A Review on Antimicrobial Activity of Mushroom (Basidiomycetes) Extracts and Isolated Compounds
For a long time, mushrooms have been playing an important role in several aspects of human activity. Edible mushrooms, for example, are used extensively in cooking and make up part of low-calorie diets. Mythology is extensively garnished by mushrooms and is typically associated with gnomes, fairies, and other fairytale personages. The psychedelic and consciousness expansion properties of some species have pushed mushrooms to become part of some religions. Even toxic mushrooms have found a place of relevance, because of the uniqueness of their compounds that evolved naturally as a protection against consumption .
Wild and cultivated mushrooms contain a huge diversity of biomolecules with nutritional  and/or medicinal properties , , . Due to these properties, they have been recognized as functional foods, and as a source for the development of medicines and nutraceuticals. Fruiting bodies, mycelia, and spores accumulate a variety of bioactive metabolites with immunomodulatory, cardiovascular, liver protective, antifibrotic, anti-inflammatory, antidiabetic, antiviral, antioxidant, antitumor, and antimicrobial properties , , , , , , , , , , , .
In particular, mushrooms could be a source of natural antibiotics, which can be LMW and HMW, respectively, compounds. LMW compounds are mainly secondary metabolites such as sesquiterpenes and other terpenes, steroids, anthraquinone and benzoic acid derivatives, and quinolines, but also primary metabolites such as oxalic acid ([Fig. 1]). HMW compounds mainly include peptides and proteins.
It is estimated that there are about 140 000 species of mushrooms on earth, and of these only 22 000 are known and only a small percentage (5 %) has been investigated. Therefore, there is much to explore about mushroom properties and potential applications
The present review focuses on the antimicrobial effects of mushrooms from all over the world, and their isolated compounds. It will certainly be useful for future scientific studies. Both edible and nonedible mushrooms showed antimicrobial activity against pathogenic microorganisms, including bacteria associated with nosocomial infections (Pseudomonas aeruginosa, Pseudomonas maltophila, Listeria monocytogenes, Staphylococcus aureus, Klebsiella pneumoniae, Morganella morganii, Proteus mirabilis, Serratia marcescens) and multiresistance (MRSA, MRSE, VREF, PRSP, ERSP).
Data available from the literature indicates that mushroom extracts and isolated compounds exhibit higher antimicrobial activity against gram-positive than gram-negative bacteria. Among all the mushrooms, Lentinus edodes is the best-studied species and seems to possess broad antimicrobial action against both gram-positive and gram-negative bacteria. Species from the genera Boletus, Ganoderma, and Lepista appear promising for future studies, if one considers the positive activity and limited number of publications. Considering the low number of studies with individual compounds, Plectasin peptide, isolated from Pseudoplectania nigrella, revealed the highest antimicrobial activity against gram-positive bacteria.
The comparison of the results reported by different authors is difficult, due to the diverse methodologies used to evaluate antimicrobial activity of mushroom extracts or isolated compounds. Therefore, the standardization of methods and establishment of cut-off values is urgent. The knowledge about the mechanisms of action of different compounds might lead to the discovery of new active principles for antimicrobial activity. Furthermore, the application of cytotoxicity assays is also important to evaluate the effects on humans in the range of the in vitro tested concentrations.
The research on mushrooms is extensive and hundreds of species have demonstrated a broad spectrum of pharmacological activities, including antimicrobial activity. Although there are a number of studies available in the literature, they are almost entirely focused on the screening of antibacterial properties of mushroom extracts. In fact, there is a gap in the identification of the individual compounds responsible for those properties, and only a few low-molecular weight compounds and some peptides and proteins have been described. After elucidation of their mechanism of action, these mushroom metabolites or other related compounds could be used to develop nutraceuticals or drugs effective against pathogenic microorganisms resistant to conventional treatments.