ARS | Publication request: Molasses Supplementation Promotes Conidiation but Suppresses Aflatoxin Production by Small Sclerotial Aspergillus flavus

Submitted to: Letters in Applied Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: September 20, 2006
Publication Date: February 1, 2007
Citation: Chang, P.-K., Hua, S.T. 2007. Molasses Supplementation Promotes Conidiation but Suppresses Aflatoxin Production by Small Sclerotial Aspergillus flavus. Letters in Applied Microbiology. 44(2):131-137.

Interpretive Summary: Asexual spores are the primary disseminating mechanism for fungal propagation. Sclerotia, pigmented mycelial aggregates are a survival structure capable of remaining dormant for long periods under unfavorable conditions. The mold Aspergillus flavus consists of two morphological groups. One group is called S strain, which produces numerous tiny sclerotia but low amounts of spores and is always aflatoxigenic. Another group is called L strain, which produces predominantly spores but only a few sclerotia. Experimental procedures such as protoplasting or field trials usually require large amounts of spores as inocula. However, no media that favor spore production by S strain A. flavus has been developed. We found that adding molasses, a cheap by-product of cane sugar processing, to commonly used culture media not only increases spore production significantly but also decreases aflatoxin production by S strain isolates. Medium containing only molasses promotes spore production and suppresses aflatoxin production at the same time. Molasses-based media thus increases handling safety and decreases disposal costs. It adds to observations suggesting that natural products from plants can have an enhancing effect in inhibiting aflatoxin production.

Technical Abstract: Aims: To find an agar medium ingredient that significantly promotes conidial production in small sclerotia (S strain) Aspergillus flavus isolates that normally produce lower numbers of conidia than large sclerotia (L strain) A. flavus isolates on routinely used growth media. Methods and Results: We examined molasses supplementation on the production of conidia, sclerotia and aflatoxins on coconut agar (CNA), potato dextrose agar (PDA), and vegetable juice agar (V8). We also determined how nitrogen sources in molasses medium (MM) affect the production of conidia, sclerotia and aflatoxins. Conidia harvested from agar plates were counted using a hemocytometer. Sclerotia were weighed after drying at 45 degrees C for five days. Aflatoxins B1 and B2 were quantified by high performance liquid chromatography. Supplementing molasses to CAN and PDA increased conidial production and decreased aflatoxin production. MM, like V8, did not support aflatoxin production and yielded two-fold conidia as that of V8. Adding ammonium to MM significantly increased the production of sclerotia and aflatoxins but slightly decreased conidial production. Adding urea to MM significantly increased the production of conidia, sclerotia and aflatoxins. Conclusions: Molasses stimulates conidial production and inhibits aflatoxin production. Its effect on sclerotial production is medium-dependent. Medium containing molasses alone or molasses plus V8 juice are ideal for conidial production by S strain A. flavus. Significance and Impact of Study: Molasses medium provides a means for producing large amounts of conidia. It increases handling safety and decreases disposal costs due to suppression of aflatoxin production.