In vitro activity of azithromycin in combination with amikacin, ceftazidime, ciprofloxacin or imipenem against clinical isolates of Acinetobacter baumannii

The in vitro activity of the two-drug combinations of azithromycin with amikacin, ceftazidime, ciprofloxacin or imipenem against five clonally unrelated strains of Acinetobacter baumannii were evaluated. Synergy studies were performed by the checkerboard microtiter method. The fractional inhibitory concentration (FIC) index was calculated for each drug combination. None of the four combinations tested was antagonistic. The combination of azithromycin and ceftazidime was synergistic (FIC index less than or equal to0.5) for one strain and partially synergistic (FIC index 0.75) for another strain. An additive effect (FIC index = 1) was observed for the combinations of azithromycin with imipenem (two strains) or ceftazidime (one strain). The activities of the other combinations were indifferent (FIC index range from 1.5 to 2.5). It is concluded that azithromycin combined with ceftazidime has moderate synergistic activity against some multiresistant A. baumannii. Copyright (C) 2003 S. Karger AG, Basel.