Fungi
Page 12
Widespread recreational use of LSD in North America and Europe began in the 1960s, at the same time that psychedelic or ‘magic’ mushrooms became a symbol of anti-establishment counterculture. The most potent psychoactive compound in these fungi is psilocybin, which is produced by more than 200 mushrooms including the Psilocybe species. Psilocybin is converted to psilocin after the mushrooms are eaten. The chemical structure of psilocin is very similar to serotonin, which is a neurotransmitter associated with feelings of happiness. When psilocin binds to serotonin receptors in the brain, the disturbance to neurological function causes euphoria and a range of hallucinatory experiences.
Clinical studies on test subjects given purified psilocybin reveal that the drug causes a temporary reduction in blood flow to certain parts of the brain that diminishes neurological activity. Stimulation of crosstalk between regions of the brain that do not normally communicate is another effect of psilocybin (psilocin). This ‘synaesthesia’ can result in the perception of colours when listening to music. Other common experiences include visions of geometric patterns, altered perception of the passage of time, and stimulation of memory. Some people who consume magic mushrooms have very frightening experiences, but the frequent positive responses to psilocybin have encouraged studies on its use to treat depression. Different compounds in fly agaric mushrooms (Amanita muscaria) raise serotonin and dopamine levels and induce a range of hallucinatory experiences.
Mushroom poisoning is a terrifying consequence of picking the wrong species when people are searching for magic mushrooms or for edible species. The horror of these life-changing and life-ending errors is one of the reasons that mycologists with skills in mushroom identification are so valuable. The most potent mushroom toxins include amatoxins, produced by the death cap (Amanita phalloides), which cause liver failure, and orellanine and cortinarins, found in webcaps (Cortinarius species), which attack the kidneys and liver. Gyromitrin is a toxin produced by a species of false morel, Gyromitra esculenta. False morels are ascomycetes. Their fruit bodies have stalks and convoluted heads that resemble the shapes of ‘standard’ basidiomycete mushrooms, but their spores are dispersed from the exposed surface of the cap rather than falling from protected gills. When these mushrooms are eaten, gyromitrin is converted into monomethylhydrazine which is a chemical used in rocket fuel. Nausea and vomiting are the usual symptoms of poisoning, but organ damage has been reported in a few cases and some victims have died. Despite the risk, false morel aficionados enjoy eating this fungus after boiling the fruit bodies and discarding the water to remove the toxin.
The biological reasons that fungi produce these remarkable hallucinogens and toxins are not understood. It seems likely that some of these molecules work as pesticides that protect fruit bodies against insects and other invertebrates. Humans, in other words, are not the intended target for these compounds. Very few of the 16,000 mushroom species are poisonous and a few, as most readers will agree, combine safety with delicious flavours. In Chapter 8 we turn to the pleasures of edible mushrooms and other ways in which fungi are used to improve our lives.
Chapter 8
Edible mushrooms and fungal biotechnology
It is appropriate to end this short book with a chapter on the uses that we have made of our mycological wisdom. The pleasure of eating wild mushrooms, tempered with awareness of the poisonous nature of a few species, is an ancient experience born from our history as hunter-gatherers in forest ecosystems. Cultivation of edible mushrooms is among the oldest biotechnological uses of fungi, although brewing and baking with yeast go back even further. Until the 19th century, beer, wine, and bread were made by people who utilized the physiological activities of fungi without knowing that microorganisms existed. Artisanal uses of fungi in cheesemaking and the fermentation of a fabulous array of Asian foods are similar instances of the practical uses of fungi in the absence of scientific knowledge. From these long-standing interactions between humans and fungi, we come to the modern intentional application of fungi for the production of antibiotics and a plethora of other medicines, fermentation of meat substitutes, synthesis of industrial enzymes and acids, and manufacture of biofuels.
Picking wild mushrooms
People pick wild mushrooms in the pursuit of mycological knowledge, to jazz up their diet, and to make money. Mushroom identification is a tricky business and there is no substitute for learning from a mycologist with years of practical experience. Some species are more difficult to identify than others. The magnification of specimens with a hand lens is often helpful and indoor study ranges from the collection of spore prints from mushroom caps and detailed examination of mushroom tissues and spores using a microscope. Photographic records are also important. Academic study involves molecular identification and the collection and drying of specimens for deposition in a herbarium. Mycological societies play an important role in teaching people to identify mushrooms and this often involves gathering large numbers of fruit bodies and their display on tables. This seems wasteful when the haul of shrivelling fungi is thrown away at the end of the day, but this is more of an aesthetic concern than an issue of sustainability. Mushroom picking for cooking is more popular in some cultures than others and there is an overlap between the study and consumption of wild mushrooms among amateur mycologists.
