Enzymatic inactivation of fungal toxins is an attractive strategy for the decontamination of agricultural commodities and for the protection of crops from phytotoxic effects of fungal metabolites. This review summarizes research on the biological detoxification of fungal toxins by microorganisms and plants and its practical applications. Some mycotoxins are detoxified during ensiling and other fermentation processes (aflatoxins, alternariol, mycophenolic acid, patulin, PR toxin) while others are transformed into toxic products or survive fermentation unchanged. Plants can detoxify fomannoxin, fusaric acid, HC-toxin, ochratoxin A and oxalate but the degradation of deoxynivalenol has yet to be proven. Microflora of the digestive tract of vertebrates and invertebrates exhibit detoxification activities towards aflatoxins, ochratoxin A, oxalate and trichothecenes. Some toxin-producing fungi are able to degrade or transform their own products under suitable conditions. Pure cultures of bacteria and fungi which detoxify mycotoxins have been isolated from complex microbial populations by screening and enrichment culture techniques. Genes responsible for some of the detoxification activities have been cloned and expressed in heterologous hosts. The detoxification of aflatoxins, cercosporin, fumonisins, fusaric acid, ochratoxin A, oxalic acid, patulin, trichothecenes and zearalenone by pure cultures is reviewed. Finally, current application of these results in food and feed production and plant breeding is summarized and expected future developments are outlined.

Biological detoxification of fungal toxins and its use in plant breeding, feed and food production.

Degradation of aflatoxin B1 by fungal laccase enzymes

Human activities affect both the size and structure of fungal populations. Construction, war, recreation, and agriculture disrupt large expanses of vegetation and soil; disruption causes redistribution of fungal propagules and makes nutrients available to fungi. Many fungi, including the aspergilli, exploit these human engineered resources. This results in the association of large fungal populations with various human activities, especially agriculture. When crops are grown or animals raised, fungi are also grown. From a human perspective, most fungi associated with cultivation increase inadvertently. Human activity, however, partly dictates which and how many fungi occur and the fungi, both directly and through fungal products, influence human activities, domestic animals, and even humans themselves.

Agriculture, Aflatoxins and Aspergillus

Biological degradation of aflatoxin B1 by Rhodococcus erythropolis cultures

Enzymes and aflatoxin biosynthesis.

Aflatoxin formation, lipid synthesis, and glucose metabolism by Aspergillus parasiticus during incubation with and without agitation

Aflatoxins B1 B2, G1, and G2 were degraded by the 8- and 16-day old but not by the 4-day old mycelium of a toxigenic strain ofAspergillus parasiticus. The 16-day old mycelium degraded the toxin more rapidly than did the 8-day old mycelium. Degradation of toxin by the mycelium was similar at pH 2.5 and 6.0.

Aflatoxin can be degraded by the mycelium of aspergillus parasiticus.

Spores of an aflatoxinogenie strain ofAspergillus parasiticus were inoculated into a glucose-salts medium and incubated at 28° C for 5 days when both mold growth and aflatoxin production were nearly maximal. The mold mycelium thus produced was recovered by filtration, washed three times with distilled water, placed in a nitrogen-free liquid medium (so growth was not possible), and held for 45 h. Samples of medium were tested periodically for aflatoxin content to determine release of toxin by the mycelium. When the mycelium was in a medium with 5% glucose and was held at 7° C; 17, 33, 39, and 56% of the aflatoxin from the mycelium appeared in the liquid after 1, 9, 21, and 45 h, respectively. Aflatoxin B1 was lost by the mycelium more slowly than were B2, G1, and G2. At 28° C, values for the same time intervals were 27, 60, 70, and 76%, respectively, and at 45° C they were 32, 71, 71, and 79%. At 28 and 45° C, aflatoxins G1 and G2 were lost by the mycelium more rapidly than were aflatoxins B1 and B2. Elimination of glucose from the medium at 28° C hastened, and use of 50% instead of 5% glucose markedly reduced release of aflatoxin by the mycelium during early stages of the incubation period.

Release of aflatoxin from the mycelium of Aspergillus parasiticus into liquid media

Spores of an aflatoxigenic strain ofAspergillus parasiticus were inoculated into a glucose-salts medium which was incubated with and without shaking at 28° C for 15 days Without shaking, maximal production of total aflatoxin and aflatoxins B1, G1, and G2, occured at 5 days, whereas the maximal amount of B2 appeared after 7 days Initially approximately 5% of the total toxins appeared in the mycelium but this increased to more than 60% after 5 days Shaking of cultures during incubation served to reduce production of total aflatoxin and of each of the individual toxins The maximal amount of total aflatoxin and of toxins B1, and G1, appeared in shaken cultures after 5 days, whereas 8 and 11 days were needed to obtain maximal amounts of B2 and G2, respectively The mycelium of shaken cultures initially retained approximately 50% of the total aflatoxin and this increased to about 80% as the incubation progressed Very little aflatoxin was synthesized at 35 and 45° C and production of total aflatoxin and of each individual toxin was less at 15° C than at 25 or 28° C

Chun-Nan Shih

Papers

Aflatoxin formation, lipid synthesis, and glucose metabolism by Aspergillus parasiticus during incubation with and without agitation

Aflatoxin can be degraded by the mycelium of aspergillus parasiticus.

Release of aflatoxin from the mycelium of Aspergillus parasiticus into liquid media

Production of aflatoxin and its partition between the medium and the mycelium ofaspergillus parasiticus during incubation under various conditions