Enzyme-producing microorganisms and mycotoxin risk management
Use of enzymes in animal feed can offer a broader armory in controlling mycotoxin contamination of feed.
Enzymes have long been used to increase the nutrient digestibility of animal feed and, therefore, the efficiency of animal production, while also reducing the industry’s environmental impact. What might be new for some is that enzyme-producing microorganisms may be incorporated into animal feeds with a different objective - namely for the degradation of mycotoxins, scientifically referred to as biotransformation.
Although many animal producers may underestimate the existence of mycotoxins in cereals and animal feeds, the truth is that they occur more often than not. Currently, the search for good quality mycotoxin-free feedstuffs is an increasingly complicated task, as global warming affects crops, as grain prices are affected by their scarcity and as by-products are increasingly used as alternative ingredients.
Mycotoxins in the 2009 US corn harvest had a significant impact worldwide. This year, Australia is struggling with mycotoxin contamination in wheat after a very wet December and January, while still to be known are the effects of Japan’s tsunami on the country’s crops. These are just three of numerous examples of a single fact: mycotoxins are here to stay, so we should think about how to manage them.
With obvious differences from country to country, mycotoxin binders are now much more acknowledged and frequently used as part of feed formulations. The role of a toxin binder in an animal diet is to adsorb mycotoxins so reducing their bioavailability for the organism or, in other words, to avoid their absorption by the animal.
Binders are important control agents in the case of adsorbable mycotoxins, such as aflatoxins. However, what is the answer in the case of other widespread mycotoxins such as zearalenone, vomitoxin, T-2 toxin, fumonisins and ochratoxin A? For those, it is widely believed that adsorption by binders is not an efficacious solution and, often, the use of certain adsorptive materials may lead to greater problems due to the unspecific binding of essential nutrients. This is why biotransforming agents are imperative.
Biotransformation stands for the conversion of mycotoxins into less toxic molecules by enzymes or microorganisms. This degradation takes place in the gastro-intestinal tract of the animal consuming mycotoxin-contaminated feed. Initial research in the field of mycotoxin biotransformation started 40 years ago; however, so far, few microorganisms have shown the capacity of degrading mycotoxins and of those, even fewer can be used safely and in a stable way as animal feed additives.
For the elimination of the toxic effects of trichothecenes - a large family of more than 200 structurally similar mycotoxins, from which vomitoxin (deoxynivalenol or DON) and T-2 toxin are the most well-known members - Eubacterium BBSH 797 was isolated. The enzymes produced by this organism within the gastro-intestinal tract (for example epoxidases) play an important role by enabling the specific disruption of the toxic epoxy ring possessed by this group of mycotoxins (Figure 1).
Some years later, the non-pathogenic yeast T. mycotoxinivorans MTV was isolated, described and patented for its ability to degrade zearalenone and ochratoxin A. For the elimination of zearalenone’s negative effects it is vital that the lactone ring within the molecule is destroyed. This reaction is once again mediated by enzymes (e.g. esterases) (Figure 2). In doing so, zearalenone’s resemblance with the sexual female hormone estradiol is lost and therefore impairment of the reproduction system is avoided. In the case of Ochratoxin A, cleavage of the phenylalanine moiety results in the derivate ochratoxin alpha (Figure 3), considerably less toxic than the original molecule.
Production of the enzymes that mediate these reactions occurs within the gastrointestinal tract of the animal and, therefore, their production and activity cannot be quantified, as is the case of purified enzymes.
Their activity is not simple. Not only must they show a rapid degradation of the mycotoxin into less- or non-toxic metabolites, but they must also remain active at various pH values and in complex environments, with the presence of various metabolites and nutrients. Furthermore, the non-toxicity of these biotransforming microorganisms must be assured.
In conclusion, mycotoxin risk management strategies should comprise several components. The elimination of adsorbable mycotoxins, such as aflatoxins and ergot alkaloids can be achieved through adsorption; however, for the elimination of the toxicity of non-adsorbable mycotoxins, such as zearalenone, ochratoxins and trichothecenes, biotransformation is crucial. Biotransformation, which can be achieved through enzyme-producing microorganisms, results in the conversion of the toxic structure of mycotoxins into non-toxic, harmless metabolites.