HPLC-DAD quantification of usnic acid and atranorin and evaluation of antibacterial and antifungal activities in eight peruvian lichens

Abstract Background The study of lichen species has accelerated significantly due to their promising potential as sources of novel bioactive compounds. Although usnic acid (UA) and atranorin (ATR) are well-documented as major metabolites in several lichen species, the biological activities exhibited by most studied species appear to derive from a synergistic effect among these compounds and other secondary metabolites. This work evaluates the antimicrobial and antioxidant activities of eight lichen extracts obtained with a MeOH: (CH₃)₂CO (1:1) mixture, providing a comprehensive overview of their biological profile and metabolite content (UA and ATR). Methods HPLC-DAD was used to quantify UA and ATR in the studied extracts. Antimicrobial activity against Staphylococcus aureus , Escherichia coli and Candida spp. was evaluated using disk diffusion and broth microdilution assays. Different Antioxidant capacities were observed through DPPH radical scavenging (1364–2461 µg/mL), ABTS cation decolorization (11.5–15.9 mg TE/g) and ferric reducing antioxidant power (FRAP) assays (169-719.79 µM Fe²⁺/g), while total phenolic content (TPC) was determined using the Folin–Ciocalteu method (11.34–332.15 µg GAE/g). Results Eight lichen species collected from San Cristóbal de Huascoy (Huaral Province) were evaluated. In all species under investigation, UA and ATR concentrations ranged from 0.024 to 1.2% and 0.018–0.46%, respectively. The lichen extracts exhibited significant antibacterial action against four Staphylococcus aureus strains (MIC = 0.1–0.35 mg/mL), showing a potency comparable to Gentamicin (0.1 mg/mL) but lower than standard UA (0.02 mg/mL). Furthermore, all extracts demonstrated to have a moderate antifungal activity against the evaluated Candida strains (MIC = 0.4–1.0 mg/mL). Given that both MBC/MIC and MFC/MIC ratios were below 4, the extracts were classified as bactericidal and fungicidal, respectively. Conclusion Among all lichens tested, the Alectoria ochroleuca extract exhibited the second-highest total phenolic content (TPC) and moderate antioxidant performance. Overall, all tested lichen extracts displayed moderate antimicrobial and antioxidant profiles across all assays. The mixture of soluble metabolites in the MeOH: (CH₃)₂CO (1:1) extract, including UA and ATR, may exerts a synergistic effect driving the observed biological activities. Future studies isolating individual metabolites and evaluating their in vivo bioactivity are needed to confirm the pharmacological relevance of these lichen extracts.