Friday, 8 February 2019

Toxins of Aspergillus

Toxins of Aspergillus

Aflatoxins are potent carcinogens (Class 1; JECFA, 1997) affecting man and all tested animal species including birds and fish. Four compounds are commonly produced in foods: aflatoxins B 1 , B 2 , G 1 and G 2 , named for the colour of their fluorescence under ultra violet light, and their relative position on TLC plates.

Major sources. Aspergillus flavus is the most common species producing aflatoxins, occurring in most kinds of foods in tropical countries. This species has a special affinity with three crops, maize,peanuts and cottonseed, and usually produces only B aflatoxins. Onlyabout 40% of known isolates produce aflatoxin.

Aspergillus parasiticus occurs commonly in peanuts, but is quite rare in other foods. It is also restricted geographically, and is rare in Southeast Asia (Pitt et al., 1993). A. parasiticus produces both B and G aflatoxins, and virtually all known isolates are toxigenic.

Minor sources. Table 1 shows the species which are known to be capable of producing aflatoxin in culture, and some details concerning their appearance and their occurrence. Note that most of the minor species are known from only a very few isolates, and their occurrence in foodstuffs or feedstuffs is at most rare. On the other hand A. nomius, A. toxicarius, and A. parvisclerotigenus may be more common than expected, because it is very difficult to distinguish between those species and isolates may easily have been identified as A. flavus or A. parasiticus.

Cyclopiazonic acid
Cyclopiazonic acid (CPA) (Holzapfel, 1968) is a potent mycotoxin that produces focal necrosis in most vertebrate inner organs in high concentrations and affects the ducts or organs originating from ducts.It was originally believed that aflatoxins were responsible for all thetoxic effects of Aspergillus flavus contaminated peanuts to turkeys in Turkey X disease, but it was later shown that cyclopiazonic acid had an additional severe effect on the muscles and bones of the turkeys (Jand et al., 2005).

Major sources. Aspergillus flavus and the domesticated form A. oryzae often produce large amounts of CPA. A. flavus is common on oil seeds, nuts, peanuts and cereals, but may also produce aflatoxin on dried fruits (Pitt and Hocking, 1997).

Minor sources. Other producers of CPA in Aspergillus include A. tamarii, A. pseudotamarii, A. parvisclerotigenus, but the role of these fungi concerning CPA production in foods or feeds is not clear.

Cytochalasin E
Cytochalasin E is a very toxic metabolite of Aspergillus clavatus. It may occur in malting barley (Lopez-Diaz and Flannigan, 1997) Major source. Aspergillus clavatus. Minor source. Rosellinia necatrix is not found in foods.

Gliotoxin is strongly immunosuppressive, but is probably only a potential problem in animal feeds (Betina, 1989). Major sources. Aspergillus fumigatus has been found in animal feeds. Minor sources. Gliocladium virens, P. lilacinoechinulatum and few other soil-borne species also produce gliotoxin.

β-Nitropropionic Acid (BNP)
β-nitropropionic acid has been reported to be involved in sugar cane poisoning of children, but may potentially also cause other intoxications, as producers are widespread (Burdock et al., 2001). Furthermore BNP has been found in miso, shoyu and katsuobushi and it can be produced by A. oryzae when artificially inoculated on cheese, peanuts etc. Unfortunately A. flavus has not been tested for the production of BNP, but BNP production by A. oryzae on peanuts indicates that A. flavus may be able to produce this mycotoxin in combination with aflatoxin B 1 , cyclopiazonic acid and kojic acid. The possible synergistic effect of these mycotoxins on mammals is unknown.

A. flavus may be an important producer of this mycotoxin in foods, but there are no surveys that include analytical determination of BNP alongside cyclopiazonic acid and aflatoxin B 1 . A. oryzae and A. sojae can produce BNP in miso and shoyu, but it is probably more important that their wild-type forms, A. flavus and A. parasiticus respectively, may produce BNP in foods. More research is needed in this area. Minor sources. Penicillium atrovenetum is another authenticated producer of BNP, but this fungus is only found in soil.

Ochratoxin A 
Ochratoxin A (OA) is a nephrotoxin, affecting all tested animal species, though effects in man have been difficult to establish unequivocally. It is listed as a probable human carcinogen (Class 2B) (JECFA, 2001). Links between OA and Balkan Endemic Nephropathy have long been sought, but not established (JECFA, 2001).

Major sources. Aspergillus ochraceus (van der Merwe et al., 1965), occurring in stored cereals (Pitt and Hocking, 1997) and coffee (Taniwaki et al., 2003). A. ochraceus has been shown to consist of two species (Varga et al., 2000a, b; Frisvad et al., 2004b). The second and new species producing large amounts of ochratoxin A consistently, has been described as A. westerdijkiae. Actually the original producer of ochratoxin A from Andropogon sorghum in South Africa, NRRL 3174, has been designated as the type culture of A. westerdijkiae (Frisvad et al., 2004b). This is interesting as A. westerdijkiae is both a better and more consistent ochratoxin producer than A. ochraceus, and it may be also more prevalent in coffee than A. ochraceus. The ex type culture of A. ochraceus CBS 108.08 only produces trace amounts of ochratoxin A.

Petromyces alliaceus (Lai et al., 1970), produces large amounts of ochratoxin A in pure culture, and OA produced by this fungus has been found in figs in California (Bayman et al., 2002). Aspergillus steynii, from the Aspergillus section Circumdati, is also a very efficient producer of OA, and has been found in green coffee beans, mouldy soy beans and rice (Frisvad et al., 2004b). As with A. westerdijkiae, A. steynii may have been identified as A. ochraceus earlier, so the relative abundance of these three species is difficult to evaluate at present.

Sterigmatocystin is a possible carcinogen. However, its low solubil-ty in water or gastric juices limits its potential to cause human ill (Pitt and Hocking, 1997).

Major sources. The major source of sterigmatocystin in foods is Aspergillus versicolor. This fungus is common on cheese, but may also occur on other substrates (Pitt and Hocking, 1997). Minor sources. A large number of species are able to produce sterigmatocystin, including Chaetomium spp., Emericella spp., Monocillium nordinii and Humicola fuscoatra (Joshi et al., 2002). These species are unlikely to contaminate foods.

Verruculogen and Fumitremorgins
Verrucologen is an extremely toxic tremorgenic mycotoxin, but it is unlikely to be founding significant levels in foods. Neosartorya fisheri may be present in heat treated foods, but N. glabra and allied species are much more common in foods, and the latter species do not pro-
duce verrucologen.

Major sources. Aspergillus fumigatus and Neosartorya fischeri are the major Aspergillus species producing verruculogen but these species are uncommon in foods. These species produce many other toxic compounds including gliotoxin, fumigaclavins, and tryptoquivalins (Cole et al., 1977; Cole and Cox, 1981; Panaccione and Coyle, 2005).

Minor sources. Aspergillus caespitosus, Penicillium mononematosum and P. brasilianum are efficient producers of verrucologen and fumitremorgins, but are very rare in foods and feeds.

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