively; S8, 8 μg of sulbactam per ml. 

^cSee footnote a of Table 2. 

^dThe number in parentheses indicates the number of strains for which the MIC of the test drug alone was ≥2 μg/ml, thus fulfilling current NCCLS screening criteria for ESBL production. These strains would therefore be false positives for ESBL production in the inhibitor-drug combination tests. 

^eThe ceftriaxone MIC for one strain of E. coli, which produced a low level of TEM-12, was ≤0.06 μg/ml. This value was too low to permit detection of an eightfold decrease in MIC when ceftriaxone was tested in combination with a β-lactamase inhibitor. 

^fE. coli (LAT-1). 

^gK. pneumoniae (ESBL plus AmpC). 

^hE. coli (LAT-1 [one strain]), K. pneumoniae (ESBL plus AmpC [one strain]), and E. coli (high-level chromosomal cephalosporinase [two strains]). 

ⁱE. coli (LAT-1 [one strain]) and E. coli (high-level chromosomal cephalosporinase [one strain]). 

^jE. coli (LAT-1 [one strain]), K. pneumoniae (ESBL plus AmpC [one strain]), and E. coli (high-level chromosomal cephalosporinase [one strain]). 

^kK. pneumoniae (PSE-1 plus SHV-1). 

^lK. pneumoniae (PSE-1 plus SHV-1 [one strain]) and K. pneumoniae (high SHV-1 [two strains]). 

^mE. coli (SHV-1). 

ⁿK. pneumoniae (high SHV-1). 

^oK. pneumoniae (high SHV-1 [two strains]). 

^pK. pneumoniae (high SHV-1 [one strain]) and E. coli (high TEM-1 [one strain]).