The team was interested in two particular lichen secondary metabolites: usnic acid, a yellow pigment famous among natural products chemists, and atranorin, a depside compound common in many lichen thalli. Both have documented biological activities, but the researchers set out with a more nuanced question. If individual compounds can kill bacteria on their own, does the full chemical cocktail found in a crude lichen extract do the job even better? The answer from their experiments appears to be a cautious yes, with evidence suggesting that mixtures of metabolites may act synergistically rather than as isolated single agents.

To get a reliable picture of what was inside each extract, the scientists used high-performance liquid chromatography coupled with a diode array detector, an analytical technique that separates complex mixtures and identifies compounds based on how they absorb ultraviolet light. After extracting the lichen material with a one-to-one mixture of methanol and acetone, they quantified usnic acid and atranorin across all eight species. The concentrations ranged from a modest 0.024 percent up to 1.2 percent for usnic acid, and from 0.018 to 0.46 percent for atranorin. These numbers matter because they allow direct comparison between the chemical composition of each species and its measured biological effect, transforming what could have been a simple screening exercise into a structured structure-activity investigation.

The antimicrobial results are where the study becomes genuinely striking. The researchers tested their extracts against four strains of Staphylococcus aureus, the notorious bacterium responsible for everything from skin infections to life-threatening sepsis and a World Health Organization priority pathogen. Using broth microdilution assays to determine the minimum inhibitory concentration, they found that all eight lichen extracts suppressed bacterial growth at concentrations between 0.1 and 0.35 milligrams per milliliter. That places the crude extracts in the same potency range as gentamicin, a standard clinical antibiotic tested at 0.1 milligrams per milliliter in the same assays. Pure usnic acid was even stronger at 0.02 milligrams per milliliter, but the fact that unfractionated extracts approached clinical-antibiotic territory is remarkable for unmodified natural material.

Crucially, the team went beyond simply asking whether the extracts stopped growth. They determined the minimum bactericidal concentration for each extract, the dose at which bacteria are actually killed rather than merely inhibited. When the ratio of the bactericidal concentration to the inhibitory concentration falls below four, microbiologists classify a substance as genuinely bactericidal. Every extract in the study met that criterion against the staphylococcal strains, meaning these lichen compounds do not just hold bacteria in check but destroy them. The same logic applied to the antifungal experiments, where minimum fungicidal concentration to minimum inhibitory concentration ratios also fell below four against the tested Candida strains.

That antifungal activity, while real, was more modest. Extracts inhibited Candida growth at concentrations between 0.4 and 1.0 milligrams per milliliter, a range the authors characterize as moderate. Candida species are among the most common fungal pathogens of humans, and rising resistance to existing antifungal drugs has made the search for new chemical scaffolds increasingly urgent. While a lichen extract would need considerable development before it could inform any clinical antifungal, the finding that these same extracts carry fungicidal activity alongside potent antibacterial effects makes them chemically richer candidates for follow-up work than many single-purpose natural products.

The study also examined antioxidant capacity, another dimension of lichen chemistry that has attracted attention for its possible health relevance. Using three complementary assays, the researchers measured the ability of the extracts to neutralize synthetic radicals and reduce iron. In the DPPH radical scavenging assay, effective concentrations fell between 1364 and 2461 micrograms per milliliter. The ABTS decolorization assay yielded values from 11.5 to 15.9 milligrams of Trolox equivalents per gram, while the ferric reducing antioxidant power assay produced readings between 169 and 719.79 micromoles of iron equivalents per gram. Total phenolic content, measured with the Folin-Ciocalteu method, ranged from 11.34 to 332.15 micrograms of gallic acid equivalents per gram, a wide spread that highlights just how chemically different these eight co-occurring species can be.

Among the species tested, one stood out in the antioxidant profiling: the extract of Alectoria ochroleuca showed the second-highest total phenolic content and a moderate antioxidant performance across the assays. Phenolic compounds are well-established antioxidants because their molecular structures can donate electrons or hydrogen atoms to neutralize reactive oxygen species, the unstable molecules implicated in cellular damage, aging, and numerous diseases. In lichens, which endure intense ultraviolet radiation and desiccation on exposed mountain surfaces, an arsenal of protective phenolics makes evolutionary sense. Atranorin and usnic acid themselves contribute to this chemical sunscreen and defense system, which is precisely why lichens have become such attractive targets for pharmacological screening.

The authors are careful about interpretation, and appropriately so. Because pure usnic acid outperformed the crude extracts against Staphylococcus aureus, and because the biological effects did not map neatly onto the concentrations of any single measured compound, the data point toward synergy among metabolites. In natural products research, synergy is both an opportunity and a complication. It can mean that combinations of molecules achieve effects no individual component could, potentially through multiple mechanisms of attack that also make resistance harder to develop. But it also means that isolating a single magic molecule may never fully reproduce the activity of the whole extract, complicating standardization and drug development. The study’s conclusion emphasizes that future work isolating individual metabolites and evaluating their bioactivity in vivo will be needed to confirm the pharmacological relevance of these extracts.

What makes this research resonate beyond the laboratory is its setting. Lichens are among the hardiest organisms on the planet, thriving on bare rock, tree bark, and harsh Andean slopes where almost nothing else grows, and their chemical inventories remain astonishingly underexplored. Studies like this one, combining rigorous chromatographic quantification with standardized susceptibility testing and antioxidant assays, provide exactly the kind of reproducible baseline data needed to turn folklore about healing lichens into testable pharmacology. With antibiotic resistance climbing worldwide and the pipeline of new drugs running thin, the humble organisms carpeting a Peruvian mountainside may hold molecular lessons that modern medicine has barely begun to read. The next chapter will depend on whether the synergistic activity seen in these crude extracts survives the journey from the extraction flask through animal studies and, eventually, toward the clinic.

Subject of Research: Antimicrobial and antioxidant activities of eight Peruvian lichen species analyzed by HPLC-DAD quantification of usnic acid and atranorin.

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

Article References: Carrasco, F., Hernández, W., Guerrero, M., Rojas, R., Tamariz-Angeles, C., Olivera-Gonzales, P., Briceño-Luna, V., Solis, E. C., Raposo, C., Rodilla, J. M., & Silva, L. (2026). HPLC-DAD quantification of usnic acid and atranorin and evaluation of antibacterial and antifungal activities in eight peruvian lichens. BMC Complementary Medicine and Therapies. https://doi.org/10.1186/s12906-026-05599-6

Image Credits: AI Generated

DOI: 10.1186/s12906-026-05599-6

Keywords: lichens, usnic acid, atranorin, HPLC-DAD, antimicrobial activity, antioxidant, Peru, Staphylococcus aureus, Candida, natural products, secondary metabolites, synergy