The effect of lichen secondary metabolites on Aspergillus fungi

A spectrophotometric analysis of extracted water-soluble phenolic metabolites of lichens

MAIN CONCLUSION

Rainwater most probably constitutes a relatively effective solvent for lichen substances in nature which have the potential to provide for human and environmental needs in the future. The aims were (i) to test the hypothesis on the potential solubility of lichen phenolic compounds using rainwater under conditions that partly reflect the natural environment and (ii) to propose new and effective methods for the water extraction of lichen substances. The results of spectrophotometric analyses of total phenolic metabolites in rainwater-based extracts from epigeic and epiphytic lichens, employing the Folin-Ciocalteu (F.-C.) method, are presented. The water solvent was tested at three pH levels: natural, 3, and 9. Extraction methods were undertaken from two perspectives: the partial imitation of natural environmental conditions and the potential use of extraction for economic purposes. From an ecological perspective, room-temperature water extraction ('cold' method) was used for 10-, 60-, and 120-min extraction periods. A variant of water extraction at analogous time intervals was an 'insolation' with a 100W light bulb to simulate the heat energy of the sun. For economic purposes, the water extraction method used the Soxhlet apparatus and its modified version, the 'tea-extraction' method ('hot' ones). The results showed that those extractions without an external heat source were almost ineffective, but insolation over 60- and 120-min periods proved to be more effective. Both tested 'hot' methods also proved to be effective, especially the 'tea-extraction' one. Generally, an increase in the concentration of phenolic compounds in water extracts resulted from an increasing solvent pH. The results show the probable involvement of lichen substances in biogeochemical processes in nature and their promising use for a variety of human necessities.

Synthesis and toxicological study of chitosan-pirul (Schinus molle L.) essential oil nanoparticles on Aspergillus flavus

In recent years, the study of essential oils as antifungal alternatives and their encapsulation to increase their properties for greater effects has been tested. In this work, nanoparticles of chitosan-Schinus molle L. essential oil (CS-PEO-Np) with a size of 260 ± 31.1 nm were obtained by ionic gelation and evaluated in some growth phases of Aspergillus flavus, a toxigenic fungus. At a concentration of 250 μg/mL of CS-PEO-Np, the A. flavus mycelial growth was inhibited at 97.1% with respect to control, at 96 h of incubation; the germination and viability of spores were inhibited at 74.8 and 40%, respectively, after exposure to 500 μg/mL of these nanomaterials, at 12 h of incubation. The fluorescence images of stained spores with DAPI showed the affectations caused by nanoparticles in the cell membrane, vacuoles and vacuolar content, cell wall, and nucleic acids. For both nanoparticles, CS-Np and CS-PEO-Np, no mutagenic effect was observed in Salmonella Typhimurium; also, the phytotoxic assay showed low-to-moderate toxicity toward seeds, which was dependent on the nanoparticle's concentration. The acute toxicity of CS-PEO-Np to A. salina nauplii was considered low in comparison to CS-Np (control), which indicates that the incorporation of Schinus molle essential oil into nanoparticles of chitosan is a strategy to reduce the toxicity commonly associated with nanostructured materials. The nanoparticulated systems of CS-PEO-Np represent an effective and non-toxic alternative for the control of toxigenic fungi such as A. flavus by delaying the initial growth stage.

Discovery and excavation of lichen bioactive natural products

Lichen natural products are a tremendous source of new bioactive chemical entities for drug discovery. The ability to survive in harsh conditions can be directly correlated with the production of some unique lichen metabolites. Despite the potential applications, these unique metabolites have been underutilized by pharmaceutical and agrochemical industries due to their slow growth, low biomass availability, and technical challenges involved in their artificial cultivation. At the same time, DNA sequence data have revealed that the number of encoded biosynthetic gene clusters in a lichen is much higher than in natural products, and the majority of them are silent or poorly expressed. To meet these challenges, the one strain many compounds (OSMAC) strategy, as a comprehensive and powerful tool, has been developed to stimulate the activation of silent or cryptic biosynthetic gene clusters and exploit interesting lichen compounds for industrial applications. Furthermore, the development of molecular network techniques, modern bioinformatics, and genetic tools is opening up a new opportunity for the mining, modification, and production of lichen metabolites, rather than merely using traditional separation and purification techniques to obtain small amounts of chemical compounds. Heterologous expressed lichen-derived biosynthetic gene clusters in a cultivatable host offer a promising means for a sustainable supply of specialized metabolites. In this review, we summarized the known lichen bioactive metabolites and highlighted the application of OSMAC, molecular network, and genome mining-based strategies in lichen-forming fungi for the discovery of new cryptic lichen compounds.

