Ultrastructural evidences of growth inhibitory effects of a novel biocide, Akacid(plus), on an aflatoxigenic Aspergillus parasiticus.

The effects of Akacid(plus), a novel member of guanidine-based polymeric compounds recently introduced as a potent inhibitor of fungal growth and aflatoxin biosynthesis were studied on Aspergillus parasiticus by transmission electron microscopy (TEM). The toxigenic fungus was cultured on yeast extract-sucrose broth in presence of serial two-fold concentrations of Akacid(plus) (1.5-96 microL/50 mL medium) for 96 h at 28 degrees C with shaking. Mycelial samples exposed to fungistatic concentrations of compound (1.5-48 microL) were processed for TEM. Corresponding to the growth inhibition, TEM observations revealed morphological anomalies in fungal compartments. The results demonstrated that Akacid(plus) targets the plasma membrane of the hyphae by its breaking down at variable intervals with the formation of small membrane-bound vesicles inside the cytoplasm, while no obvious damage was observed on the cell wall. A marked depletion of cytoplasmic contents of hyphae accompanied with lysis and disruption of membranes of major organelles such as nuclei, mitochondria and endoplasmic reticulum indicates that in high fungistatic concentrations, Akacid(plus) passes not only through the cell wall but also through the plasma membrane and then interact with membranous structures of the cytoplasmic organelles. Ultrastructural changes of fungal compartments exposed to Akacid(plus) in relation to the fungal growth and aflatoxin biosynthesis are discussed.