Novel Microdilution Method to Assess Double and Triple Antibiotic Combination Therapy In Vitro

Table 4

Triple combinations of the antibiotics against the clinical isolates.

(a)

Isolate number	Antibiotics (µg/mL)
	CIP + AMK + CAZ				CIP + AMK + PIP				CIP + AMK + IMP				CIP + CAZ + PIP				CIP + CAZ + IMP
	MIC	MBC	IC	IT	MIC	MBC	IC	IT	MIC	MBC	IC	IT	MIC	MBC	IC	IT	MIC	MBC	IC	IT
PA2	0.031	0.031	2	S	0.062	0.125	1	I	0.031	0.031	2	S	0.031	0.125	2	S	0.062	0.125	1	I
PA3	0.031	0.031	2	S	0.015	0.031	3	S	0.125	0.125	1	I	0.031	0.031	2	I^1	0.062	0.125	1	I
PA9	0.125	0.25	3	S	0.062	0.25	3	S	0.125	0.25	2	S	0.125	0.25	2	I^1	0.25	1	1	I
PA11	2	4	2	S	2	16	2	S	8	16	1	I	8	16	1	I	8	>16	2	I^1
PA14	0.062	1	5	S	0.50	8	2	S	0.50	8	2	S	0.50	4	3	I^1	1	8	1	I
PA15	0.031	0.031	2	S	0.015	0.125	3	S	0.125	0.125	0	I	0.125	0.125	0	I	0.015	0.062	3	S
PA18	0.0078	0.062	2	S	0.0078	0.062	1	I	0.031	0.062	0	I	0.007	0.015	2	S	0.015	0.062	1	I
PA19	0.015	0.031	2	S	0.015	0.031	2	S	0.062	0.125	0	I	0.015	0.031	3	S	0.031	0.125	1	I
PA20	1	8	2	S	2	16	1	I	1	16	2	S	>16	>16	0	I	16	>16	1	I
PA21	0.015	0.062	3	S	0.031	0.25	2	S	0.125	0.25	0	I	0.031	0.062	2	S	0.062	0.25	1	I

(b)

Isolate number	Antibiotics (µg/mL)
	CIP + PIP + IMP				AMK + CAZ + PIP				AMK + CAZ + IMP				CAZ + PIP + IMP
	MIC	MBC	IC	IT	MIC	MBC	IC	IT	MIC	MBC	IC	IT	MIC	MBC	IC	IT
PA2	0.062	0.125	1	I	0.125	1	3	S	0.5	4	1	I	0.125	4	3	S
PA3	0.031	0.031	2	I^1	0.5	4	2	S	2	8	0	I	2	8	2	I^1
PA9	0.125	0.25	2	I^1	0.50	1	3	S	0.5	4	3	S	2	8	1	I
PA11	>16	>16	0	I	4	16	1	I	4	16	1	I	32	32	1	I
PA14	2	8	0	I	0.50	8	2	S	1	16	1	I	0.25	4	4	S
PA15	0.062	0.125	1	I	0.50	4	3	S	0.5	4	3	S	2	16	1	I
PA18	0.007	0.015	3	S	0.25	1	3	S	0.25	1	3	S	0.50	0.50	2	I^1
PA19	0.031	0.125	1	I	0.5	2	2	S	0.50	2	2	S	1	2	1	I
PA20	16	>16	1	I	2	8	1	I	2	8	1	I	32	32	1	I
PA21	0.125	0.25	0	I	0.125	1	4	S	0.25	1	3	I^1	2	4	1	I

(i) The IC (Interaction Code) is the change in the MIC of the most potent antibiotic in double or triple combination compared with the antibiotic alone.
(ii) The IC is assigned zero value if the MIC is not changed in combination, while a value of 1 or −1 is given if the MIC value decreases or increases by one dilution, respectively, in combination.
(iii) The IT (interaction type) is indifferent (I) when the IC value lies between −1 and <2, synergistic (S) if the IC value is ≥2, and antagonistic (A) if the IC value is ≤−2.
¹The interaction is indifferent because the IC of the triple combination is not better than the ones of the double combinations with respect to the most potent antibiotic.