Lichens: An update on their ethnopharmacological uses and potential as sources of drug leads

ETHNOPHARMACOLOGICAL RELEVANCE

Lichens, a unique symbiotic association between an alga/cyanobacterium and a fungus, produce secondary metabolites that are a promising source of novel drug leads. The beauty and importance of lichens have not been adequately explored despite their manifold biological activities such as anticancer, antimicrobial, antioxidant, anti-inflammatory, analgesic, antipyretic and antiparasitic.

AIM OF THE STUDY

The present review collates and discusses the available knowledge on secondary metabolites and biological activities of lichens (in vitro and in vivo).

MATERIALS AND METHODS

Using relevant keywords (lichens, secondary metabolites, bioactivity, pharmacological activities), five electronic databases, namely ScienceDirect, PubMed, Google Scholar, Scopus and Recent Literature on Lichens, were searched for past and current scientific contributions up until May 2022. Literature focusing broadly on the bioactivity of lichens including their secondary metabolites were identified and summarized.

RESULTS

A total of 50 review articles and 189 research articles were searched. Information related to antioxidant, antimicrobial, anti-inflammatory, anticancer and insecticidal activities of 90 lichen species (from 13 families) and 12 isolated metabolites are reported. Over 90% of the studies comprised in vitro investigations, such as bioassays evaluating radical scavenging properties, lipid peroxidation inhibition and reducing power, cytotoxicity and antimicrobial bioassays of lichen species and constituents. In vivo studies were scarce and available only in fish and rats. Most of the studies were done by research groups in Brazil, France, Serbia, India and Turkey. There were relatively few reports from Asia and Africa despite the ubiquitous nature of lichens and the high occurrence in these continents.

CONCLUSION

Secondary metabolites from lichens are worthy of further investigation in terms of their potential therapeutic applicability, including better understanding of their mechanism(s) of action. This would be of great importance in the search for novel drugs.

A review of the potential of lichen substances as antifungal agents: the effects of extracts and lichen secondary metabolites on Fusarium fungi

The present meta-analysis provides literature data on the effect of lichen extracts and single secondary metabolites used against Fusarium spp. moulds. Lichen extracts were obtained from 51 corticolous, 17 terricolous and 18 saxicolous lichen species and 37 secondary compounds were tested against eight fungal species, i.e., Fusarium acuminatum, F. avenaceum, F. culmorum, F. fujikuroi, F. oxysporum, F. roseum, F. solani and F. udum. The researchers used several test methods, mostly to determine MIC and IZ. Extracts were obtained using several solvents, mainly organic ones with use of the Soxhlet apparatus. The most frequently tested species was F. oxysporum, against which lichen substances from Alectoria sarmentosa, Cladonia mitis, C. rangiferina, Flavoparmelia caperata, Hypotrachyna cirrhata, Leucodermia leucomelos, Parmotrema austrosinense, P. reticulatum, Physcia aipolia, Pseudevernia furfuracea, Roccella montagnei and Umbilicaria nylanderiana and secondary metabolites such as 2-hydroxy-4-methoxy-3,6-dimethylbenzoic acid, atranorin, lecanoric and (+)-usnic acids showed the highest antifungal potential. These agencies could compete with the potential of fungicides, such as flucytosine and fluconazole. Other species have been poorly investigated. Statistical analysis of literature data showed that the fungistatic potential of lichen extracts is significantly different from individual secondary metabolites. Similarly, the potential of secondary metabolites often differs significantly from that of non-lichen substances. This meta-analysis indicates the potential of lichen substances as future anti-fusarial agents.