Aspergillus flavus is a very important toxigenic fungus that produces aflatoxins, a group of extremely toxic substances to man and animals. Toxigenic fungi can grow in feed cropsAflatoxins constitute a group of low molecular weight compounds known to be extremely toxic to animals and man (Bennett & Klich 2003). They are secondary metabolites produced mainly by Aspergillus flavus and A. parasiticus, which are ubiquitous and cosmopolitan fungi able to grow on a wide variety of substrates. However, not all members of each species are toxigenic (Thapar 1988) and, even within potentially toxigenic strains, growth is not necessarily accompanied by toxin production, which depends on favorable environmental conditions such as temperature and humidity (Correa 2000). The economic losses due to fungal contamination and the high costs of control measures are enormous (Bennett & Keller 1997) and add up to billions of dollars worldwide (Pozzi 2000). For this reason, efforts on scientific research have been towards prevention rather than remedial action (Bennett & Keller 1997).The key action for preventing mycotoxins is to block the growth of toxigenic fungi (Fonseca et al. 1974, Bennett & Keller 1997. But this is not an easy task, since these microorganisms are widely spread through several ecosystems, covering vast geographical areas. Furthermore, they can be easily disseminated by wind and may contaminate food crops at different stages, including harvest, post-harvest, processing, transportation, and even at storage conditions in supermarkets.Several approaches can be adopted to prevent or minimize food contamination by aflatoxins, such as controlling potential toxigenic-fungi carriers (e.g., insects and rodents), reducing the storage period and controlling humidity and temperature (Correa 2000). The use of antifungal compounds and insecticides, in turn, has been increasingly criticized from the standpoint of environmental concerns, as well as being costly. The production of resistant-plant varieties, although not having yielded strong positive results, is considered to be a promising alternative (Trail et al. 1995) to prevent both A. flavus substrate colonization and aflatoxin production upon fungal proliferation. However, it is important to bear in mind that the host-parasite co-evolution frequently leads to an "arms race" (Ridley 2001), and it is thus unlikely that fungi remain stable with respect to their invasive or toxigenic potential, as they continuously adapt.The use of non-toxigenic strains for the biological control of toxigenic ones has already been suggested by Egel and collaborators (1994). The non-toxigenic varieties would be artificially disseminated in nature (Egel et al. 1994, Trail et al. 1995 to compete with the toxigenic strains, driving them out of their ecological niches (Trail et al. 1995). In experiments with plants artificially infected with non-toxigenic fugal strains, a reduction by up to 90% in the aflatoxin contamination was observed (Cotty 1989 apud Tran-Dinh et al. 1999). Yet promising, this ...

Two strains of Aspergillus flavus, non-toxigenic NRRL 6550 and toxigenic NRRL 5940, were studied over a period of 44 days, in order to detect the presence of virus-like particles (VLPs) by means of electron microscopy (EM) and nucleic acids electrophoresis. Only the toxigenic strain contained VLPs, presenting three-segmented dsRNA. An increase in VLPs number was observed during the exponential phase of fungal growth, up to day 12 of culture; after this, higher levels of aflatoxin production in toxigenic NRRL 5940 mycelia occurred in parallel with decreased VLPs replication.