Introduction to Fungi and their Nutritional, Medicinal and Technological Uses

In this introductory article we will briefly define fungi, outline the history of mycology, examine some of their interactions with other organisms before discussing the significance of fungi in nutrition, medicine and technology and industry. A fungus is a member of a large group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, Fungi, which is separate from plants, animals and bacteria - please see Simon Harding's article on bacteria in this series. One major difference is that fungal cells have cell walls that contain chitin, unlike the cell walls of plants, which contain cellulose. These and other differences show that the fungi form a single group of related organisms, named the Eumycota (true fungior Eumycetes), that share a common ancestor. This fungal group is distinct from the structurally similar slime molds and water molds. The discipline of biology devoted to the study of fungi is known as mycology, which is often regarded as a branch of botany, even though genetic studies have shown that fungi are more closely related to animals than to plants !

Abundant worldwide, most fungi are inconspicuous because of the small size of their structures, and their hidden lifestyles in soil, on dead matter, and as symbionts of plants, animals, or other fungi. They may become noticeable when fruiting either as mushrooms or molds. Fungi perform an essential role in the decomposition of organic matter and have fundamental roles in nutrient cycling and exchange. They have long been used as a direct source of food, such as mushrooms and truffles, as a leavening agent for bread, and in the fermentation of various food products, such as wine, beer and soy sauce which we will discuss in our nutrition section below.

Since the 1940s, fungi have been used for the production of antibiotics, and, more recently, various enzymes produced by fungi are used industrially and in detergents. Fungi are also used as biological pesticides to control weeds, plant diseases and insect pests. Many species produce bioactive compounds called mycotoxins that may be toxic to animals including humans. The fruiting structures of a few species contain psychotropic compounds and are consumed recreationally or in traditional spiritual ceremonies. Fungi can break down manufactured materials and buildings, and become significant pathogens of humans and other animals. Losses of crops due to fungal diseases (e.g. rice blast disease) or food spoilage can have a large impact on human food supplies and local economies.


History

As far as we can tell mushrooms were probably first written about in the works of Euripides (480-406 B.C.). The Greek philosopher Theophrastos of Eressos (371-288 B.C.) was perhaps the first to try to systematically classify plants; mushrooms were considered to be plants that were missing certain organs. Pliny the Elder (23–79 A.D.), also who wrote about truffles in his encyclopedia Naturalis historia.

The Middle Ages saw little advancement in the body of knowledge about fungi. Rather, the invention of the printing press allowed some authors to disseminate superstitions and misconceptions about the fungi that had been perpetuated by the classical authors.

The start of the modern age of mycology begins with Pier Antonio Micheli's 1737 publication of Nova plantarum genera. Published in Florence, this seminal work laid the foundations for the systematic classification of grasses, mosses and fungi. The term mycology and the complementary mycologist were first used in 1836 by M J Berkeley.

Ecology of Fungi

Although often inconspicuous, fungi occur in almost every environment on Earth and play very important roles in most ecosystems. Along with bacteria, fungi are the major decomposers in most terrestrial (and some aquatic) ecosystems, and therefore play a critical role in biogeochemical cycles and in many food webs. See Dr Harding's essay on food webs in this series. As decomposers, they play an essential role in nutrient cycling, especially as saprotrophs and symbionts, degrading organic matter to inorganic molecules, which can then re-enter anabolic metabolic pathways in plants or other organisms

Symbiosis

Many fungi have important symbiotic relationships with organisms from most if not all Kingdoms. These interactions can be mutualistic or antagonistic in nature, or in the case of commensalfungi are of no apparent benefit or detriment to the host. Lets look at some example with plants and insects.

With plants

Symbiosis between plants and fungi is one of the most well-known plant–fungus associations and is of significant importance for plant growth and persistence in many ecosystems; over 90% of all plant species engage in mycorrhizal relationships with fungi and are dependent upon this relationship for survival. Mycorrhizal symbiosis is ancient, dating to at least 400 million years ago. It often increases the plant's uptake of inorganic compounds, such as nitrate and phosphate from soils having low concentrations of these key plant nutrients. The fungal partners may also mediate plant-to-plant transfer of carbohydrates and other nutrients. Such mycorrhizal communities are called "common mycorrhizal networks". A special case of mycorrhiza is myco-heterotrophy, whereby the plant parasitizes the fungus, obtaining all of its nutrients from its fungal symbiont. Some fungal species inhabit the tissues inside roots, stems, and leaves, in which case they are called endophytes. Similar to mycorrhiza, endophytic colonization by fungi may benefit both symbionts; for example, endophytes of grasses impart to their host increased resistance to herbivores and other environmental stresses and receive food and shelter from the plant in return.

