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 pathogens 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 combination 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 between 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 inhibitory 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.