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Background: Carbapenem-resistant A. baumannii (CRAB) bacterium is difficult to treat with available antimicrobial agents leading to use a few antibiotics such as colistin as an option for treatment. However, the use of the colistin has serious side effects and leads to developing bacterial resistance. Objective: This study aimed to determine effectiveness of phage/colistin combination to inhibit biofilm formation of CRAB. Methods: Sixty clinical A. baummanii isolates were identified with the VITEKII system and 16S rRNA gene. The antibiotic susceptibility and detection of Oxacillinases genes were tested for all isolates. Results: The antibiotic sensitivity of A. baumannii isolates showed a high resistance percentage to Ceftriaxone (CRO) with 92% (55 isolates), and Cefotiam (CTF) with 87% (52 isolates), while the lowest percent related to Colistin (CO) with 17% (10 isolates). The results of antibiotic resistance and Oxacillinases genes reported that only 14 isolates from the current study were CRAB. Phage and host A. baumannii of phage were isolated and characterized previously. Biofilm production assay of CRAB isolates were showed that among 14 isolates including the phage host: 57% (8) was weak and 43% (6) was moderate biofilm producer. Therefore, the synergistic effect of a combination ΦKAB phage in MOI (10) and Colistin with MIC=8 µg/ml against CRAB isolates was evaluated and showed complete transfer of isolates by 100 % to weak biofilm producer compared with CRAB isolates. Conclusions: Among 60 isolates of A. baumannii, most isolates were MDR 44 (73.3%) and only 14 (23.3%) isolates were CRAB that were 57% weak and 43% moderate biofilm producers. The mixture of Colistin/ΦKAB phage could inhibit and reduce biofilm forming in CRAB isolates. Overall, the present study may provide evidence on the capability of the isolated phage to serve as a novel strategy to treat infections caused by MDR A. baumannii. [ABSTRACT FROM AUTHOR]