[Medium optimization for antagonistic Streptomyces S24 and its inhibition on Aspergillus flavus]

Streptomyces S24 has broad spectrum against Aspergillus spp. in food and feed, such as Aspergillus flavus, Aspergillus niger and Asperegillus alutacells. The objective of this study was to improve the production of antifungal substances produced by S24 and to test their inhibitory effects on Aspergillus flavus. By using one-factor-at-a-time experiment and orthogonal design method, we optimized the fermentation medium. The composition of an optimized medium for the production of antifungal substances contained (g/L): starch soluble, 10; Glucose, 40; yeast extract, 8; soybean powder, 24; KH2PO4 4; and CaCO3 0.8. As a result, the productivity of antifungal substances could reach to 10 235.45 microg/mL, and this value was 2.81 times higher than that of initial medium before optimization. Additionally, inhibitory effects of the products on Aspergillus flavus were analyzed. Antagonistic tests indicated that the antifungal substances greatly inhibited mycelium growth and spores germination of Aspergillus flavus. We observed through microscope that the mycelia grew abnormally, such as contorting, bulging, vacuole increasing and the cytoplasmic contents inside effusing and the spores appeared unusual, such as gathering, deforming, cytoplasmic contents inside effusing and fracturing.