Synergistic antimicrobial and antibiofilm effects of plant-active ingredients and antibiotics on multidrug-resistant Acinetobacter baumannii.

Aims: The increasing antimicrobial resistance, particularly in Acinetobacter baumannii, complicates the treatment of infections, leading to higher morbidity, mortality, and economic costs. Herein, we aimed to determine the in vitro antimicrobial, synergistic, and antibiofilm activities of colistin (COL), meropenem, and ciprofloxacin antibiotics, and curcumin, punicalagin, geraniol (GER), and linalool (LIN) plant-active ingredients alone and in combination against 31 multidrug-resistant (MDR) A. baumannii clinical isolates.

Methods and results: The combinations were tested to identify alternative approaches to conventional antibiotic therapy by reducing the Minimum Inhibitory Concentration (MIC) values of antibiotics when used with plant-active ingredients. Synergistic interactions were evaluated by checkerboard assay and interpreted via the Fractional Inhibitory Concentration Index (FICI). Antibiofilm activity was assessed using crystal violet microtiter plate method. Notably, COL with GER (83.87%) and with LIN (77.42%) exhibited strong synergistic interactions, with FICI values between 0.12 and 0.5. Where synergism was observed, antibiotic MICs were reduced by 2- to 128-fold, indicating substantial enhancement in bacterial efficacy. Synergistic and additive interactions were more prevalent than indifference, and no antagonism was detected. Biofilm formation inhibition assays further demonstrated that these combinations significantly suppressed biofilm production in A. baumannii isolates. Biofilm eradication rates were consistently highest at 2 × MIC concentrations for all tested antibiotics and plant-active ingredients. Cytotoxicity tests on L929-fibroblast cell lines confirmed the safety at the tested concentrations.

Conclusion: Combining conventional antibiotics with plant-active ingredients offers a novel approach to mitigating the impact of MDR A. baumannii.