Synergistic Effect between Usnic Acid and Polymyxin B against Resistant Clinical Isolates of Pseudomonas aeruginosa

Table 3

Evaluation of usnic acid and polymyxin B interaction by the Checkerboard method.

Clinical isolates	MIC (μg/mL)		MIC of the combination (μg/mL)	FICI	Interaction
	Polymyxin B	Usnic acid	Polymyxin B + usnic acid
PA05	4	500	0.25/15.625	0.094	Synergism
PA07	4	500	0.25/15.625	0.094	Synergism
PA18	8	500	0.25/31.25	0.094	Synergism
PA19	4	500	0.5/15.625	0.156	Synergism
PA46	32	250	0.25/7.8125	0.039	Synergism
PA56	8	250	0.25/31.25	0.094	Synergism
PA57	32	500	0.25/15.625	0.039	Synergism
PA78	32	500	0.25/15.625	0.039	Synergism
PA82	4	250	0.25/15.625	0.125	Synergism

MIC: minimum inhibitory concentration; FICI: fractional inhibitory concentration index.