Bioactive compounds in Diospyros mafiensis roots inhibit growth, sporulation and aflatoxin production by Aspergillus flavus and Aspergillus parasiticus

Abstract

Diospyros mafiensis F. White is a medicinal shrub or small tree (6 m tall) widely distributed in the Zanzibar-Inhambane regional mosaic and traditionally used to treat leprosy, diarrhoea, and skin fungal infections in Tanzania and Mozambique. Our objective was to determine the anti-aflatoxigenic properties of compounds from D. mafiensis root bark against vegetative growth, sporulation and aflatoxin production by Aspergillus flavus and Aspergillus parasiticus. Bioassay-guided extraction, fractionation, and isolation of bioactive compounds using A. parasiticus B62 were employed. The bioactive compounds were elucidated using 1H and 13CNMR and LC-MS. Growth inhibition was determined by measuring the colony diameter of A. flavus AF3357 and A. parasiticus SU-1 ATCC56775. Inhibitory effects on sporulation were estimated using a haemocytometer. Total aflatoxin was quantified by direct competitive enzyme-linked immunosorbent assay (ELISA). Bioactive compounds diosquinone (DQ) and 3-hydroxydiosquinone (3HDQ) were identified. DQ weakly inhibited A. flavus and A. parasiticus vegetative growth (MIC50 > 100 µg/ml) and 3HDQ strongly inhibited A. flavus (MIC50 = 14.9 µg/ml) and A. parasiticus (MIC50 = 39.1 µg/ml). DQ strongly reduced total aflatoxin production by A. flavus from 157 to 36 ng/plate, and by A. parasiticus from 1,145 ng/plate to 45 ng/plate at 100 µg/ml. 3HDQ reduced total aflatoxin production by A. parasiticus from 1,145 to 32 ng/plate; stimulated production by A. flavus from 157 to 872 ng/plate at 12.5 µg/ml but reduced to 45 ng/plate at 100 µg/ml. In summary, DQ and 3HDQ could be used as natural antifungal compounds to prevent mould growth and aflatoxin accumulation in food and feed.