Effect of In Vitro Testing Parameters on Ceftazidime-Avibactam Minimum Inhibitory Concentrations

Table 3

Meropenem MICs with variable testing parameters.

Organism	Isolate #	Genotype	Meropenem MIC (µg/mL)
			Standard	5 × 10^4 CFU/mL	5 × 10^6 CFU/mL	pH 5	pH 6	pH 8	CAMHB + 2.5% LHB	HTM	Trace calcium	50 mg/L calcium	CO2
E. coli	ATCC 25922	WT	0.016	0.016	0.125	0.125	0.06	0.03	0.03	0.03	0.016	0.03	0.03
E. coli	CML2255	CTX-M-15, TEM-1, OXA-1	≤0.06	≤0.06	0.25	0.125	≤0.06	≤0.06	≤0.06	≤0.06	≤0.06	≤0.06	≤0.06
E. coli	CML1468	KPC	4	1	>8	1	4	4	4	8	2	4	4
K. pneumoniae	ATCC 700603	SHV-18	0.03	0.016	0.06	0.06	0.03	0.06	0.25	0.03	0.016	0.016	0.03
K. pneumoniae	CML138	CTX-M-15	≤0.06	≤0.06	2	0.125	0.125	≤0.06	≤0.06	≤0.06	≤0.06	≤0.06	≤0.06
K. pneumoniae	CML128	KPC-2, SHV-27, TEM-1	>64	32	>64	64	>64	>64	64	>64	32	64	64
K. pneumoniae	CML148	ΔKPC-2, SHV-27, TEM-1	0.25	0.25	4	0.25	0.25	0.5	0.25	0.25	0.125	0.25	0.125
K. pneumoniae	CML2280	CTX-M-15, TEM-1, OXA-1	0.06	0.03	0.5	0.125	0.06	0.06	0.06	0.06	0.06	0.125	0.06
K. oxytoca	CML2251	TEM-129	≤0.06	≤0.06	4	0.125	≤0.06	≤0.06	2	≤0.06	0.25	0.5	≤0.06
P. aeruginosa	ATCC 27853	WT	0.5	0.5	0.5	0.5	0.25	0.5	0.5	0.5	0.5	0.25	0.25
P. aeruginosa	CML1039	AmpC	64	32	64	8	32	64	32	32	32	32	32
P. aeruginosa	CML1040	AmpC, TEM-24	4	4	8	4	4	8	4	4	4	8	8
P. aeruginosa	CML1047	AmpC	16	16	16	4	4	32	16	16	8	16	8

MICs obtained under standard CLSI conditions are indicated by the “standard” column header. Parameters that were varied include inoculum density (5 × 10⁴ CFU/mL and 5 × 10⁶ CFU/mL), pH of medium (pH 5, 6, and 8), medium supplements (2.5% laked horse blood (LHB) and HTM), cation content (trace amounts of calcium and 50 mg/L calcium), and incubation with 5% CO₂. Bolded values indicate a change in MIC of ≥4-fold.