In vitro synergistic activity of tigecycline and colistin against XDR-Acinetobacter baumannii

The emergence of extensive drug-resistant (XDR) Acinetobacter baumannii limits the therapeutic options and leads to high mortality in intensive care units. Combined antibiotic therapy is frequently recommended for the treatment of these infections. Colistin (CO) and tigecycline (TIG), alone or in combination with other antimicrobials, are the most commonly used antibiotics in the treatment of these resistant infections. In this study, the in vitro synergistic activity of TIG and CO were tested for 25 XDR-A. baumannii strains isolated from ventilator-associated pneumonia by the Etest method. Resistance to CO was not detected, whereas 8% of the strains were resistant to TIG. The TIG-CO combination was more synergistic than TIG-rifampin and CO-rifampin according to the fractional inhibitory concentration index. No antagonism was detected between the drugs in the study. There was no strong correlation between the activity of the combinations with reference to strains or genotypes. Our results suggest that the combined use of TIG and CO may be useful for the treatment of XDR-A. baumannii infections.