With algae and cyanobacteria

Lichens are formed by a symbiotic relationship between algae or cyanobacteria (referred to in lichen terminology as "photobionts") and fungi , in which individual photobiont cells are embedded in a tissue formed by the fungus. Lichens occur in every ecosystem on all continents, play a key role in soil formation and the initiation of biological succession and are the dominating life forms in extreme environments, including polar, alpine and semiarid desert regions ( see Dr Harding's articles on polar and desert environments in this series). They are able to grow on inhospitable surfaces, including bare soil, rocks, tree bark, wood, shells, barnacles and leaves. The photobiont provides sugars and other carbohydrates via photosynthesis, while the fungus provides minerals and water. The functions of both symbiotic organisms are so closely intertwined that they function almost as a single organism; in most cases the resulting organism differs greatly from the individual components. Characteristics common to most lichens include obtaining organic carbon by photosynthesis, slow growth, small size, long life, long-lasting (seasonal) vegetative reproductive structures, mineral nutrition obtained largely from airborne sources, and greater tolerance of desiccation than most other photosynthetic organisms in the same habitat.
With insects

Many insects also engage in mutualistic relationships with fungi. Several groups of ants cultivate fungi as their primary food source, while ambrosia beetles cultivate various species of fungi in the bark of trees that they infest. Similarly, females of several wood wasp species inject their eggs together with spores of the wood-rotting fungus into the sapwood of pine trees - the growth of the fungus provides ideal nutritional conditions for the development of the wasp larvae. Termites on the African savanna are also known to cultivate fungi,  and yeasts of the genera Candida and Lachancea inhabit the gut of a wide range of insects, including beetles and cockroaches - it is not known whether these fungi benefit their hosts.

However, we should also consider fungi as pathogens and parasites as well as in symbiotic relationships

As pathogens and parasites

Many fungi are parasites on plants, animals (including humans), and other fungi. Serious pathogens of many cultivated plants causing extensive damage and losses to agriculture and forestry include the rice blast fungus; tree pathogens causing Dutch Elm Disease and chestnut blight and other plant pathogens. Some carnivorous fungi, like Paecilomyces lilacinus are predators of nematodes, which they capture using an array of specialized structures such as constricting rings or adhesive nets.

Some fungi can cause serious diseases in humans, several of which may be fatal if untreated. These include aspergilloses, candidoses, cryptococcosis etc. Furthermore, people with immuno-deficiences are particularly susceptible to disease by genera such as Aspergillus, Candida and Pneumocystis. Other fungi can attack eyes, nails, hair, and especially skin, the so-called dermatophytic and keratinophilic fungi, and cause local infections such as ringworm and athlete's foot. Fungal spores are also a cause of allergies, and fungi from different taxonomic groups can evoke allergic reactions. We now move on to the second part of this essay - a brief description of the nutritional, medical and technological uses of fungi.

Human use


The human use of fungi for food preparation or preservation and other purposes is extensive and has a long history. Mushroom farming and mushroom gathering are large industries in many countries. The study of the historical uses and sociological impact of fungi is known as ethnomycology. Because of the capacity of this group to produce an enormous range of natural products with antimicrobial or other biological activities, many species have long been used or are being developed for industrial production of antibiotics, vitamins, and anti-cancer and cholesterol-lowering drugs. More recently, methods have been developed for genetic engineering of fungi, enabling metabolic engineering of fungal species. For example, genetic modification of yeast species which are easy to grow at fast rates in large fermentation vessels—has opened up ways of pharmaceutical production that are potentially more efficient than production by the original source organism. Let us for clarity divide the discussion into three main sections. Nutrition, Medicine and Industrial.


1) Nutrition

Within nutrition we can consider fungi as a direct food source and also in cultured foods:

Edible and poisonous species

Edible mushrooms are well-known examples of fungi. Many are commercially raised, but others must be harvested from the wild. Agaricus bisporus sold as button mushrooms when small or Portobello mushrooms when larger, is a commonly eaten species, used in salads, soups, and many other dishes. Many Asian fungi are commercially grown and have increased in popularity in the West, including straw mushrooms, oyster mushrooms, and shiitakes. There are many more mushroom species that are harvested from the wild for personal consumption or commercial sale. Milk mushrooms, morels, chanterelles, truffles, black trumpets and porcini mushrooms ( Boletus edulis) (also known as king boletes) demand a high price on the market. They are often used in gourmet dishes. Certain types of cheeses require inoculation of milk curds with fungal species that impart a unique flavor and texture to the cheese. Examples include the blue color in cheeses such as Stilton or Roquefort. Molds used in cheese production are non-toxic and are thus safe for human consumption; however, mycotoxins (e.g., aflatoxins, roquefortine C, patulin, or others) may accumulate because of growth of other fungi during cheese ripening or storage.

