Usnic Acid Activity on Oxidative and Nitrosative Stress of Azole-Resistant Candida albicans Biofilm.

Several studies report that (+)-usnic acid, a lichen secondary metabolite, inhibits growth of different bacteria and fungi; however, the mechanism of its antimicrobial activity remains unknown. In this study, we explored the ability of usnic acid, obtained from , as an antibiofilm agent against azole-resistant and azole-sensitive Candida albicans strains by studying the cellular stress and antioxidant response in biofilms. The biofilm inhibitory concentration of usnic acid (4 µg/mL) exhibited a significant biofilm inhibition, 71.08 % for azole-resistant and 87.84 % for azole-sensitive C. albicans strains. Confocal scanning laser microscopy showed that the morphology of mature biofilm was altered (reduced the biomass and thickness) in the presence of usnic acid. The antifungal effect was mediated by an oxidative and nitrosative stress, with a significant accumulation of intracellular and extracellular reactive oxygen species detected by confocal scanning laser microscopy and by nitro blue tetrazolium, respectively. In fact, azole-resistant and azole-sensitive C. albicans biofilms treated at the biofilm inhibitory concentration of usnic acid presented 30-fold and 10-fold increased reactive oxygen species measurements compared to basal levels, respectively, and important nitric oxide generation, showing 25-fold and 60-fold increased reactive nitrogen intermediates levels with respect to the controls, respectively. Nonenzymatic and enzymatic antioxidant defenses were increased in both strains compared to biofilm basal levels as response to the increase of oxidant metabolites. The present study shows for the first time that usnic acid can alter the prooxidant-antioxidant balance, which may be the cause of the irreversible cell damage and lead to cell death. Our results suggest that usnic acid could be an alternative for the treatment of infections, which deserves further investigation.