Morphological changes of a toxigenic isolate of Aspergillus parasiticus cultured in the presence of aqueous leaf and seed extracts obtained from neem, Azadirachta indica A. Juss (syn. Melia azadirachta L.), a potent inhibitor of aflatoxin biosynthesis, was studied. Mycelial samples obtained from A. parasiticus cultures exposed to an effective concentration of the leaf and seed extracts (50% v/v in culture media) produced approximately 90 and 75%, less aflatoxins respectively. Under these conditions semi-thin longitudinal and cross sections of the mycelia and vesicles showed attenuation of the cell wall at variable intervals causing deformation of the mycelium, vacuolation of the mycelial cytoplasm and vesicles. Herniation of the cytoplasmic contents which were protruding from the mycelium resulting in irregular mycelial shape. In addition, some mycelia showed a cleft between the cell wall and cytoplasm. Data obtained from microscopic observations may suggest that irreversible inhibition in aflatoxin biosynthesis by fungi due to neem extracts is partly due to mycelial cell wall damages.

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