Antibacterial, anti-quorum sensing, and antibiofilm effects of ascorbic acid against multidrug-resistant Acinetobacter baumannii

Acinetobacter baumannii is a major cause of healthcare-associated infections and is increasingly associated with multidrug resistance. Because quorum sensing plays a central role in biofilm formation and bacterial virulence, targeting this system may provide an alternative strategy for infection control. This study evaluated the antibacterial, antibiofilm, and anti-quorum sensing activities of ascorbic acid against multidrug-resistant A. baumannii . Ten clinical isolates and two reference strains (ATCC 19606 and ATCC 17978) were confirmed by PCR. Antibacterial activity was determined by broth microdilution to determine the minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC). The interaction between ascorbic acid and cefepime was evaluated by the checkerboard assay. Biofilm inhibition was assessed using the crystal violet microtiter plate method, and the expression of the quorum sensing genes abaI and abaR was quantified by real-time PCR. All strains were resistant to ciprofloxacin, imipenem, and piperacillin/tazobactam. Ascorbic acid exhibited MICs of 1.367–10.938 mg/mL, with MIC 50 and MIC 90 values of 5.469 and 10.938 mg/mL, respectively. Identical MIC and MBC values indicated bactericidal activity. Combined treatment with cefepime showed synergistic or additive effects in 72.7% of strains. Ascorbic acid reduced biofilm formation by 54–65% at MIC and significantly downregulated abaI and abaR expression by 68.0% and 74.5%, respectively. Ascorbic acid demonstrated antibacterial, antibiofilm, and anti-quorum sensing activities against multidrug-resistant A. baumannii and enhanced the antibacterial activity of cefepime in vitro. These findings support further investigation of ascorbic acid as a potential adjunctive anti-virulence strategy.