Synergistic Activities of Colistin Combinations with Meropenem, Sulbactam, Minocycline, Disodium Fosfomycin, or Vancomycin Against Different Clones of Carbapenem-Resistant Acinetobacter baumannii Strains

Aims: Colistin became the primary treatment option for Acinetobacters that had developed a high rate of resistance to carbapenems which were the first-line therapy in the past, and now Acinetobacters become resistant to nearly all antibiotics. Because of the resistance potential to colistin and the concerns about toxicity, especially for high doses, colistin combination therapies are preferred nowadays. In this study, we aimed to investigate whether combinations of colistin with meropenem, sulbactam, fosfomycin, vancomycin, and minocycline are synergic or not and to determine minocycline susceptibility rate, which is not in use in our country. Results: For the studied 23 Acinetobacter strains, the highest synergy was between colistin and vancomycin, which was shown in 4 (17.4%) strains. The synergy of colistin with meropenem and fosfomycin was detected for 1 (4.3%) strain, the synergy of colistin with minocycline was detected for 2 (8.6%) strains, and no synergy was detected for colistin-sulbactam combination. All the strains were susceptible to minocycline. Conclusion: None of the antibiotic combinations was antagonistic. They had synergistic and additive interactions. Thus, these combinations can be used in clinical practices. The remarkable synergistic interaction of colistin-vancomycin combination and high susceptibility to minocycline highlight the need for more researches on these subjects.