Nannocystis exedens as potential biocompetitive agent against toxigenic Aspergillus flavus and Aspergillus parasiticus

Aflatoxins, secondary metabolites produced by toxigenic strains of Aspergillus flavus, Aspergillus parasiticus and Aspergillus nominus, are potent hepatocarcinogens found in foods and feeds. Since the discovery of aflatoxins in 1960, no effective approach has been found to eliminate or decrease the population of the molds responsible for aflatoxin contamination at the field level. The objective of this study was to identify a biocompetitive agent that would parasitize toxigenic strains of A. flavus and A. parasiticus. Nannocystis exedens was selected as the biocompetitive agent because it is a predator bacterium which lyse other bacteria and yeasts. In addition, the bacterium is found commonly in soils and dung of herbivorous animals and it is nonpathogenic.

Aspergillus flavus ATCC 1687, Aspergillus flavus ATCC 26946 and Aspergillus parasiticus NRRL 3145 were grown in the presence of Nannocystis exedens ATCC 25963, and growth and production of aflatoxin B₁ and G₁ were determined after 14 days of incubation at 28°C. N. exedens inhibited growth and aflatoxin B₁ and G₁ production by all three aflatoxigenic molds. When N. exedens was grown in close proximity with each of the molds on 0.3% trypticase-peptone yeast extract agar, zones of inhibition developed between the bacterium and each mold colony. Flattening of the mold colony on the sides nearest N. exedens, growth of mold colony away from the bacterial colony, and general stunting of growth of the mold colony were observed. When N. exedens was added to the center of the cross streak of a mold colony, lysis of the mold colony by the bacterium was observed after 24 hours. Bacterial growth was only observed in the center of each mold colony. Microscopic observations revealed that N. exedens grew parasitically on spores, germinating spores, hyphae and sclerotia of the aflatoxigenic molds. N. exedens developed as streaming colonies which surrounded the propagules of each mold as a sheath of tightly packed rods, clumps of rods, or as individual rods.

These results indicate that Nannocystis exedens ATCC 25963 may be effective in interfering with and/or inhibiting growth and aflatoxin production by toxigenic strains of A. flavus and A. parasiticus in the field.