Doctoral Degrees (Microbiology)

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Browsing Doctoral Degrees (Microbiology) by Subject "Aflatoxins"

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    Microbial degradation of mycotoxins
    (Stellenbosch : Stellenbosch University, 2007-04) Alberts, Johanna Francina; Van Zyl, Willem Heber; Botha, Alfred; Stellenbosch University. Faculty of Science. Dept. of Microbiology.
    ENGLISH ABSTRACT: Aflatoxins are mycotoxins predominantly produced by the filamentous fungi Aspergillus flavus and Aspergillus parasiticus. Aflatoxin B1 (AFB1), the most abundant aflatoxin, is highly mutagenic, toxic, carcinogenic and teratogenic to humans and animals and is particularly correlated with the incidence of hepatocellular carcinoma in parts of Africa, China and South East Asia. In this regard aflatoxin is classified as a type I human carcinogen by the International Agency for Research on Cancer. Furthermore, aflatoxin contamination of food and feed is responsible for extensive economic losses due to loss of crops and farm animals. In spite of regulations regarding acceptable levels of aflatoxin in food, aflatoxin contamination remains a serious worldwide problem, especially in developing countries where it occurs predominantly in dietary staples. Inactivation of aflatoxin by physical and chemical methods has not yet proved to be effective and economic. However, biological detoxification offers an attractive alternative for eliminating toxins as well as safe-guarding the desired quality of food and feed. In this study, the biological degradation of AFB1 by bacteria and fungi was investigated. Several bacteria, including Rhodococcus spp., as well as white rot fungi have the potential to degrade a wide range of polycyclic hydrocarbon compounds due to the large repertoire of enzymes they produce and therefore the ability of some of these microorganisms to degrade AFB1 was investigated. Effective degradation of AFB1 by intracellular extracts of Mycobacterium fluoranthenivorans sp. nov. DSM 44556T, Nocardia corynebacterioides DSM 20151 and N. corynebacterioides DSM 12676 was demonstrated. Furthermore, AFB1 was effectively degraded by liquid cultures as well as intra- and extracellular extracts of Rhodococcus erythropolis DSM 14303. Significant (P<0.001) reduction in AFB1 was observed following treatment with R. erythropolis extracellular extracts with only 33.20% residual AFB1 after 72 h. Results indicated that the degradation by R. erythropolis DSM 14303 is enzymatic and that the enzymes are constitutively produced. The degradation of AFB1 when treated with R. erythropolis DSM 14303 extracellular extract coincided with a total loss of mutagenicity. In addition, treatment of AFB1 with culture fractions containing recombinant 2,3-dihydroxybiphenyl dioxygenase, which was produced through extracellular expression of the bphC1 gene of R. erythropolis DSM 14303 in Escherichia coli BL21, resulted in significant (P<0.0001) degradation (49.32%) and reduced mutagenic potency (42.47%) of the molecule. Significant (P<0.0001-0.05) degradation of AFB1 was obtained following treatment with culture extracts containing laccase enzyme produced by white rot fungi (17.10- 76.00%), purified fungal laccase from Trametes versicolor (1 U/ml, 87.34%) as well as with recombinant laccase produced by Aspergillus niger (118 U/L, 55.00%). Furthermore, treatment of AFB1 with purified fungal laccase enzyme (1 U/ml) resulted in loss of the mutagenic potency of the molecule. The decrease in the fluorescence and mutagenic properties of AFB1 following treatment with the microbial preparations imply changes to the furofuran- and/or lactone rings of the molecule. The current study contributes towards developing genetic engineered microbial strains which could be applied as an important bio-control measure. Such strains could exhibit multifunctional technological properties including degradation of AFB1, to significantly improve the quality, safety and acceptability of food.