Quercetin and carvacrol act synergistically to inhibit Candida albicans biofilms in vitro via membrane disruption and oxidative stress.

Candida albicans biofilms are a major cause of device-associated infections and treatment failure due to high antifungal tolerance. Here, we evaluated the synergistic antibiofilm activity of quercetin and carvacrol against a catheter-derived C. albicans isolate (CRL7) and delineated the underlying cellular and structure-based mechanisms. Combination therapy markedly enhanced antifungal potency, reducing MICs from 200 μg/mL (carvacrol) and 240 μg/mL (quercetin) to 17 μg/mL and 10.9 μg/mL, respectively (FICI = 0.13), indicating strong synergy. The quercetin-carvacrol combination reduced biofilm biomass by ∼82% at ½ MIC (vs. 51-69% for monotherapy) and decreased metabolic activity by ∼68% at ½ MIC (vs. 40-42%). Mechanistically, the combination caused profound membrane destabilization, evidenced by increased nucleic acid/protein leakage and a pronounced reduction in DPH membrane fluorescence, accompanied by extensive disruption of biofilm architecture on SEM. The combination also triggered oxidative stress, increasing intracellular ROS by ∼64.4% (vs. ∼56.5% for carvacrol and ∼51.5% for quercetin alone) and inducing apoptosis-like cell death with robust metacaspase activation supported by nuclear condensation signatures. Consistently, qPCR analysis demonstrated downregulation of key biofilm and virulence determinants, including adhesion and hyphal-associated genes (ALS1/HWP1/ECE1/LIP3) and oxidative-stress regulators (CAP1/SOD1), indicating suppression of biofilm-associated transcriptional programs. To complement experimental findings, structure-based computational analysis (molecular docking and normal mode analysis) predicted stable binding of quercetin and carvacrol to virulence-linked targets (ALS3, HWP1, CAP1, SOD1), with quercetin showing denser hydrogen-bond/π-interaction networks and higher complex rigidity signatures relative to carvacrol. Collectively, these results support a dual mechanism in which quercetin and carvacrol synergistically dismantle catheter-derived C. albicans biofilms through membrane disruption, ROS-mediated apoptosis-like cell death, virulence gene suppression, and structure-based interference with adhesion and redox-defense pathways, supporting the quercetin-carvacrol combination as a candidate warranting further preclinical evaluation. Findings are preliminary and limited to in vitro assays; in vivo efficacy and safety remain to be established.