Fungal SMs are formed as families of closely related compounds resulting from biosynthetic enzymes less specific to their substrate than those of primary metabolism (Hanson, 2003). The yield of different family members can be modified by changing the composition of the growth medium, the environmental conditions and by mimicking biotic interactions (i.e., co-cultures) (Vinale et al., 2017; Pan et al., 2019). Different members of a family frequently demonstrate diverse biological activities (Hanson, 2003).
The majority of SMs belong to one of the following families: (1) Polyketides and fatty acids; (2) Terpenoids and steroids; (3) Phenylpropanoids; (4) Alkaloids; (5) Specialized amino acids and peptides (Hanson, 2003).
Though not crucial for their survival, fungi bio-synthetize numerous SMs of the mentioned families with different chemical structures and properties. Especially filamentous Asco- and Basidiomycota are rich sources of polyketides, non-ribosomal peptides, terpenes and indole alkaloids that are biosynthesized via specialized pathways (Herbert, 1994).
Fungal secondary metabolism often is connected to specific stages of morphological differentiation such as asexual sporulation. The produced SMs are incorporated into structural elements of the cell or released into the environment as volatiles or dissoluble substances. They hence play important roles in regulating interactions between organisms. Examples of fungal compounds involved in biotic interactions are phytotoxins, produced by fungal pathogens as virulence factors for plants, mycotoxins, produced during the colonization of crops by specific fungi and capable of causing disease and death in humans and other animals, pigments, with antioxidant activity, and antibiotics, that are considered natural products capable of killing or inhibiting microbes. It is important to underline that biological activities are not essentially limited to one specific group or single metabolites and a single fungal metabolite can produce different effects on specific targets (Karlovsky, 2008).
Antibiotics are produced by fungi for gaining an advantage in competition, permitting the microbe to occupy space and to acquire access to nutrients. Most of the useful fungal SMs have been isolated and characterized via a screening approach following “bioassay-guided fractionation”, meaning that the bioactivity of the compound(s) are constantly monitored during the isolation procedure by employing in vitro or in vivo test systems for detecting the biological activity of an extract or a pure substance (bioassay). All generated fractions are tested for biological activity, and those showing the desired bioactivity are further processed until the bioactive compound is obtained in a pure form (Ghisalberti, 2003; Colegate and Molyneux, 2007).
References:
Colegate, S.M., Molyneux, R.J., 2007. Bioactive Natural Products: Detection, Isolation, and Structural Determination. Boca Raton, Florida: CRC press.
Ghisalberti, E.L., 2003. Bioactive natural products (Part I). Studies in Natural Products Chemistry. Amsterdam: Elsevier.
Hanson, J.R., 2003. Natural Products: The Secondary Metabolites. 17. Cambridge, UK: Royal Society of Chemistry.
Herbert, R.B., 1994. The Biosynthesis of Secondary Metabolites, second ed. London, UK: Chapman & Hall.
Karlovsky, P., 2008. Secondary Metabolites in Soil Ecology. Berlin, Heidelberg: Springer, pp. 1–19.
Pan, R., Bai, X., Chen, J., Zhang, H., Wang, H., 2019. Exploring structural diversity of microbe secondary metabolites using OSMAC strategy: A literature review. Front. Microbiol. 10, 294.
Vinale, F., Nicoletti, R., Borrelli, F., et al., 2017. Co-culture of plant beneficial microbes as source of bioactive metabolites. Sci. Rep. 7, 1–12.
