In vivo activity of vancomycin combined with colistin against multidrug-resistant strains of Acinetobacter baumannii in a Galleria mellonella model

Background: With increasing antibiotic resistance, the selection of effective treatment of A. baumannii infections is particularly challenging. Methods: This study assessed the activities of the combination of vancomycin and colistin combination in vitro and in vivo using a Galleria mellonella model against four colistin-susceptible or colistin-resistant A. baumannii strains. Results: In checkerboard assays, synergy was observed between vancomycin and colistin for all four strains tested (0.156 <= Fractional inhibitory concentration indices [FICI] <= 0.281). In time-kill assays, the combination showed continued bactericidal activity and synergy after 24 h for colistin-susceptible strains. For colistin-resistant strains, the combination resulted in bactericidal activity within 8 h, but sustained bacterial re-growth was then observed. Treatment of G. mellonella larvae infected with lethal doses of A. baumannii (except 19606R) resulted in significantly increased survival rates when vancomycin was given with colistin compared to colistin treatment alone (p<0.05). Conclusions: These findings suggest that regimens containing vancomycin may be useful for infections due to multidrug-resistant Acinetobacter baumannii.