Natural Product Usnic Acid as an Antibacterial Therapeutic Agent: Current Achievements and Further Prospects

Antimicrobial resistance (AMR) poses a significant global public health threat, endangering both human and animal health. In clinical environments, AMR often undermines the effectiveness of antibacterial treatments, underscoring the urgent need to discover and develop new antibacterial agents or alternatives to antibiotics. Usnic acid, a secondary metabolite derived from lichens, has emerged as a promising candidate owing to its diverse pharmacological properties, which include antibacterial, immune-regulating, antiaging, and anti-inflammatory activities. Extensive research has shown that usnic acid exhibits strong direct antibacterial effects against Gram-positive bacteria and acts as an antimicrobial adjuvant to enhance the therapeutic efficacy of antibiotic drugs against Gram-negative pathogens. Its mechanisms of action are multifaceted, encompassing the inhibition of RNA, DNA, and protein synthesis; suppression of bacterial efflux pump protein expression and membrane-localized drug-resistant enzyme activity; disruption of cell membrane integrity and metabolic homeostasis; and reduction of virulence factor production and biofilm formation. Despite its potential, the clinical application of usnic acid as an antibacterial agent faces significant challenges including poor aqueous solubility, low bioavailability, and dose-dependent toxicity. To overcome these limitations, nanodelivery systems such as liposomes and polymeric nanoparticles have been developed to enhance solubility, improve targeted delivery, and reduce toxicity, thereby expanding its therapeutic potential. Structural modification can also enhance the antibacterial activity and address solubility issues. This review systematically consolidates current knowledge on usnic acid's antibacterial properties, molecular mechanisms, and combinatorial therapies. It critically evaluates advancements in nanoformulation strategies, assesses safety and toxicity profiles, and identifies obstacles to its development as a clinically viable antibacterial agent. By addressing these aspects, this review aims to provide actionable insights, foster interdisciplinary dialogue, and catalyze further innovation in leveraging this natural product to combat AMR.