Fungi
Given the large number of fungal species, remarkably, only a very few species are deadly poisonous after ingestion. A more insidious risk for humans comes from various fungi that produce toxins in foods. Of particular importance are the fungal species that produce mycotoxins.
Mycotoxins is the name given to a group of potentially toxic substances produced by certain fungal species that grow on food crops pre- or post-harvest. Mycotoxins which have given rise to most concerns in terms of risk to health include aflatoxins, ochratoxins, deoxynivalenol, fumonosins, tricothecenes and ergot alkaloids. The fungal species associated with the production of these toxins include Aspergillus spp., Fusarium (Gibberella zeae) spp., Penicillium verruculosum and Claviceps purpurea. Mycotoxins are often present in foods at low levels. Consumption of these foods generally does not give rise to acute, and hence readily detectable, symptoms of poisoning although some cases of acute exposure have been reported. Instead, chronic exposure to mycotoxins is more likely. Mycotoxins are linked with producing neurotoxic, carcinogenic or teratogenic effects together with suppression of the immune system. Aflatoxins are produced by Aspergillus spp., mainly A. flavus, A. parasiticus and A. nomius that are found as contaminants in both human food stuffs and animal feed, particularly in maize, groundnuts and other nuts such as pistachios. Exposure to the toxins occurs through consumption of contaminated food and also during the handling and processing of aflatoxin contaminated crops. Aflatoxins have been detected in human milk, urine and blood samples. It has also been shown that ingestion of contaminated feed by cattle can produce milk contaminated with aflatoxin metabolites and that these substances pass to cheese and other dairy products during manufacture. Aflatoxin-B1 is a very potent human carcinogen that reacts with DNA once it has been bioactivated to the epoxide and is thought to cause hepatocellular carcinoma.
Ochratoxin-A (Fig. 4.4) is a widespread myco toxin produced mainly by the fungi A. ochraceus and Penicillium verruculosum during the storage of cereals, cereal products, herbs, spices and other plant-derived products like coffee. It has been found primarily in northern temperate barley- and wheat-growing areas. Consumption of mouldypig feed may result in detectable levels of ochratoxin-A in pork-derived products. Since ochratoxin-A is hydrolysed rapidly by ruminal flora, it is unlikely to be found in milk or meat from cattle. When ingested by humans, ochratoxin-A is very persistent, with an elimination half-life of about 35 days attributed to very strong binding to plasma proteins. Deoxynivalenol (DON) is a mycotoxin belonging to the trichothecene type of toxins produced by Fusarium graminearum (Gibberella zeae) and F. culmorum. These fungi grow on cereal crops, mainly wheat and maize. As wheat and maize products form a considerable part of the diet in many regions, the toxicity and the content of DON is an important issue in food safety control. DON can affect the immune system, and both suppression and activation have been reported, but the toxin is not considered to be mutagenic or carcinogenic. Outbreaks of human disease related to trichothecenes have been reported. Consumption of mouldy wheat or maize results within 5–30 minutes in nausea, vomiting, abdominal pain, diarrhoea, dizziness and headache.
Zearalenone is a benzoxacyclotetradecin derivative produced in F. graminearum (Gibberella zeae) and related species, and is primarily associated with maize (Zea mais). It is among the most widely distributed mycotoxins. Toxic effects in humans are extremely difficult to assess as, in cases of contaminated cereals, several mycotoxins are present simultaneously. Endocrine disrupting effects occur in animals, but these were not reported in two outbreaks in which both DON and zearalenone were involved (IARC 1993).
A number of analytical methods have been described for the analysis of aflatoxins in food stuffs. HPLC with fluorescence detection is considered to be the method of choice, with pre or post-column detection with bromine or iodine for the B and G aflatoxins, or trifluoroacetic anhydride for the M aflatoxins to improve sensitivity. More recently, LC-MS/MS methods have been developed. Immunoassay methods are available and TLC can also be used and is particularly useful as a screening procedure. Several analytical procedures have been proposed for the identification and measure ment of ochratoxin-A in food products, as reviewed by Van Egmond (1991), Valenta (1998) and Gilbert and Vargas (2003).
Poisoning by ergot alkaloids produced by the mould Claviceps purpurea is usually referred to as ergotism, and is probably the oldest recorded food-borne disease of fungal origin. Cl. purpurea grows on food grain, particularly rye, during wet seasons. Symptoms of ergotism include erythema, diarrhoea, vomiting, a burning sensation of the limbs and, eventually, gangrene. CNS effects are convulsions, catalepsy, dullness or maniacal excitement. These symptoms can be explained by the pharmacological properties of the ergot alkaloids, which may cause an alphaadrenergic blockade, as well as serotonin (5 hydroxy tryptamine; 5-HT) antagonism. The ergot alkaloids ergotamine and ergometrine, or ergonovine, have been used for centuries as therapeutic agents to stimulate uterine contractions and in the treat ment of migraine attacks. Overdose with these agents, and intoxication with ergot alkaloids from other sources, can be treated symptomatically with potent vasodilator drugs, such as sodium nitroprusside, and by maintaining adequate circulation. The method of choice for analysis of ergot alkaloids in serum is HPLC with fluorescence detection after liquid–solid extraction. More recently a number of LC-ESI-MS/MS methods have been developed. The structural chemistry and a comparison of the available analytical techniques for ergot alkaloids have been reviewed extensively by Flieger et al. (1997).
