In-vitro effects of various antimicrobial combinations against multidrug- resistant Acinetobacter baumannii strains

In recent years, Acinetobacter strains have emerged as one of the most important nosocomial pathogens, especially in patients admitted to an intensive care unit (ICU). The progressively increasing antibiotic resistance against A. baumannii is now a major problem in our country as it is throughout the world. This resistance against A.baumannii has increased and led clinicians to find alternative antibiotics or antibiotic combinations. In the present study, it is aimed to evaluate the interaction between colistinrifampicin, colistin-imipenem, tigecycline-rifampicin and tigecycline-imipenem antibiotic combinations using microdilution checkerboard and E-test methods against ten multidrug resistant A. baumannii strains. Since A. baumannii strains have become frequently observed as an infection factor and since antimicrobial resistance rates have increased, there should be newly developed drugs for better treatments. In this study, 50 A. baumannii strains were isolated from various clinical specimens between June 2005 and September 2009 in Celal Bayar University, Faculty of Medicine, Department of Microbiology and Clinical Microbiology, and in Bacteriology Laboratory. Isolation and identification procedures were performed by using conventional biochemical tests as well as by BBL Crystal GN; N/F ID (Becton Dickinson, USA) or Phoenix 100 BD systems (Becton Dickinson, USA). The antibiotic susceptibilities of strains were investigated by using the disk diffusion method according to the recommendations of the Clinical and Laboratory Standards Institute. According to our in vitro study results, the checkerboard method, which was used to examine the synergy between colistin-rifampicin and colistin-imipenem, showed 80% synergistic activity. Tigecycline-imipenem combination had lowest synergetic (10%) efficiency and highest antagonistic effect (30%). The consistency between checkerboard and E-test methods was 52,5% (range 10-70%). Further comparison studies of the E-test synergy technique with the checkerboard and time-kill methods are warranted.