Decontamination of Mycotoxin-contaminated Commodities by Biological Agents
Biological detoxification can be defined as the enzymatic degradation or biotransformation of mycotoxins that can be obtained by either the whole cell or an enzyme system. Other approaches that can be regarded as biological include the use of biocompetitive agents and genetically engineered plants for reducing mycotoxin contamination.
Aflatoxins
Field treatments with non-toxigenic strains of Aspergillus flavus and A. parasiticus have been shown to reduce aflatoxin contamination by successfully competing with the indigenous strains. This approach produced very good results for soilborne crops, such as peanuts and cottonseed, with a reduction of aflatoxin contamination of up to 99.9%. For airborne crops, such as maize, a lower efficacy was obtained with a reduction of aflatoxin contamination up to 87%. Field application of non-toxigenic strains had a carry-over effect and reduced aflatoxin contamination in peanuts in storage conditions.
Only one bacterium, Flavobacterium aurantiacum, among one thousand microorganisms, including yeast, moulds, and bacteria, screened for their activity to degrade aflatoxins, was able to irreversibly remove aflatoxin B1 (AFB1) from both solid and liquid media. The ability of this microorganism to remove AFB1 from foods was demonstrated in vegetable oil, peanut, corn, peanut butter and peanut milk. Recent studies indicated that the factor responsible for degradation of AFB1 by extract of F. aurantiacum may be a protein with enzyme characteristics. The possibility of using the purified crude protein extracts in the food industry was suggested as a means of removing aflatoxins from contaminated foods.
Other microorganisms including Rhizopus spp., Corynebacterium rubrum, Candida lipolytica, Aspergillus niger, Trichoderma viride, Mucor ambiguous, Neurospora spp., Armillariella tabescens, and lactic acid bacteria, have been tested in in vitro systems with varying results. When degradation compounds were investigated, formation of aflatoxicol, a compound having the same carcinogenicity of AFB1, was observed.
Fumonisins
Two species of black yeast fungus (Exophiala spinifera, Rhinocladiella atrovirens) and a Gram-negative bacterium (Caulobacter spp.) isolated from moldy maize kernels have been found to extensively metabolize fumonisins to CO2 in liquid culture. These microorganisms produce fumonisin catabolizing enzymes, such as esterase which lead to the formation of hydrolyzed Fumonisin B1 (AP1) plus tricarballylic acid and deaminase, which in turn form N-acetyl AP1 plus 2-OP1 hemiketal. AP1, has reduced toxicity in comparison to the intact FB1. Fumonisin esterase enzymes were expressed in transgenic maize plants. Lower levels of fumonisin B1 accompained by an accumulation of AP1, were observed in kernels of transgenic maize plants as compared to conventional maize plants.
Maize kernel injuries caused by insects, such as the European corn borer and pink borer, promote Fusarium infection with consequent accumulation of fumonisins. Levels of fumonisins in transgenic maize hybrids with kernel expression of insecticidal Bacillus thuringiensis protein, CryIA(b) were lower than in conventional maize hybrids. In particular, fumonisin levels were 4.9-11.8 microgrammes/g and 1.2-1.3 microgrammes/g in conventional and transgenic maize hybrids respectively, in field experiments carried out over two years.
The major objective of an ongoing EU funded research project (Safemaize, INCO-DEV programme) is to develop improved maize genotypes with increased resistance to Fusarium verticillioides, a well known fumonisin producer, using traditional breeding as well as biotechnology. More details on the results obtained so far can be found at:
http://www.up.ac.za/academic/botany/safemaiz.html
http://www.agron.missouri.edu/mnl/77/39lanzanova.html
http://www.agron.missouri.edu/mnl/77/40conti.html
Trichothecenes
The 12,13-epoxide ring is essential for the toxicity of these mycotoxins, and removal of this ring results in a significant loss of toxicity. Ruminal or intestinal microflora are capable of detoxifying deoxynivalenol (DON) by enzymatic reduction of the epoxide ring resulting in the metabolite DOM-1 that is known to be non-toxic. A pure anaerobic bacterial strain (Eubacterium), capable of the biotransformation of DON to DOM-1, was isolated from an enriched mixed culture obtained from bovine rumen content. This bacterium transformed DON and other trichothecenes within 24-48h in in vitro experiments using pieces of pig intestine.
Treatments of moldy corn contaminated with approximately 5 microgrammes/g of DON with microbial inoculum from the digestive tract of poultry reduced the DON concentration by 54%. This decontamination process also partially alleviated the toxic effects of this diet on feed intake and body weight gain in young pigs.
A bacterium isolated from soil and belonging to the Agrobacterium-Rhizobium group was found to transform 70% of DON to 3-keto-DON after a 1-day incubation. This metabolite exhibited a reduced immunosuppressive toxicity as compared to DON. The bacterium showed the same activity against 3-acetyl-DON but not against other trichothecenes such as nivalenol and fusarenone X.
The selection and field-testing of competitive fungi for controlling ear infection by toxigenic Fusarium spp. in cereals have been performed in a recent EU-funded project (Control Mycotox Food). The studies have shown that some antagonists, including non-toxigenic Fusarium species, can effectively decrease Fusarium Head Blight on wheat and significantly control DON production by >70%, as well as that obtained with present fungicides.
The objective of the ongoing EU-funded project FUCOMYR is to reduce mycotoxins contamination in wheat, Europe’s most important cereal crop, at the pre-harvest stage by means of improved Fusarium Head Blight resistance.
Zearalenone
A variety of microorganisms including bacteria, yeasts and
fungi are able to convert Zearalenone (ZEA) to
-
and
-zearalenol.
However this transformation cannot be regarded as detoxification since the
oestrogenicity activity of these metabolites is similar to that of ZEA.
Ochratoxin A
Of the microorganisms screened for their ability to degrade
ochratoxin A (OTA), Acinetobacter calcoaceticus,
Phenylobacterium immobile, and a non-toxigenic strain of
Aspergillus niger were reported to convert OTA to the less toxic
-OTA.
The microbial flora of the mammalian gastrointestinal tract, including rumen
microorganisms of the cow and sheep, were also reported to degrade OTA to-OTA
although the microorganisms responsible for OTA degradation were not identified.
References
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