Background: Over the last decade Acinetobacter baumannii has emerged as one of the most problematic nosocomial pathogens.Successful extensively drug-resistant (XDR) clones have been disseminated worldwide.Tigecycline provided new hope for the treatment of MDR A.baumannii infections,but isolates showing reduced susceptibility have emerged in many countries,further limiting the therapeutic options.Empirical combination therapy has become a common practice to treat patients infected with XDR A.baumannii,although the limited microbiological and clinical evidence supporting its efficacy.Here,the in vitro interaction of tigecycline with Cefoperazone-Sulbactam has been assessed.Methods: Seven-two MDR A.baumannii isolates from hospital patients were tested.MICs were determined by the broth microdilution method.Antibiotic interactions were determined by chequerboard and time-kill assays.Only antibiotic combinations showing synergism or antagonism in both chequerboard and time-kill assays were accepted as authentic synergistic or antagonistic interactions,respectively.Results: Considering cefoperazone-Sulbactam in combination with tigecycline,chequerboard analysis showed 29.17% synergy,41.67% partial synergy,23.67% additive,5.56% indifference,and no antagonism.Time-kill assays confirmed the synergistic interaction between tigecycline and Cefoperazone-Sulbactam for 3 of 4 selected isolates.No antagonism was confirmed by time-kill assays.Conclusion: This study demonstrates the in vitro synergistic activity of tigecycline in combination with Cefoperazone-Sulbactam seven tigecycline intermediate-susceptible A.baumannii strains,opening the way to a more rationale clinical assessment of novel combination therapies to combat infections caused by XDR A.baumannii.