Trade in wild mushrooms occurs in many regions, with markets in France and elsewhere in Europe for a range of edible species including ceps or porcini (Boletus edulis), girolles (chanterelles, Cantharellus cibarius), and the beautiful orange Caesar’s mushroom (Amanita caesarea). Specialized seasonal markets sell white truffles (Tuber magnatum) in northern Italy and black Périgord truffles (Tuber melanosporum) in southern France. Restaurateurs bid for these delicacies in international auctions. Species of Terfezia and Tirmania are desert truffles that grow in association with Helianthemum bushes in semi-arid and arid deserts from Morocco to Saudi Arabia. These expensive ‘mushrooms’ are very popular in North Africa and the Middle East. The reproductive mechanisms of fungi that rely on the production of vast numbers of spores by short-lived fruit bodies encourages the idea that mushroom picking is a harmless pastime. There are some limits to this assumption.
When mushroom picking becomes a commercial activity that encourages the wholesale removal of fruit bodies from particular areas every year, the sustainability of the practice is questionable. A thirty-year experiment on the impact of mushroom picking in Switzerland showed that intensive picking had no effect on subsequent crops as long as the pickers worked from a catwalk. This indicated that mycelia can be very resilient. But because mushroom spores are dispersed by wind, the loss of the genetic diversity derived from fruit bodies in one patch might have wider consequences that are not seen in a study confined to one area. There are more immediate concerns raised by the Swiss study. When the investigators left the catwalk to pick normally, fewer mushrooms were formed in subsequent years in the trampled areas. This means that intensive picking has the potential to do serious damage to mushroom populations.
Mushroom harvesting in the Pacific Northwest of the United States is one area of concern, and in China, where annual exports of wild mushrooms may run into the hundreds of thousands of tons of fruit bodies, the future of the ‘industry’ seems bleak. Ophiocordyceps sinensis is an interesting case. This parasite of moth caterpillars is related to the zombie ant fungus described in Chapter 7. It fruits in alpine meadows in the Tibetan Plateau and Himalayas and is used in Chinese and Tibetan medicine. Trade in the ‘caterpillar fungus’ supports a global market valued between US$5 billion and $11 billion. One study showed that the harvest in Nepal fell by 50 per cent between 2009 and 2011. Management of this natural resource is complicated by the reliance of many people on the fungus as a source of income. The same issue is raised in the Pacific Northwest as a defence of people who gather chanterelles, boletes, and matsutake mushrooms. Without management, however, these mushroom grounds, like many commercial fisheries, will collapse.
Mushroom culti
vation
Mushroom cultivation offers a sustainable source of fruit bodies of a limited number of domesticated fungi. The white button mushroom, Agaricus bisporus, is the most popular species, followed by shiitake, Lentinula edodes. Agaricus bisporus is grown on straw blended with animal manure and gypsum. This mixture is composted by bacteria and then pasteurized by steaming before inoculation with mushroom spawn. Mushroom spawn is prepared by growing Agaricus bisporus on cereal grains. On modern farms, beds of compost are supported on metal shelves in growing rooms in which the environmental conditions are controlled to optimize mycelial development. After two to three weeks, fruiting is stimulated by casing the compost with a layer of peat and limestone, lowering the temperature in the growing rooms, and increasing airflow to drop carbon dioxide levels. Mushrooms appear in a few weeks and three flushes can be picked from a typical bed.
Agaricus bisporus is the mycological equivalent of the banana, an agricultural oddity with too little genetic diversity. Unlike wild species, mycelia that grow from single spores of the white button mushroom can form a new generation of fruit bodies without mating. Reproduction is clonal, which is a good thing from the perspective of product consistency but worrying from the viewpoint of pest resistance. Mushroom breeders have had some successes in strain development and different varieties of this fungus have been produced that form darker crimini mushrooms rather than white buttons. Older fruit bodies of the crimini strains are sold as portabellos.
Shiitake mushrooms are grown on slender hardwood logs arranged in stacks or propped against one another. Plugs of shiitake spawn are pressed into holes drilled into the logs and sealed with wax. If everything goes well, the mycelium will rot the wood over the following six to eighteen months and crops of mushrooms can be harvested for up to five years from a productive stack of logs. Shiitake was grown in this fashion in China for more than 1,000 years before it became popular in Western countries. China ‘owns’ the cultivated mushroom market today, producing 90 per cent of the world’s shiitake and more than one-third of white button mushrooms. Other cultivated mushrooms that account for slivers of the market share include the wood ear (Auricularia auricula) and silver ear (Tremella fuciformis). Both of these species are wood-rotting basidiomycetes that form jelly-like fruit bodies. They are sold in a dried form and are popular in Asian cuisine.
Medicinal mushrooms and antibiotics
Mushrooms have been used as medicines since the Neolithic. Two species of bracket fungi, or polypores, threaded on strips of animal hide, were found on the mummified body of ‘Ötzi the iceman’, who died 5,300 years ago in the Alps. One of these fruit bodies was probably used as tinder, but the dried chunk of the birch polypore, Piptoporus betulinus, may have been carried for its medicinal properties. Today’s controversy surrounding the use of medicinal mushrooms arises from conflict between the traditions of East Asian medicine and the evidence-based medicine of the West. Shiitake, for example, has been used since the Ming Dynasty as an energy boosting ‘tonic’ that is recommended for the elderly and for patients convalescing after serious illnesses. It is also prescribed as a specific treatment for respiratory disease, liver disease, and parasitic worms.
