The ongoing crisis of antimicrobial resistance demands novel combinations between antimicrobials and nonantimicrobials to manage infections caused by highly resistant pathogens. This study aimed to evaluate the effect of combining sodium ascorbate and/or apo-transferrin with imipenem, forming double and triple combinations, against 20 multiple-carbapenemase-producing Acinetobacter baumannii strains using the checkerboard test, time-kill assay, and disc diffusion test. The results of the checkerboard assay revealed that all double combinations showed indifference, while only triple combination recorded a synergistic effect (fractional inhibitory concentration index [FICI] < 0.8) in 95% the test isolates. Moreover, the MIC of imipenem (MICimp) was strongly reduced (up to 128-fold reduction) after treatment with the triple combination against highly resistant isolates and reached the susceptible range. The time-kill assay revealed that the triple combination led to a 4-log10 reduction in the CFU at 8 h compared with the initial bacterial count, and no viable count was recorded at 10 h. The mouse pneumonia model showed restoration of lung function and structure, with mild to moderate residual inflammation and moderately congested vessels observed 8 h following administration of the triple rescue therapy. Additionally, normal lungs with normal patent alveoli were detected 72 h following treatment. Accordingly, sodium ascorbate and apo-transferrin are promising adjunct biological agents with the potential to restore the effectiveness of critically essential antibiotics like imipenem, commonly used for the treatment of A. baumannii infections. IMPORTANCE Combination therapy provides a perspective to threat multidrug-resistant (MDR) strains. The present study sheds light on a novel and effective triple combination against carbapenem-resistant A. baumannii. Our in vitro results showed that combining imipenem with apo-transferrin and sodium ascorbate yielded synergism in 95% of test isolates, and this was associated with a marked reduction in imipenem MIC, shifting it below the breakpoint. Furthermore, a bactericidal effect was recorded, with no viable count detected at 10 h. An in vivo murine model of pneumonia was induced to mimic human disease. The triple combination therapy restored lung function and structure, with mild to moderate residual inflammation and moderately congested vessels observed 8 h following the initiation of therapy. Therefore, our findings suggest novel insights about a promising new combination therapy against highly resistant carbapenemase-producing A. baumannii to restore the effectiveness of imipenem.
Keywords: A. baumannii; carbapenemases; pneumonia; synergism; triple combination.