# Lichens as a Natural Source of Compounds Active on Microorganisms of Human Health Interest
## 4 Discussion  
The research and the use of new drugs from natural sources have been accelerated in recent years. Researchers are increasingly focusing on the discovery of new compounds, which could be used for the treatment of various infectious diseases, especially those caused by antibiotic-resistant microorganisms. A few chemical compounds have been associated with the antimicrobial activity of lichens. Usnic acid is the most examined among lichen compounds, and several studies have been developed to investigate its antimicrobial properties [43; 44]. Results obtained in our study demonstrate the presence in the investigated lichen of bioactive compounds with a wide spectrum of activity against yeasts and bacteria, which could be associated with a scabrosin derivative on the basis of the chemical characterization. The scabrosin esters belong to the epipolythiodixopiperazine (ETP) family of fungal metabolites, and they are the first ETP compounds isolated from lichens [45]. ETP compounds display an extensive range of biological activity, including antibacterial and antifungal properties [46]. ETP scabrosin ester inhibits mitochondrial function, which results in the release of calcium and magnesium, leading to cell death by apoptosis [47]. Our results suggest that natural compounds extracted from lichens can be effective against pathogenic microorganisms, thus providing an interesting alternative to the use of traditional antimicrobials. In detail, the two lichen extracts were able to inhibit the growth of bacteria of interest in human health, showing the same activity, especially against Gram-positive bacteria. This can be explained by the fact that LC-MS analysis \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Lichen Extracts** & **Solvent** & **\% Yield** & **Main Metabolites** \\ \hline LC1 & Cyclohexane & 1.0 & variadic acid scabrosin derivates \\ LC2 & Acetone & 2.4 & variadic acid scabrosin derivates \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Extraction yields and main metabolites in LC1 and LC2 lichen extracts. Figure 6: Example of a LC1 fraction HPLC chromatogram (fraction 8) showing the antimicrobial activity against _Candida albicans_ ATCC 10231. The peak at a retention time (\(\rm t_{R}\)) of 10.8 min corresponds to a scabrosin derivative. Figure 7: Example of the mass spectrometry (MS) spectrum related to the peak at 10.8 min. revealed the two extracts to contain the same components. Evidently, the two solvents were both effective in extracting the bioactive metabolites. The most interesting and relevant result was obtained against _Candida albicans_, which was susceptible to the extracts up to 30 experimental days. The _Candida_ genus is responsible for many fungal infections, and the species _C. albicans_ and _Candida glabrata_ are involved in the 65-75% of infections in immunocompromised patients [48; 49]. In addition, considering the emergence of drug-resistant _C. albicans_ strains, especially in clinical settings, due to the excessive use of fluconazole and to the restricted number of new antifungal drugs, the development of new effective anti-_Candida_ approaches is necessary to target fungal diseases. Many studies have demonstrated the antimicrobial activity of lichen extracts against _Candida_ species, used alone or in combination with other drugs [17; 50; 51]. Some studies on the anti-_Candida_ activity of usnic acid showed how this compound can alter the pro-oxidant-antioxidant balance, which causes cell death due to irreversible tissue damage [52]. Furthermore, usnic acid was able to reduce _Candida_ biofilm by decreasing the amount of sugars present in the exopolysaccharideaccharide (EPS) layer [53]. With regard to the amount of extracted active metabolites, as for other plant derived natural compounds, the different environmental factors (light, temperature, precipitation rates, and humidity) are particularly important in determining the growth differences of lichens and the consequent production of secondary metabolites [54].