Antibacterial, antibiofilm, and antioxidant impacts of usnic acid against vancomycin-resistant enterococcus

Diabetic foot ulcer caused by vancomycin-resistant Enterococcus strains is a troublesome concern that threatens public health. Due to the few treatment options, it is essential to develop a new alternative antibacterial agent with high efficacy. In the current study, we evaluated the antibacterial and antibiofilm properties of usnic acid (UA), a secondary metabolite of lichens. Microdilution broth and crystal violet methods were applied to determine the antibacterial and antibiofilm activities, respectively. To further elucidate the biofilm inhibition, scanning electron microscopy imaging was performed. In addition, the antioxidant activity of UA was investigated by 2, 2-diphenyl-1-picrylhydrazyl assay. The bacteriostatic and bactericidal activity of UA was observed as an amount of 23.02 and 312.5 μg/mL, respectively. UA suppressed biofilm formation and motivated the reduction of free oxygen radicals. The present study may open up a new perspective to combat biofilm-forming enterococci.