In vitro activities of colistin combined with imipenem, tigecycline or cefoperazone-sulbactam against multidrug-resistant Acinetobacter baumannii blood-stream isolates

Acinetobacter baumannii has emerged as one of the most important nosocomial patho- gens and multi-drug resistant (MDR) isolates are of great concern worldwide. The aim of the present study was to investigate the in vitro synergistic activity of colistin in combi- nation with other antibiotics against MDR A. baumannii blood stream isolates. A total of 54 non-duplicate, MDR A. baumannii isolates from blood culture specimens obtained be- tween June 2011 and July 2012 were included in the study. In vitro synergistic activity of colistin in combination with imipenem, tigecycline or cefoperazone-sulbactam against study isolates was investigated by Etest superimposing method and the fractional in- hibitory concentration (FIC) index was calculated for each antibiotic combination. The most frequent synergistic effect of colistin was found in combination with tigecycline in only 7 isolates (13.0%). All three antibiotics were found to have synergistic effect with colistin in four isolates (7.4%). Of isolates, 46 (85.2%) showed additive effect of colistin in combination with cefoperazone-sulbactam or tigecycline, 45 (83.3%) with imipenem. We found synergistic activity of colistin with other antibiotics in only a small number of isolates. Although Etest method is a practical method to investigate the synergistic activity, in case of choosing empirical treatment, colistin in combination with another antibiotic may be preferred.