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Background:Acinetobacter baumannii has emerged as an important pathogen causing a variety of infections.Using data from the China Surveillance of Antimicrobial Resistance Program conducted biennially,we investigated the secular changes in the resistance of 2917 isolates ofA.baumannii from 2004 to 2014 to differ antimicrobial agents.Methods:Pathogen samples were collected from 17 to 20 hospitals located in the east,central,and west regions of China.Minimum inhibitory concentrations (MICs) were determined by a 2-fold agar dilution method,and antimicrobial susceptibility was established using the 2014 Clinical Laboratory Standards Institute-approved breakpoints.Isolates not susceptible to all the tested aminoglycosides,fluoroquinolones,β-lactams,β-lactam/β-lactam inhibitors and carbapenems were defined as extensively drug resistant.Results:The rates of nonsusceptibility to common antimicrobial agents remained high (＞65％) over the years with some fluctuations to certain agents.The prevalence of imipenem-resistant A.baumannii (IRAB) increased from 13.3％ in 2004 to 70.5％ in 2014 and that of extensively drug-resistant A.baumannii (XDRAB) increased from 11.1％ in 2004 to 60.4％ in 2014.The activity of tigecycline was stable with MIC9o ≤4 mg/L against A.baumannii from 2009 to 2014.Susceptibility to colistin remained high (97.0％) from 2009 to 2014.The prevalence of XDRAB increased in all the three surveillance regions over the years and was significantly higher in Intensive Care Unit (ICU) wards than non-ICU wards.Conclusions:This longitudinal multicenter surveillance program revealed the nationwide emergence of A.baumannii in China and showed a significant increase in prevalence from 2004 to 2014.High levels of bacterial resistance were detected among samples collected from clinical settings in China,with IRAB and XDRAB being especially prevalent.This study will help to guide empirical therapy and identify at-risk groups requiring more intense interventional infection control measures,while also helping to focus surveillance efforts.