Lentinula edodes and Pleurotus ostreatus: functional food with antioxidant - antimicrobial activity and an important source of Vitamin D and medicinal compounds

Abstract

Background: Mushrooms produce a large amount of medicinal compounds, and are also an optimal source of fibres, proteins, vitamins (like groups B and D), and other micronutrients including potassium, magnesium, etc. Consequently, mushrooms are commonly considered to be functional foods. Many works report the high biological potentials of medicinal mushrooms involving their antibacterial, hypoglycaemic, anticholesterolemic, radical scavenging, and anti-inflammatory effects.

Context and purpose of this study: First off, this work aimed to find strains of Lentinula edodes and Pleurotus ostreatus from a bank of edible mushrooms bought from international strain banks (Table I) that could possess health benefit related properties, such as a radical scavenging activity (antioxidant effect), antibacterial effects against common pathogenic bacteria, and being able to produce interesting nutrients and secondary metabolites. As the fungal bank comprises of 20 strains of L. edodes and 20 strains of P. ostreatus, a first screening was made by the selection of 13 strains for each mushroom able to grow in multiple wood types or that were particularly productive and had proved good growth reproducibility over the last 5 years. This work also studied the correlation between culture conditions and mushroom quality in terms of the previously reported properties. Comparison among the selected strains was operated by the assessment of antioxidant and antimicrobial activities after different sample treatments. Furthermore, an initial optimization of the analytic techniques was produced for the direct estimation of important secondary metabolites and nutrients by means of HPLC-MS/MS technique. Further research will encompass an evaluation of transformation processes (drying, freezing, rehydration, cooking, etc.) impact on radical scavenging, antibacterial activity, and possible degradation/loss of nutraceutically important substances such as vitamin D2, ergothioneine, eritadenine, lovastatin, lentinan, and lenthionine. 

Results: 13 strains of each mushroom species have been cultivated on different wood logs. Seven strains of shiitake and six strains of oyster mushroom were able to produce sporocarps. Antioxidant levels in water extracts from dried mushrooms produced significatively different results on the basis of strains and of wood. Both mushrooms demonstrated higher radical scavenging activity in log cultivation than substrates cultivation, which was subsequently used as reference. Furthermore, all strains of P. ostreatus demonstrated the lowest level of antioxidant activity at 4°C, a significant increase towards 50°C and a limited decrease towards 80°C. The same trend was observed for shiitake extracts. Concerning the shiitake mushroom only, crude water extracts showed an interesting antibacterial activity against the model microorganisms Pseudomonas aeruginosa and Staphylococcus aureus. A comparison was also performed between the best performing strain extract and the commercial antibiotic Ceftriaxone against P. aeruginosa, assessing that 20 mg of crude extract corresponds to 0.2 mg of the pure antibiotic when studied by means of disk diffusion assay.

Conclusions: The results suggested that the cultivation of both shiitake and oyster mushrooms on logs could enhance the content of antioxidant and antibacterial activities, compared to the cultivation of mushrooms on sawdust substrates. Radical scavenging and antibacterial activity depends both on L. edodes strain and the log type. The bacteriostatic/bactericidal activity of the best performer strain may depend on a pH and solvent treatment sensitive substance. Secondary metabolites such as ergothioneine and vitamin D2 from both shiitake and oyster were released just after water extraction: this suggests that the transformation/cooking processes may produce a loss of characteristic mushroom biological properties in water. Further evaluation of biologically relevant compounds content and loss during different food transformation and cooking processes will be assessed.

Key words: Shiitake, Oyster, Pleurotus ostreatus, Lentinula edodes, Pseudomonas aeruginosa, Staphylococcus aureus, Log grown mushroom, antioxidant, radical scavenging, total phenolic content, DPPH, ABTS, Folin-Ciocalteu, antibacterial, fruitbodies, sporocarps.