Nitrogen source and pH influence aflatoxin production by a previously unrecognized A. flavus morphotype .

P. J. Cotty, K.E. Kobbeman, J.E. Mellon, T. Feibelman, K. F. Cardwell, and K.E. Ehrlich.

Author address: 

USDA, ARS, Division of Plant Pathology and Microbiology, University of Arizona, Tucson, AZ


An Aspergillus that produces B aflatoxins and elongate tan sclerotia was isolated from soils collected in North America, Asia, and West Africa. Phylogenetic analysis of the aflJ/aflR A. flavus. All isolates belonged to the same vegetative compatibility group and shared greater than 99% sequence similarity. This morphotype (morphotype P) produced elongate sclerotia, up to 5 mm in length, with a bulbous base and reduced melanin. In contrast to other aflatoxin-producing A. flavus, P morphotype isolates produced 8 to 190 fold more B aflatoxins in a medium with nitrate as the sole nitrogen source than in media with either ammonium or urea as sole nitrogen sources. Other A. flavus isolates and isolates of A. nomius and A. parasiticus produced either similar quantities or more aflatoxins in ammonium-based medium than in nitrate based medium. Low aflatoxin production in ammonium-based medium resulted from sensitivity to low pH created during fermentation on ammonium. Buffering with either succinate or citrate resulted in increased aflatoxin production. Aflatoxin production by both the P morphotype and A. parasiticus (NRRL 2999) was greater in nitrate than in ammonium at pH 2.5. However, at pH 7.5, both fungi initially produced greater quantities of aflatoxins on ammonium medium. Variation in pH sensitivity may cause variation among A. flavus isolates in aflatoxin production. The P morphotype may be useful for studying interrelationships among sclerotial morphogenesis, aflatoxin biosynthesis, and melaninization.

abstract No: 


Full conference title: 

23rd Fungal Genetics Conference
    • Fungal Genetics Conference 23rd (2002)