Many mushroom species are poisonous to humans, with toxicities ranging from slight digestive problems or allergic reactions as well as hallucinations to severe organ failures and death. Genera with mushrooms containing deadly toxins include Conocybe, Galerina, Lepiota, and most infamously, Amanita. The latter genus includes the destroying angel (A.virosa) and the death cap ( A. phalloides), the most common cause of deadly mushroom poisoning. The false morel is occasionally considered a delicacy when cooked, yet can be highly toxic when eaten raw. Fly agaric mushrooms (Amanita muscaria) also cause occasional non-fatal poisonings, mostly as a result of ingestion for use as a recreational drug for its hallucinogenic properties. Historically, fly agaric was used by different peoples in Europe and Asia and its present usage for religious or shamanic purposes is reported from some ethnic groups such as the Koryak people of north-eastern Siberia.

Cultured foods

Baker's yeast, a single-celled fungus, is used to make bread and other wheat-based products. Yeast species of the genus Saacharomyces are also used to produce alcoholic beverages through fermentation. Shoyu koji mold is an essential ingredient in brewing Shoyu (soy sauce) and sake, and the preparation of miso while Rhizopus species are used for making tempeh. Several of these fungi are domesticated species that were bred or selected according to their capacity to ferment food without producing harmful mycotoxins.


2) Medicine Here we should briefly discuss the use of fungi as drugs and also as model organisms

Many species produce metabolites that are major sources of pharmacologically active drugs. Particularly important are the antibiotics, including the penicillins. Although naturally occurring penicillins such as penicillin G have a relatively narrow spectrum of biological activity, a wide range of other penicillins can be produced by chemical modification of the natural penicillins. Modern penicillins are semisynthetic compounds, obtained initially from fermentation cultures, but then structurally altered for specific desirable properties. Other antibiotics produced by fungi include: ciclosporin, commonly used as an immunosuppressant during transplant surgery and fusidic acid, used to help control infection from certain bacteria. Widespread use of these antibiotics for the treatment of bacterial diseases, such as  tuberculosis, syphilis, leprosy and many others began in the early 20th century and continues to play a major part in anti-bacterial chemotherapy. In nature, antibiotics of fungal or bacterial origin appear to play a dual role: at high concentrations they act as chemical defense against competition with other microorganisms in species-rich environments, such as the rhizosphere, and at low concentrations as quorum-sensing molecules for intra- or interspecies signaling. Other drugs produced by fungi include griseofulvin used to treat fungal infections, and statins used to inhibit cholesterol synthesis.

Model organisms

Certain mushrooms enjoy usage as therapeutics in folk medicines such as traditional Chinese medicines. (see Simon Harding's essay on World Remedies in this series) Research has identified compounds produced by some fungi that have inhibitory biological effects against viruses and cancer cells. Specific metabolites, such as ergotamine, are routinely used in clinical medicine. The shiitake emushroom is a source of lentinan, a clinical drug approved for use in cancer treatments in several countries, including Japan.

Several pivotal discoveries in biology were made by researchers using fungi as model organisma, that is, fungi that grow and sexually reproduce rapidly in the laboratory. For example, the one gene-one enzyme hypothesis was formulated by scientists who used bread mold to test their biochemical theories  Other important model fungi are used to investigate issues in cell biology and genetics, such as cell cycle regulation, structure, and gene regulation. Other fungal models have more recently emerged that each address specific biological questions relevant to medicine, plant pathology, and industrial uses.


3) Industrial/Technological


Pest control

In agriculture, fungi may be useful if they actively compete for nutrients and space with pathogenic microorganisms such as bacteria or other fungi via the competitive exclusion principle or if they are parasites of these pathogens. For example, certain species may be used to eliminate or suppress the growth of harmful plant pathogens, such as insects, mires, weeds, nematodes and other fungi that cause diseases of important crop plants.This has generated strong interest in practical applications that use these fungi in the biological control of these agricultural pests. Entomopathogenic fungi can be used as biopesticides, as they actively kill insects. Some endophytic fungi of grasses produce alkaloids that are toxic to a range of invertebrate and vertebrate herbivores.  Infecting cultivars of pasture or forage grasses  endophytes is one approach being used in grass breeding programs; the fungal strains are selected for producing only alkaloids that increase resistance to herbivores such as insects, while being non-toxic to livestock

Bioremediation

Certain fungi, in particular "white rot" fungi, can degrade insecticides, herbicides, creosote, coal tars and heavy fuels and turn them into carbon dioxide, water, and basic elements. Fungi have been shown to also biomineralize uranium oxides, suggesting they may have application in the bioremediation of radioactively polluted sites. Fungi also have other industrial and technological uses. Fungi are used extensively to produce industrial chemicals like citric, gluconic. lactic and malic acids and industrial enzymes used in detergents, cellulases and amylases.


In conclusion we can see the enormous importance of fungi to our ecosystem, and their incredible value in nutrition, medicine and technology   Dr Simon Harding  www.chronosconsulting.com  www.biblon.com