Objective  To analyze the in vitro antibacterial effects of colistin (COL) or tigecycline (TGC) in combination with imipenem (IPM), amikacin (AMK), and cefoperazone-sulbactam (CSL) against carbapenem-resistant Acinetobacter baumannii(CRAB), so as to provide evidence for screening effective combination regimens for clinical microbial infections.

Methods  Non-duplicate CRAB strains isolated from clinical specimens between June and December 2023 at Huai'an First People's Hospital Affiliated to Nanjing Medical University were retrospectively collected. The minimum inhibitory concentrations of commonly used antimicrobial agents against these strains were determined using the broth microdilution method. In vitro combination susceptibility testing was performed using the checkerboard dilution method, and the fractional inhibitory concentration index was calculated to evaluate the combined effects (synergistic, additive, indifferent, antagonistic).

Results  A total of 48 CRAB strains were included. COL-based combination regimens all exhibited favorable synergistic or additive effects against CRAB. Among them, the COL+IPM group showed the highest synergistic rate (83.3%), while the COL+AMK and COL+CSL groups primarily demonstrated additive effects (additive rates of 66.7% and 70.8%, respectively). TGC-based combination regimens mainly exhibited additive or indifferent effects, with the TGC+CSL group showing the highest additive rate (75.0%) and the TGC+AMK group showing the highest indifferent rate (70.8%). No antagonistic effects were observed in any of the six combination regimens. Further analysis revealed that the antibacterial effect of the COL+IPM group against CRAB was significantly different from that of the other combination regimens (P < 0.001). The synergistic rate of the COL+IPM group was significantly higher than that of the other five combination regimens (83.3% vs. 12.5%, P < 0.001) and that of the other COL-based regimens (83.3% vs. 31.3%, P < 0.001). The additive rate of the COL+IPM group was significantly lower than that of the other five combination regimens (16.7% vs. 58.7%, P < 0.001) and that of the other COL-based regimens (16.7% vs. 68.7%, P < 0.001). Additionally, no indifferent effects were observed in any of the COL-based combination regimens.

Conclusions  The combination of COL and IPM demonstrates the optimal synergistic antibacterial effect against CRAB, with the highest synergistic rate, and can be used as an empirical combination regimen for the clinical treatment of CRAB infections.