A white Maitake (Grifola frondosa): nutritional value and antibacterial preliminary activity test

Grifola frondosa (Dicks.) Gray (Meripilaceae) is a big size, perennial, infrequent lignicolous basidiomycetes, traditionally known as Maitake. Brown or grayish overlapped caps, with a wavy margin, characterize the sporophore. In the wild, it is usually found at the base of living oak trees but it is also cultivated. G. frondosa is mainly distributed in Japan, Asia, Eastern America, and Europe. In Italy, is infrequent but widely distributed in all regions. Field investigation carried out in autumn 2016 in Sicily (southern Italy) permitted to observe an unusual white G. frondosa growing at the base of a living tree of Quercus pubescens Willd. s.l. Once harvested, the mushroom sample was cleaned of debris with a knife, cut into pieces and placed in plastic bags for freezing, then transported to the laboratory and preserved at -80 °C. Finally the frozen mushroom was lyophilized.The morphological studies were carried out under the microscope using KOH 3% and ammoniacal Congo Red and performed in a combination with DNA sequence analyses of the Internal Trancribed Spacer (ITS) region. The chemical and nutritional composition (ashes, water, proteins, fats, carbohydrates, vitamins, nitrogen, and organic and inorganic substances) was evaluated according to standard methodologies. The antibacterial activity against four Gram-positive and Gramnegative bacteria strains: Staphylococcus aureus ATTC 25923, S. epidermidis ATTC 12228, Pseudomonas aeruginosa ATCC 15442, and Escherichia coli ATCC 25922 was also evaluated. The minimal inhibitory concentration (MIC) of G. frondosa extracts was determinated by a micro-method by diluiting in a range from 50% to 0,6% v/v. Tryptic soy broth (TSB, Sigma) was used as media. As a result of morphological analysis the white G. frondosa showed basidiospores of slightly narrowed shape than the typical Maitake (5 - 7 x 3.2 – 4 μm). Besides, sterigmata are long up to 6 μm, sturdy, with the base of diameter up to 1.5 μm. These microscopical features are not reported in Ryvarden while Bernicchia illustrated for the typical Maitake sterigmata “thin and very divaricate”. Clamp connections are present but rare in the contest. Cystidia and cystidioles are absent. The identity of the white G.frondosa was confirmed by analysis of ITS sequence. The results of nutritional composition analysis (Table 1) showed that the white G. frondosa is richer in nutritional elements than the cultivated brown Maitake analyzed by Stamets. In particular the white Maitake possess a very high content of Ca, Fe, K, Na, and Cu. As regards vitamins only the value of vitamin D2 is similar to the value reported by other authors. The other vitamin contents are lower. The cold extract of G.frondosa has been showed to be effective in inhibiting the growth of S. epidermidis 12228 and P. aeruginosa 15442 at the maximum screening concentration of 50% v/v. In our opinion the white Maitake is a promising and potential new medicinal mushroom. Further investigations are currently in progress to evaluate potential application as cultivated and medicinal mushroom.