Acinetobacter baumannii which is an opportunistic pathogen leading to nosocomial epidemics, exhibit high rates of antimicrobial resistance. Treatment of
Acinetobacter infections is a challenge since most of the isolates are multiple
antibiotic resistant. The aim of this study was to investigate minimum inhibitory concentrations (MICs) of
sulbactam,
imipenem,
meropenem, and
cefoperazone and in vitro synergistic activity of
sulbactam in combination with
imipenem,
meropenem and
cefoperazone against A.baumannii isolates of hospitalized patients. Forty A.baumannii strains isolated from various clinical specimens and found to be resistant to
carbapenems by disc diffusion method, were included in the study. The isolates were identified by conventional methods and VITEK 2 (bioMerieux, France) automated identification system. MICs of
sulbactam,
imipenem,
meropenem, and
cefoperazone were determined by the broth microdilution method according to the standards of CLSI and in vitro synergy test was performed using the checkerboard microdilution method. Synergistic, partial synergistic, additive, indifferent and antagonistic effects of
drug combinations were evaluated with the fractional inhibitory concentration index (FICI). Interpretation of the FICI was as follows: ≤ 0.5 synergy; > 0.5 to < 1 partial synergy; 1 additive; > 1 to < 4 indifference; and ≥ 4 antagonism. Forty A.baumannii isolates were resistant to
imipenem and
cefoperazone, but two were susceptible, seven were moderately susceptible and 31 were resistant to
meropenem with the microdilution method. MIC values of the isolates for
sulbactam were found to be 4 μg/ml in two, 8 μg/ml in five, 16 μg/ml in three, 32 μg/ml in 13, 64 μg/ml in three, 128 μg/ml in six and > 128 μg/ml in eight isolates. According to the FICI;
imipenem/
sulbactam combination exhibited synergy in 18 (45%), partial synergy in 4 (10%) and indifferent effect in 2 (5%) isolates, the combination of
meropenem and
sulbactam showed synergy in 19 (48%), partial synergy in 3 (7.5%), and indifferent effect in 3 (7.5%) isolates, the combination of
cefoperazone/
sulbactam demonstrated synergy in 18 (45%), partial synergy in 2 (5%), and indifferent effect in 2 (5%) isolates. There was no antagonistic effect with the tested combinations. In conclusion, MIC values of
sulbactam were generally high in
carbapenem-resistant A.baumannii strains. However, synergistic effect was detected in approximately half of the strains with the
sulbactam/
carbapenem combinations. The data obtained in this study should be supported by further advanced in vitro and clinical studies to predict the accurate clinical efficacy of
sulbactam containing combinations on A.baumannii
infections.