Ascorbic Acid Enhances the Inhibitory Effect of Theasaponins against Candida albicans

Candida albicans (C. albicans) is a main cause of hospital-acquired fungal infections. Combination therapy is promising as a novel anti-C. albicans strategy because of its better efficacy. Theasaponins are pentacyclic triterpenes in the Camellia genus with multiple biological activities. Our previous studies prove that theasaponins display inhibitory activity against C. albicans. Ascorbic acid (VC) is a vitamin found in many plants that shows potential in combination therapy. However, whether VC enhances the activity of theasaponins remains unclear. In this study, the checkerboard micro-dilution method was used to assess the effect of VC (0-80 mmol/L) on the anti-C. albicans effect of theasaponins (0-1000 μg/mL). Then, the effects of theasaponins (31.25 μg/mL), VC (80 mmol/L), and theasaponins (31.25 μg/mL) + VC (80 mmol/L) on C. albicans planktonic cells and different stages of biofilm formation were assessed. Transcriptomic analysis was conducted to investigate the molecular mechanisms. According to the results, VC enhanced the anti-planktonic and anti-biofilm effect of theasaponins against C. albicans. The minimum inhibitory concentration of theasaponins was significantly decreased and the fungicidal efficiency was increased with the addition of VC. VC remarkably aggravated the suppression of theasaponins with regard to various virulence factors of C. albicans, including adhesion, early biofilm formation, mature biofilm, cell surface hydrophobicity, and phospholipase activity. Compared with the theasaponins or VC groups, the level of intracellular reactive oxygen species was higher, while the levels of mitochondrial membrane potential and adenosine triphosphate were lower in the combination group, suggesting more severe oxidative stress, mitochondrial injury, and energy deficiency. Transcriptomic analysis revealed that the combination predominantly suppressed the pathways of glycolysis, glycerophospholipid metabolism, glutathione metabolism, and cysteine and methionine metabolism. This implied that energy deficiency and redox imbalance were associated with the anti-C. albicans activity of the combination. These results prove that VC enhances the inhibitory effect of theasaponins against C. albicans and that the combination has the potential to be used as a topical antifungal therapy or disinfectant.