In Vitro Antibacterial Interaction of Doripenem and Amikacin against Multidrug-Resistant , , and Isolates.

Evaluation of the in vitro interaction of doripenem and amikacin against , , and was done by classifying them into four groups: doripenem and amikacin sensitive (DOR-S/AMK-S), doripenem sensitive and amikacin resistant (DOR-S/AMK-R), doripenem resistant and amikacin sensitive (DOR-R/AMK-S), and both doripenem and amikacin resistant (DOR-R/AMK-R). The MIC of each antibiotic and their combination was obtained using the Etest method. The fractional inhibitory concentration index was calculated to classify the results as synergistic, additive, indifferent, or antagonistic interaction. In the DOR-S/AMK-S class, 1 isolate of showed synergy and the other 5 showed additive results, 5 isolates of showed additive and 1 isolate showed indifferent result, and 2 isolates of showed additive and the other 4 showed indifferent results. In the DOR-S/AMK-R class, 3 isolates of showed additive and the other 3 showed indifferent results, 2 isolates of showed indifferent results, and 1 isolate of showed additive and the other 5 showed indifferent results. In the DOR-R/AMK-S class, 1 isolate of showed additive and the other 5 showed indifferent results, 1 isolate of showed additive and the other 5 showed indifferent results, and 4 isolates of showed additive and the other 2 showed indifferent results. In the DOR-R/AMK-R class, 6 isolates of showed indifferent results, 1 isolate of showed additive and the other 5 showed indifferent results, and 1 isolate of showed additive and the other 5 showed indifferent results. Synergy occurred in only 1 (1.5%) isolate. Additive interaction occurred in 24 (35.3%) isolates, and indifferent interaction occurred in 43 (63.2%) isolates. Doripenem sensitive combined with amikacin sensitive reduced MIC significantly in all bacterial isolates when compared to single MIC of each antibiotic.