1.1 BRIEF EXPLANATIONS ON FUNGI
Fungi are an important group of microorganisms that lack photosynthetic ability but possess tremendous economic, health and scientific value. First, some fungi such as mushroom are a source of protein and food for humans; and other forms of fungi especially yeasts are used in the production of many food products that benefits man and animals. For example, Saccharomyces cerevisiae is yeast that has immense benefit in bread production where they serve as leavening agent in baking. This yeast, which is a type of fungi also, has benefits in brewery where it is used for the production of alcoholic and non-alcoholic beverages. Fungi are important agents of decay or decomposition because they help in the degradation of dead organic matter in the environment. This way, fungi play important role in maintaining the biosphere and tranquility of the earth. Generally, fungi are saprophytic microorganisms because of their ability to live and feed on dead or decaying organic matter including those of animal and plant origin from where they derive their energy and nutrient source as heterotrophs. As agents of disease and infections, fungi cause a number of clinically important disease and infections in humans, plants and animals. Fungi are also food spoilage organisms that cause the spoilage of food and food products. However, majority of fungi are of immense economic, industrial, medical and pharmaceutical importance because they are sources from which major antibiotics used for treating infectious diseases of man are sourced from. For example, penicillin, an important antibiotic used clinically for treating and managing infectious diseases was first isolated from a fungus known as Penicillium chrysogenum (also known as P. notatum). Fungi have been known since prehistoric times, and their economic value have since been exploited by mankind even till date.
1.2 BRIEF HISTORY OF MYCOLOGY?
What is mycology? Mycology is simply defined as the study of fungi. Fungi (singular: fungus) are eukaryotic and heterotrophic microorganisms that do not contain chlorophyll but obtains its nutrient through the absorption of food and/or nutrients from dead or decaying organic matter in its environment. The study of fungi actually began in the early 1830’s following the serendipitous discovery of fungi as the causative agent of the white muscardine disease of silkworm. Nonetheless, the benefit and use of fungi by mankind have been noted even before this time period, but the discovery and description of fungi could not be clarified until the discovery of the microscope. This discovery that fungi were responsible in causing the white muscardine disease of silkworm was made by Agostino Bassi (1773-1856) in 1835. Agostino Bassi was perhaps the first to discover that some fungal organisms are pathogenic to other living organisms including humans. Agostino Bassi demonstrated experimentally that a type of silkworm disease was caused by a parasitic fungus known as Beauveria bassiana, which he successfully isolated and used the parasitic fungus to infect a healthy animal in order to confirm his observation and discovery. His work set the foundation for the discovery and development of other aspects of microbiology especially the discovery and explanation that a particular disease is caused by a particular microbe, as opined in the works of Robert Koch (1843-1910), another founding father of microbiology. Beauveria bassiana possess economic and agricultural value because the spores of the fungus are used today as a biological pest control agent especially in the control of termites, aphids and beetles. This way, they are important biological insecticides for farming activities since they prevent the destruction caused by insect pests on crops. This notable discovery of fungi as a pathogen by Agostino Bassi was given impetus following other discovery in the field of microbiology which made further fungi investigation possible. Most importantly was the discovery of Sabouraud dextrose agar (SDA) medium (which does not readily support bacterial growth) for the selective isolation of fungi by the French dermatologist and mycologist, Dr. Raymond Jacques Adrien Sabouraud (1864-1938) in the early 1900’s.
1.3 SOME FUNGAL FEATURES
Fungi possess certain salient characteristics that differentiate them from other forms of microbes (bacteria, viruses, algae, protozoa). These salient features of fungi are often seen or observed on growth media; and they are also expressed or observed in the infections caused by pathogenic fungi. In the environment, the growth of fungi can also be observed by the production of these salient features or properties of fungi. Fungi generally grow on culture media that specifically support the growth of fungi. The Sabouraud dextrose agar is one typical example of a culture medium that supports the growth of fungi. Sabouraud dextrose agar (SDA) medium which is widely used today to recover and cultivate fungi from both clinical and environmental samples was discovered by Dr. Sabouraud who used Sabouraud dextrose agar to cultivate the ringworm fungi he was working with as at the time. Sabouraud dextrose agar was named after Dr. R.J.A Sabouraud, and he is often regarded as the father of mycology because of his discovery of Sabouraud dextrose agar media, a vital tool for fungal studies even till date. The discovery of Sabouraud dextrose agar medium for fungal cultivation, coupled to the discovery of fungi as pathogens of humans, animals and plants set the foundation for the development of the field of microbiology known today as mycology. Most fungi are saprophytic in nature and they degrade complex organic matter in the environment to release simpler organic and inorganic molecules which also serve as part of their nutrient. In their saprophytic nature, fungi especially those found in the soil help in nutrient recycling in the ecosystem. Fungi are generally non-motile and they reproduce sexually and asexually. They are mostly aerobic eukaryotic organisms but some fungi are facultative anaerobes. Fungi grow best at room temperature (i.e., 25oC to 30oC) but some dimorphic (diphasic) fungi can grow in temperatures as high as 35oC. They live mostly in moist environments but can also be found in varied environments including aquatic, marine and terrestrial habitats especially in the soil where they normally colonize. Some fungi such as Candida are found on the human body as normal microflora. Fungi have enormous impact on the environment as mutualistic or symbiotic organisms, decomposers and pathogens of animals, plants and humans. Fungi basically exist in the natural environment in two morphological forms known as yeast and moulds.
