In vitro analysis of synergistic combination of polymyxin B with 12 other antibiotics against MDR Acinetobacter baumannii isolated from a Chinese tertiary hospital

In clinical practice, polymyxins are suggested to be used in combination with other antibiotics for improving their antibacterial efficacy and preventing the emergency of antibiotic-resistant strains. However, even though synergistic combination of polymyxin B with many antibiotics have been confirmed in various studies with different bacterial species and analyzing methods, which antibiotic is the best option for combination therapy of polymyxin B against MDR A. baumannii remains uncertain. In this study, we systematically analyzed the synergistic combination of polymyxin B with 12 other antibiotics against MDR A. baumannii isolated from a Chinese tertiary hospital using the checkerboard assay. The results suggest that, for polymyxin B-based combination therapy against MDR A. baumannii as characterized in this hospital, cefperazone-sulbactam may be the best partner, since it has the highest synergistic rate and the best synergistic effect with polymyxin B. Minocycline, imipenem, meropenem, ceftazidime, cefepime, amikacin and sulfamethoxazole also have some synergistic effects with polymyxin B, but piperacillin-tazobactam, ciprofloxacin, levofloxacin and tobramycin show no synergism. None of these 12 antibiotics has an antagonistic effect when combined with polymyxin B.