The absence of critical experimental support for the effectiveness of shiitake in any of these applications is of little interest to advocates of ‘Chinese’ medicine. They argue that their approaches to patient care are so different from those practised in modern medicine that they cannot be assessed using the same standards. There is some truth to this objection. Prescription drugs used in Western medicine contain one or a few active ingredients and are subject to testing in placebo-controlled clinical trials. It is difficult to apply the same criteria for medicinal mushrooms because they may contain many active ingredients whose levels differ from sample to sample. Yet without clinical trials, physicians and patients have no basis for using mushrooms other than trust in the stories told by people who claim to have benefited from their use. And one of the problems with this faith in anecdote is that companies that market mushroom extracts prey upon consumers by making unsubstantiated claims. This is particularly distressing when people with serious illnesses are fooled into believing that mushrooms can compensate for the inevitable limitations of modern medicine.
Putting aside the claims about miracle cures offered by mushroom extracts, there are good reasons for believing that fruit bodies do contain compounds with powerful pharmacological properties. The toxins of death caps and web caps are obvious examples of molecules that have a powerful effect on human physiology and mushrooms produce a treasure trove of metabolites of unknown function that might have beneficial medical applications. Most of the research on the properties of specific mushroom extracts has been performed on cultured cells and on laboratory mice. These studies have shown that cell wall components from shiitake and the turkey tail mushroom (Trametes versicolor) stimulate cells in the immune system and may have some protective effects against tumour growth. Experimental treatment of cancer patients with these compounds has had mixed results, but these limited trials certainly encourage further investigation. The combined use of mushroom extracts with established chemotherapeutic agents has produced some of the most encouraging results.
While the medicinal value of mushrooms is unproven, other fungi are a verified source of ‘miracle’ drugs. The discovery and development of antibiotics synthesized by filamentous fungi are triumphs of Western medicine. In a time of increasing concerns about antibiotic-resistant bacteria it is easy to forget how the discovery of penicillin in 1928 by Alexander Fleming changed our lives. Fleming shared the Nobel Prize in Physiology or Medicine in 1945 with Howard Florey and Ernst Chain, who developed penicillin for clinical use. Florey and Chain led a research group at Oxford University and tested the antibiotic on the first patient in 1941. Penicillins are secreted by species of the ascomycete Penicillium and belong to a class of antibiotics that share a common motif called the β-lactam ring. These compounds work against bacteria by weakening their cell walls, which causes them to burst. Resistant bacteria produce enzymes that inactivate the antibiotics by opening their ring structure.
Penicillin is produced by Penicillium chrysogenum grown on lactose (carbon source) and yeast extract (nitrogen source) in industrial fermenters. Synthesis of penicillin rises as the fungus depletes the lactose and production is stimulated by aeration and agitation. Meticillin and ampicillin are examples of semisynthetic antibiotics produced by the addition of side chains to the core ring structure of penicillin. Meticillin (formerly methicillin) has been replaced with other antibiotics with the emergence of meticillin-resistant strains of bacteria including Staphylococcus aureus (MRSA). Ampicillin remains effective against a wide range of bacteria. Cephalosporins are β-lactam antibiotics produced by another ascomycete, Acremonium chrysogenum. These are used as the last line of defence against MRSA. Filamentous fungi are a source of many other pharmacological agents. Lovastatin is a cholesterol-lowering drug that is produced by Aspergillus terreus. This is marketed as Mevacor® and a synthetic derivative is trademarked as Zocor®. Cyclosporin A is synthesized by a soil fungus called Tolypocladium inflatum. This compound suppresses the immune system and is used to support patients following bone marrow and organ transplantation. Cyclosporin is also an effective treatment for psoriasis, severe dermatitis, and rheumatoid arthritis.
In addition to their importance in pharmaceutical synthesis, fungi are a key source of industrial enzymes that are used in food and beverage industries, personal care products, laundry detergents, textiles, and leather manufacture. Fungal cellulases and phytases are used in the production of livestock feed, and laccases are added to breath fresheners to destroy the compounds responsible for halitosis. Laundry detergent formulations contain fungal cellulases that increase the softness and brightness of clothing. These enzymes work in cold water, which saves energy, and their effectiveness at removing dirt reduces water consumption. Other products containing fungal enzymes are effective at bleaching raw cotto
n before it is dyed, whitening paper, and removing gums from natural fibres.
Genetic engineering has become very important in fungal technology. Various methods are used to transform fungi with foreign genes so that they produce enzymes that are not part of their natural repertoire of proteins. This is necessary because some species that are sources of valuable proteins do not grow well in fermentation vessels or do not produce commercial levels of their products. If another species can be coerced to make more of the foreign protein it may serve as a very profitable living factory. Trichodermi reesei is a cellulose-decomposing fungus that has been transformed in this fashion to generate high yields of enzymes that are used in many industries. Organic acids are another fungal product category with wide applications. Aspergillus niger is used to produce citric acid, which is used as a preservative and food flavouring, and is an additive to water softeners and fertilizers. Gluconic acid and itaconic acid produced by Aspergillus and Penicillium species have applications in metal finishing, cement manufacture, and polymer synthesis.