Fungi pyrrolizidine

Some natural products, or their degradation products, represent a hazard for mammals not because of general toxicity but for subtle, adverse properties, such as carcinogenicity and tumor promotion. They are best known fi om marine dinoflagellates (okadaic acid and structural analogues), filamentous fungi (trichothecenes and ochratoxins), and plants (pyrrolizidine alkaloids). 

In relation with Crotalaria spp. secondary metabolites, it is well known that these plants produce pyrrolizidine alkaloids and monocrotaline which have high vertebrate toxicity and could potentially be toxic to nematodes but it is possible also that the low C/N ratio of Crotalaria may also contribute to its allelopathic effect against nematodes. Materials with very low C/N or high content of ammonia will either result in plasmolysis of nematodes, or proliferation of nematophagous fungi due to the release of NH4+-N (Rich and Rahi, 1995). 

We have seen how micro-organisms such as fungi, bacteria, and plankton can cause toxic contamination of food. Plants themselves may also have naturally occurring toxic constituents. For example, as we saw in the discussion on herbal remedies in Chapter 6, the pyrrolizidine alkaloids are found in many plants which may contaminate crops such as wheat and hence find their way into the food we consume. 

Macrolide-producing organisms are classified as shown in Table 11. Actino-mycetes produce the largest number of macrolides (more than 800). Myxobacte-ria produce about 100 macrolides, and the other bacteria produce about only 40 macrolides. A rather large number of macrolides have been isolated from fungi (about 200). Lichens produce only three compounds. About 70 macrolides have been isolated from algae. Though plants produce about 700 macrocyclic lactone compounds, most of them are tannins or alkaloids such as pyrrolizidines. So they... 

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