Evaluation of the in vitro interaction of doripenem and amikacin against Acinetobacter baumannii, Pseudomonas aeruginosa, and\nKlebsiella pneumoniae was done by classifying them into four groups: doripenem and amikacin sensitive (DOR-S/AMK-S),\ndoripenem sensitive and amikacin resistant (DOR-S/AMK-R), doripenem resistant and amikacin sensitive (DOR-R/AMK-S), and\nboth doripenem and amikacin resistant (DOR-R/AMK-R). The MIC of each antibiotic and their combination was obtained using\nthe Etest method. The fractional inhibitory concentration index was calculated to classify the results as synergistic, additive,\nindifferent, or antagonistic interaction. In the DOR-S/AMK-S class, 1 isolate of A. baumannii showed synergy and the other 5\nshowed additive results, 5 isolates of P. aeruginosa showed additive and 1 isolate showed indifferent result, and 2 isolates of\nK. pneumoniae showed additive and the other 4 showed indifferent results. In the DOR-S/AMK-R class, 3 isolates of A. baumannii\nshowed additive and the other 3 showed indifferent results, 2 isolates of P. aeruginosa showed indifferent results, and 1 isolate of\nK. pneumoniae showed additive and the other 5 showed indifferent results. In the DOR-R/AMK-S class, 1 isolate of A. baumannii\nshowed additive and the other 5 showed indifferent results, 1 isolate of P. aeruginosa showed additive and the other 5 showed\nindifferent results, and 4 isolates of K. pneumoniae showed additive and the other 2 showed indifferent results. In the DORR/\nAMK-R class, 6 isolates of A. baumannii showed indifferent results, 1 isolate of P. aeruginosa showed additive and the other 5\nshowed indifferent results, and 1 isolate of K. pneumoniae showed additive and the other 5 showed indifferent results. Synergy\noccurred in only 1 (1.5%) isolate. Additive interaction occurred in 24 (35.3%) isolates, and indifferent interaction occurred in 43\n(63.2%) isolates. Doripenem sensitive combined with amikacin sensitive reduced MIC significantly in all bacterial isolates when\ncompared to single MIC of each antibiotic.
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