TABLE 2.

MICs for the clinical isolate HS37, the corresponding DH5α transformant, and CTX-M-producing DH5α clones

Antibiotics	MIC (μg/ml)a						CLSI resistance breakpoint
	HS37	DH5α(pHS37)b	DH5α(pCTX-M-190)	DH5α(pCTX-M-55)	DH5α(pCTX-M-15)	DH5α
Ampicillin	>1,024	>1,024	256	>1,024	1,024	8	≥32
Ampicillin-sulbactam	64	32	64	8	8	2	≥32/16
Piperacillin	>1,024	512	512	512	512	2	≥128
Piperacillin-tazobactam	128	128	256	4	4	1	≥128/4
Amoxicillin	>1,024	>1,024	512	>1,024	>1,024	2	NAc
Amoxicillin-clavulanate	8	4	2	4	4	1	≥32/16
Ceftazidime	64	32	32	32	16	0.125	≥16
Ceftazidime-clavulanate	0.125	0.125	0.25	0.125	0.125	0.125	NA
Cefotaxime	128	128	128	256	256	0.125	≥4
Cefotaxime-clavulanate	0.25	0.125	0.125	0.125	0.125	≤0.06	NA
Aztreonam	128	8	4	32	16	≤0.06	≥16
Cefepime	64	32	32	4	2	≤0.06	≥16
Cefoxitin	16	2	2	2	2	2	≥32
Imipenem	0.125	≤0.06	≤0.06	≤0.06	≤0.06	≤0.06	≥4
Ertapenem	0.5	0.125	0.125	0.125	0.125	≤0.06	≥2

^aFor amoxicillin-clavulanate and ampicillin-sulbactam, the combinations were tested with concentrations at a 2:1 ratio (antibiotic: inhibitor). For piperacillin-tazobactam, ceftazidime-clavulanate, and cefotaxime-clavulanate, the inhibitors were tested at a fixed concentration of 4 μg/ml.

^bThe corresponding DH5α transformant of E. coli HS37, with plasmid pHS37 harboring bla_CTX-M-190.

^cNA, breakpoint criterion was not available in the CLSI interpretive standards.