Aflatoxin in Food: What Role for Biotechnology? - OFAB

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Toxicological manifestation: include cancer, mutagenicity, birth defects (teratogenic) and estrogenic, gastrointestinal, urogenital, vascular, kidney and nervous system disorders. The compromised immune response that accompanies chronic exposure to some mycotoxins reduces resistance to infectious disease and may be the most important and the most underappreciated of the health problems associated with mycotoxins Toxicological effects on children include reduction of growth. Development of children exposed to aflatoxins may reduce quality of life and limit an individual’s ability to reach their potential. Aflatoxin Pathogenicity Dr Maina discussed some of the diseases inflicted by Aspergillus. Toxicological manifestations inflicted by the fungus include cancer, birth defects, gastrointestinal, urogenital, vascular, kidney and nervous system disorders. He affirmed that the compromised immune response that accompanies chronic exposure to some mycotoxins reduces resistance to other infectious diseases as well. He was of the opinion that the later is one pathogenicity aspect which may be the most important and the most underappreciated of the health problems associated with mycotoxins. Exposing some of the strains responsible to cause illnesses, Dr Maina listed A. flavus as second only to Aspergillus fumigatus among Aspergillosis strains causing Aspergillosis. He added A. flavus as the most frequently reported Aspergillus species causing keratitis, an inflammation of the eye cornea. Detection of Aflatoxigenic fungi Dr Maina shared the methods used so far in detecting Aflatoxigenic fungi, including the use of UV light, Polymerase Chain Reaction, Transcription Polymerase chain reaction and Multiple Transcription Polymerase chain reaction Detection of Mycotoxigenic Fungi by Fluorescence under UV light Aflatoxins can be identified in colonized corn kernels by viewing under UV light. However, De Maina pointed out that the detection of Aflatoxins as judged by fluorescence of fungal colonies is not conclusive, in that non-aflatoxigenic strains of Aspergilli, such as A. parasiticus and A. niger fluoresce also under UV. This could lead to wrong interpretation and conclusion that there is presence of Aflatoxins. Further diagnostic test for the presence of Aflatoxins in fungal colonies can be determined by observation of continued phosphorescence of Aflatoxins at room temperature, after switching off the UV light, the fluorescence will last for 0.5 seconds. Without the UV light, Non-Aflatoxigenic Aspergilli do not phosphoresce. With the help of pictures, Dr Maina took time to illustrate, by photos, the difference between the appearance of non-aflatoxigenic and Aflatoxigenic colonies under UV light and after switching off the UV lamp. Detection of Mycotoxigenic Fungi by Polymerase Chain Reaction Another way to detect mycotoxins was found to be by Polymerase Chain Reaction (PCR). 2
This is possible since at least 25 genes are involved in the biosynthesis of Aflatoxins and its regulation. Primers pertaining to sequences of afl-2, aflD, aflM and aflP, (apa-2, nor-2, ver-2, omt-2, respectively) have been used to detect and identify aflatoxigenic strains of A. flavus and A. parasiticus among isolated colonies or in DNA extracts from food and feedstuff. Briefly, DNA of Aspergilli is used as template for the amplification of genes involved in Aflatoxin biosynthesis. Sequencing of the amplified fragments therefore will confirm the identity of aflatoxin biosynthetic genes. However, the mere presence of the genes reflects only the potential of the fungus to produce aflatoxins. Application of Reverse Transcription Polymerase chain reaction Application of Reverse Transcription Polymerase chain reaction (RT–PCR) for the characterization of aflatoxigenic Aspergilli relies on the presence of mRNAs pertaining to Aflatoxin biosynthesis genes. RT–PCR is indicative of the presence of the aflatoxigenic fungus and of the Aflatoxin biosynthetic enzymes Application of Multiple Transcription Polymerase chain reaction Multiplex RT–PCR containing 4–5 primer pairs of various combinations of aflD, aflO, aflP, aflQ, aflR and aflS (aflJ) are used to detect toxigenic fungi. Non-aflatoxigenic strains lack one or some Aflatoxin biosynthesis genes and their mRNA products Regulation of Aflatoxin Biosynthesis Determination of the molecular basis for the phenomenon of non-production of aflatoxin in certain members of the A. flavus group of the fungi with a view leads to a better understanding of the global regulation of toxin synthesis and the convergent evolution of aflatoxin biosynthesis. Gene Characterization The characterization of genes of the aflatoxin biosynthetic pathway has provided a foundation for a genomic investigation aimed at understanding the biochemical function and genetic regulation of aflatoxin biosynthesis Aflatoxin Metabolic Pathway Aflatoxins are synthesized by a polyketide metabolic pathway. Mapping of overlapping cosmid clones of A. parasiticus and A. flavus genomic DNA has established that the genes in the aflatoxin biosynthetic pathway are clustered. In general, the aflatoxin gene cluster in A. parasiticus and A. flavus consists of 25 genes spanning approximately 70 kb Role of biotech in controlling Aflatoxin contamination Gene Cluster With the characterization of the gene cluster, new insights into the cellular processes that govern the genes involved in aflatoxin biosynthesis have been revealed, but the signaling processes that turn on aflatoxin biosynthesis during fungal contamination of crops are still not well understood. Exploiting the Aflatoxin biosynthetic pathway The fully characterized aflatoxin biosynthetic pathway and gene cluster comprising genes that govern this pathway, including the key regulatory gene (aflR), provides a sound basis for studies 3
Page 1: OFAB 38 - JUNE 2010 Aflatoxin in Fo
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Aflatoxin in Food: What Role for Biotechnology? - OFAB

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