Taxonomic Studies of the Aflatoxin-Producing Strains in the Aspergillus Flavus Group

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Abstract
Twenty-eight representative strains of the Aspergillus flavus group including A. flavus, A. flavus var. columnaris, A. parasiticus, A. oryzae, A. tamarii, and a new taxon were examined for their ability to utilize a variety of carbon sources. The objective was to find some method which would separate these taxa on other than morphological grounds. Of the 36 compounds tested practically all supported growth of the A. flavus group except cellulose and dextran. Utilization of carbon sources does not offer much promise in separating species and varieties but does emphasize the close relationship of all the members of the group. One strain of A. flavus var. columnaris appears to have a vitamin deficiency. Most of the A. flavus strains produced kojic acid. When strains of the A. flavus group were grown at various temperatures, all grew at 37 C and only one grew very poorly at 45 C. The number of sclerotia could be altered drastically by changing the amounts of nitrate and sucrose with the optimal amounts appearing to be about 3% sucrose and 0.5% NaNO3. All strains tested for their ability to digest gelatine were positive, and all formed amylase. Twelve selected strains were examined for their ability to produce aflatoxin on rice and wheat with constant agitation. No aflatoxin was formed by two A. oryzae and two A. flavus var. columnaris strains. One strain of A. parasiticus, which had been in continuous pure culture for 46 years, produced as much as 378 μg/g of total aflatoxin on wheat.