Objective: Acinetobacter baumannii strains are widely distributed in the nature and can survive a long period on hard surfaces. Most of them have the ability to acquire resistance to different classes of antibiotic and such strains are defined as multi-drug resistant A. baumannii (MDRAB) which exhibit limited treatment options. MDRAB strains have been reported increasingly during the last decade. In this study, in vitro activities of 75 MDRAB strains isolated from various clinical samples in the last four years were evaluated against various antibiotics, including colistin, polymyxin B and tygecycline.

Materials and Methods: Identification of the strains was perfomed by automated systems. Antibiotic susceptibility tests were evaluated according to the Clinical and Laboratory Standards Institute (CLSI) criteria, except for tigecycline. Minimal Inhibitory Concentration (MIC) values for tigecycline were interpreted according to Food and Drug Administration (FDA) criteria.

Results: All of the strains were identified as MDRAB. According to antibiotic susceptibility testing, all strains were found to be resistant to at least three antibiotic classes and 95% of them were resistant to at least one of the carbapenems. All strains were susceptible to colistin and polimyxin B. Sixty percent of the strains were found to be resistant to ampicillin-sulbactam, amikacin, imipenem triplet. Three of all strains were susceptible to either imipenem or meropenem, four of them were susceptible to both of them and the rest (91%) were resistant to carbapenems. Forty six (61%) of the strains were susceptible to tigecyclin, 27 (36%) were moderately susceptible, and two (3%) were resistant. Among the carbapenem resistant strains 38% were found to be resistant or moderately susceptible to tigecycline. MIC₅₀ and MIC₉₀ values for colistin, polimyxin B and tigecycline were found to be 0.19-0.50μg/ml, 0.38-0.50μg/m, and 2-3μg/ml, respectively.

Conclusion: It was found that colistin and polimyxin B had better in vitro activity than tigecyclin in MDRAB strains. Emergence of resistance to tigecycline among MDRAB strains seems to be a major concern about the increasing resistance problem that might develop in the future.