Isolation and characterization of extracts from wild edible and non-edible mushrooms in Zimbabwe

Abstract

Poor nutrition and an increasing emergence of infectious diseases, particularly in developing countries, represent major threats to human health. Mushrooms are known to possess numerous bioactivities such as antioxidant and antimicrobial activities. However, the role of local Zimbabwean mushrooms in human health remains largely unexplored. In this study, ten local mushrooms, namely; Amanita zambiana, Boletus edulis, Cantharellus heinemannianus, C. miomboensis, C. symoensii, Lactarius kabansus, Amanita species, Coprinus species, Ganoderma lucidum and Trametes strumosa, collected from selected areas of Zimbabwe, were characterised for nutritional, secondary metabolites and biological activity. The main research hypothesis was that the extracts of different wild edible and non-edible mushrooms of Zimbabwe contained nutritional, antibacterial and anti-HIV components. Determination of protein content of mushroom powder was carried out using the Kjeldhal method while the qualitative analysis of carbohydrates was done using Molisch’s and Benedict’s tests. The lectin content was detected by the haemagglutination activity of mushrooms against sheep and goat erythrocytes. The total protein content ranged from 9.3% ± 0.99 to 30.8% ± 1.27. Amanita sp., B. edulis and L. kabansus were able to agglutinate both sheep and goat erythrocytes. Amanita zambiana showed high levels of both carbohydrates and reducing sugars. Crude extracts from 50 mushrooms,were obtained using hot water, cold water, methanol, ethanol or acetone were used to determine the protein and total phenol contents using the Folin Ciocalteu (FC) and Lowry C methods, respectively. Boletus edulis had high protein and total phenolic content (2.02 mg ± 0.1 and 503.70 mg ± 20.7, respectively). The antibacterial effect of the crude extracts against Escherichia coli, Salmonella typhi, Staphylococcus aureus and Streptococcus pneumoniae was determined using the agar disc diffusion method. The extracts exhibited antibacterial properties against the four bacteria tested. Sixteen of the extracts that showed high levels of bacterial growth inhibition were selected for further characterization. A total of 131 compounds (CP1 – CP131) were isolated using Preparative Thin Layer Chromatography from the 16 extracts. Thirteen of the isolated compounds exhibited high inhibitory activity against the growth of S. typhi (82 to 99.8%). One of the compounds (CP50) inhibited S. aureus growth (87.5%). Identification of compounds responsible for the high antibacterial activity was carried out using LC-MS. The tepernoids (boviquinone 4, cavipetin D, goshonoside, lucidenic acid M, 26-methyl nigranoate and notoginsenoside); phospholipid (C16 sphinganine) and fatty acid derivatives (11-aminoundecanoic acid, z-13-oxo-9-octadecanoic acid, palmitic amide, sorbitan oleate and stearamide) were identified as compounds partly responsible for the antibacterial activity observed. The effect of the crude extracts against HIV replication was determined using the anti-HIV-1c reverse transcriptase (RT) and HIV-1c p24 ELISA assays. The cold water extract of L. kabansus demonstrated the highest level of HIV-1 RT inhibitory activity (92.6%), whilst the hot water extracts from Coprinus species and C. heinemannianus exhibited high potent levels of HIV-1c p24 inhibitory activity, with IC50 values of 24.3 µg/ml and 33.8 µg/ml, respectively. This study revealed for the first time the presence of bioactive compounds in the local Zimbabwean mushrooms studied. The hypothesis that the extracts of different wild edible and non-edible mushrooms of Zimbabwe contained nutritional, antibacterial and antiHIV components was proven to be true. The information obtained will potentially enable development of more efficient foods as medicine in the country, based on the wealth of information generated on the health-promoting properties of the ten mushrooms studied. From this study, the development of anti-bacterial and anti-HIV therapeutic agents from the local mushrooms is recommended, due to the presence of antimicrobial compounds identified in the mushroom extracts. The study also recommends the development of edible local mushrooms such as A. zambiana, B. edulis and L. kabansus into functional food products due to their high nutritive value.