Table 2.

Outcomes of various antibiotic combinations using the Checkerboard assay in XDR and PDR A. baumannii isolates (n = 8).

Isolate	Fos MIC (mg/L)		Fold Decline	MEM MIC (mg/L)		Fold Decline	FICI (x̄)
	Alone	Combined with MEM		Alone	Combined with FOS
Ab1	128	32	4	16	4	4	0.50 (S)
Ab2	64	1	≥64	32	16	2	0.52 (PS)
Ab3	64	16	4	32	8	4	0.50 (S)
Ab4	64	16	4	32	8	4	0.50 (S)
Ab5	64	16	4	32	8	4	0.50 (S)
Ab6	128	32	4	32	8	4	0.50 (S)
Ab7	32	8	4	32	8	4	0.50 (S)
Ab8	64	16	4	64	16	4	0.50 (S)
	FOS MIC (mg/L)			AK MIC (mg/L)
	alone	Combined with AK		alone	Combined with FOS
Ab1	128	128	0	>1024	≤8	≥256	1.00 (IN)
Ab2	128	128	0	>1024	≤8	≥256	1.00 (IN)
Ab3	128	128	0	>1024	≤8	≥256	1.00 (IN)
Ab4	64	64	0	1024	≤4	≥256	1.00 (IN)
Ab5	64	64	0	1024	≤4	≥256	1.00 (IN)
Ab6	128	128	0	1024	≤4	≥256	1.00 (IN)
Ab7	32	32	0	>1024	≤4	≥512	1.00 (IN)
	MEM MIC (mg/L)			CL MIC (mg/L)
	alone	Combined with COL		alone	Combined with MEM
Ab 1	16	0.5	32	8	≤1	≥8	0.16 (S)
Ab 2	32	4	8	≤1	0.25	4	0.37 (S)
Ab 3	32	8	4	1	0.25	4	0.50 (S)
	FOS MIC (mg/L)			TGC MIC (mg/L)
	alone	Combined with TGC		alone	Combined with FOS
Ab 1	128	≤2	≥64	2	1	2	0.52
Ab 2	128	≤4	≥32	2	1	2	0.53
Ab 3	128	≤4	≥32	4	2	2	0.53
Ab 4	64 64	≤1 32	≥64 2	4 4	2 0.25	2 16	0.52 0.56
Ab 5	64 64	≤1 32	≥64 2	2 2	1 ≤0.06	2 ≥32	0.52 0.53
Ab 6	128 128	≤1 64	≥128 2	4 4	2 ≤0.06	2 ≥64	0.51 0.52
Ab 7	32	≤1	≥32	4	2	2	0.53

Abbreviation: Ab = A. baumannii, FICI = fractional inhibition concentration index, x̄ = mean value, S = synergy, PS = partial synergy, MIC = minimal inhibition concentration, x̄ = mean value. FOS, fosfomycin; IMI, imipenem; MEM, meropenem; CL, colistin; AK, amikacin; TGC, tigecycline; synergy (SY), partial synergy (PS), indifference (IN) and antagonism (AN).