Abstract
Emerging resistance to colistin in clinical Acinetobacter baumannii isolates is of growing concern. Since current treatment options for these strains are extremely limited, we investigated the in vitro activities of various antimicrobial combinations against colistin-resistant A. baumannii. Nine clinical isolates (8 from bacteremia cases and 1 from a pneumonia case) of colistin-resistant A. baumannii were collected in Asan Medical Center, Seoul, South Korea, between January 2010 and December 2012. To screen for potential synergistic effects, multiple combinations of two antimicrobials among 12 commercially available agents were tested using the multiple-combination bactericidal test (MCBT). Checkerboard tests were performed to validate these results. Among the 9 colistin-resistant strains, 6 were pandrug resistant and 3 were extensively drug resistant. With MCBT, the most effective combinations were colistin-rifampin and colistin-teicoplanin; both combinations showed synergistic effect against 8 of 9 strains. Colistin-aztreonam, colistin-meropenem, and colistin-vancomycin combinations showed synergy against seven strains. Colistin was the most common constituent of antimicrobial combinations that were active against colistin-resistant A. baumannii. Checkerboard tests were then conducted in colistin-based combinations. Notably, colistin-rifampin showed synergism against all nine strains (100%). Both colistin-vancomycin and colistin-teicoplanin showed either synergy or partial synergy. Colistin combined with another β-lactam agent (aztreonam, ceftazidime, or meropenem) showed a relatively moderate effect. Colistin combined with ampicillin-sulbactam, tigecycline, amikacin, azithromycin, or trimethoprim-sulfamethoxazole demonstrated limited synergism. Using MCBT and checkerboard tests, we found that only colistin-based combinations, particularly those with rifampin, glycopeptides, or β-lactams, may confer therapeutic benefits against colistin-resistant A. baumannii.
Highlights
Emerging resistance to colistin in clinical Acinetobacter baumannii isolates is of growing concern
The following 12 antimicrobial agents were selected based on previous studies suggesting their antimicrobial efficacy against MDR A. baumannii: colistin, ampicillin-sulbactam, amikacin, azithromycin, aztreonam, ceftazidime, meropenem, rifampin, tigecycline, trimethoprim-sulfamethoxazole, vancomycin, and teicoplanin [15,16,17,18,19,20,21,22,23,24,25,26,27]
Results of Multilocus sequence typing (MLST), carbapenemase types, and MICs of antimicrobials against each strain are summarized in Table 1 and in Table S1 in the supplemental material
Summary
Emerging resistance to colistin in clinical Acinetobacter baumannii isolates is of growing concern. Since current treatment options for these strains are extremely limited, we investigated the in vitro activities of various antimicrobial combinations against colistin-resistant A. baumannii. Using MCBT and checkerboard tests, we found that only colistin-based combinations, those with rifampin, glycopeptides, or -lactams, may confer therapeutic benefits against colistin-resistant A. baumannii. A few previous studies evaluated the in vitro synergism of antimicrobial combinations against colistin-resistant A. baumannii [9,10,11] In those studies, the number of antimicrobial agents tested did not exceed four, and only colistin-based combinations were tested. The aim of this study was to assess the in vitro efficacy of antimicrobial combinations, among 12 commercially available antimicrobial agents, against clinical isolates of colistin-resistant A. baumannii using the multiple-combination bactericidal test (MCBT) and checkerboard method
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
