Submitted to: Mycopathologia
Publication Type: Peer reviewed journal
Publication Acceptance Date: 6/15/2004
Publication Date: 1/15/2005
Citation: Ehrlich, K., Montalbano, B.G., Cotty, P.J. 2005. Divergent regulation of aflatoxin production at acidic pH by two Aspergillus strains. Mycopathologia. 159:579-581. Interpretive Summary: Aflatoxins are cancer-causing toxic products made by some Aspergillus (a type of mold) species under favorable conditions. Various environmental factors affect the production of aflatoxins by Aspergillus species, and may, in part, determine under what circumstances susceptible plant species such as corn, cottonseed, and tree and ground nuts become contaminated with aflatoxins. The acidity or basicity of the environment in which the fungus grows is one such environmental factor. Several previous reports have suggested that aflatoxin production depends on the medium acidity (pH). We found that a type of aflatoxin-producing Aspergillus from West Africa makes less aflatoxin at more acidic than at less acidic conditions, whereas typical Aspergillus from North America produce aflatoxins under exactly the reverse conditions. We studied whether the effect on aflatoxin production by medium acidity also affects the way the genes responsible for making the enzymes involved in aflatoxin production are turned on. We found that one of them, the gene that makes the enzyme involved in the last step of aflatoxin synthesis is very differently turned on in the two cultures, but the other genes were not similarly affected. This knowledge allows a better understanding of how soil acidity may affect aflatoxin production by certain types of fungi. This knowledge should be of value to farmers and research scientists trying to develop simple, cost-effective methods to prevent aflatoxin contamination in cottonseed and corn.
Technical Abstract: Aflatoxins (AF) are toxic and carcinogenic metabolites produced by Aspergillus species. Aflatoxin production depends on a number of nutritional factors, including pH. AF production by West African Aspergillus SBG isolates grown on unbuffered ammonium-based medium was much lower than that of A. flavus SB isolates under the same conditions. When the medium was buffered or the pH of the medium was raised to pH >7, aflatoxin yields from SBG isolates were almost the same as those for SB isolates grown on the same medium. The SBG isolates produce both AFB1 and AFG1. AFG1 biosynthesis was inhibited more by lower pH than that of AFB1. Quantitative PCR was used to examine influences of acidic pH (pH 2.5 to pH 5.5) on expression of six AF biosynthesis genes, including the transcriptional regulator gene (aflR), two genes (fasA and pksA) for polyketide formation, and three genes for AFB1 and AFG1 formation (omt1, ordA, and ordB). For the SBG isolate BN008, only expression of omt-1 paralleled the decrease in AF production with decreasing pH. For both types of isolate, expression of omt-1 and ordA at pH 7.5 was much lower than it was at pH 3.2. These results confirm previous studies that neutral to alkaline media inhibit expression of AF biosynthesis genes and they provide evidence that the inhibition in ammonium medium of aflatoxin production by SBG isolates results, in part, from a decrease in expression of aflatoxin biosynthesis genes.