In vitro activity of colistin in combination with various antimicrobials against Acinetobacter baumannii species, a report from South Iran Raziyeh Kheshti,1 Bahman Pourabbas,2 Maryam Mosayebi,3 Afsaneh Vazin1 1Department of Clinical Pharmacy, School of Pharmacy, Shiraz University of Medical Sciences, Shiraz, Iran; 2Professor Alborzi Clinical Microbiology Research Center, Shiraz University of Medical Sciences, Shiraz, Iran; 3Student Research Committee, Shiraz University of Medical Sciences, Shiraz, Iran Introduction: Acinetobacter baumannii is a gram-negative,opportunistic pathogen responsible for resistant nosocomial infections especially in the intensive care units (ICUS).One reason for the failure in the treatment of A. baumannii is its ability of develop resistance against several antimicrobials. combination of different antimicrobials can be used to overcome such a resistance. This study was done to evaluate the in vitro synergistic activity of colistin in combination with six different antimicrobials, including ciprofloxacin, levofloxacin, imipenem, meropenem, ampicillin-sulbactam, and rifampin against A. baumannii species isolated from blood culture of patients admitted to ICUs of Nemazee hospital, Shiraz, Iran. Method: After performing biochemical identification assays on 20 isolates of A. baumannii, minimum inhibitory concentrations were determined by E- test method and antibiotic interactions were assessed using broth microdilution checkerboard method. Results: Combinations of colistin with all six studied antimicrobials had some synergistic effect. Conclusion: clinical studies are required to clarify the therapeutic potential of these antimicrobial combinations Keywords: Acinetobacter baumannii, colistin, synergism, anti-infective agents, intensive care unit, drug resistance