1.4 YEAST AND MOULDS
Yeast and mould are the two basic morphological forms in which fungal organisms are known to grow or develop into. And this does not include mushrooms (Figure 1.1), which are usually macroscopic in nature. Mushrooms are of immense economic importance, and some mushrooms are edible and can be consumed by man as a source of protein, fat, carbohydrate, vitamins and minerals. Mushrooms are described later in this section. Yeasts are single-celled or unicellular fungi, and they are usually round to oval in shape. Yeast is the single-celled growth form of some fungi. They reproduce sexually (by spore formation) and asexually (by budding).
Moulds are filamentous-branching forms of fungi that also bear conidia or fungal spores. Moulds are multicellular fungal cells and they appear as threadlike structures. Mould is the filamentous form of fungi that exist as hyphae or mycelia colonies. They are multicellular forms of fungi. Moulds grow as filamentous, branching strands of hyphae. Fungi are ubiquitously distributed in the universe, and they directly or indirectly affect our daily lives even though we may be unaware about some of their activities. For example, the change in the colour of a loaf of bread kept in the refrigerator or in the open after some days is due to fungal activity (moulds in particular). About 100,000 species of fungi have been described and classified by mycologists, and only a small fraction of these actually cause disease in humans and animals. Table 1.1 elucidates some key terminology associated with the study of fungi.
Figure 1.1: Schematic illustration of a mushroom. Mushrooms grow in the natural environment on tree trunks, in association with other vegetations. They also grow in the soil on their own but they can also be cultivated for edible and other economic purposes.
Table 13.1: EXPLANATION OF SOME FUNGAL TERMS
|Yeast||Yeast is the single-celled growth form of some fungi. They reproduce sexually (by spore formation) and asexually (by budding).|
|Hyphae||Hyphae is the long branching filaments or structures of fungi. It is a network of filaments (with cross-walls) formed by multicellular fungi (i.e., moulds). Some fungal hyphae known as coenocytic hyphae are without cross-walls and contain more than one nucleus (i.e., they are multi-nucleated). Fungal hyphae without cross-walls are generally called coenocytic hyphae or non-septate hyphae while those with cross-walls are known as septate hyphae.|
|Mould||Mould is the filamentous form of fungi that exist as hyphae or mycelia colonies. They are multicellular forms of fungi. Moulds grow as filamentous, branching strands of hyphae.|
|Thallus (plural: thalli)||Thallus is the vegetative body of fungi. It can also be known as fungal colony. It is the visible structures of fungi. Fungal thallus structure or organization comprises of fungal hyphae (septate & non-septate), mycelium and fungal spores or conidia.|
|Conidia (singular: conidium)||Conidia are the asexual reproductive spores of fungi. They are generally known as fungal spores in some cases. However, the term “spores” is usually used to describe the sexual forms of fungi in terms of their reproduction. Thus the term “conidium” is strictly used for the asexual reproductive forms of fungi. In fungi, spores are formed sexually by meiosis and asexually by mitosis.|
|Sporangiospores||Sporangiospores are asexual spores produced by zygomycetes. They are borne or enclosed in the sporangia (singular: sporangium) of a fungal thallus.|
|Dermatophytes||Dermatophytes are fungal cells that cause skin infections or diseases. They are a group of fungal organisms that can degrade the keratin layers or tissues of animals and humans. Dermatophytes mainly affect the nails, skin and hair, causing a range of infections generally known as dermatophytosis (for example, ringworm). Typical examples of dermatophytes are Microsporum species, Epidermophyton species and Trichophyton species.|
|Dematiaceous fungi||Dematiaceous fungi are fungal organisms with cell walls that contain the skin pigment, melanin. Such fungi leave a characteristic black colouration on the skin after an infection or disease.|
|Septum (plural: septa)||Septa are the cross-walls that form in a fungal hypha. They help to differentiate coenocytic hyphae from septate hyphae.|
|Macroconidia||Macroconidia are large conidia of fungi.|
|Microconidia||Microconidia are small conidia of fungi.|
|Pseudohyphae||Pseudohyphae are chains of an elongated bud. They are usually formed from fungal species whose buds fail to separate from the parent cell, but instead continue to extend to form an elongated budding yeast cells known as pseudohyphae. Pseudohyphae generally have cell walls that are less-rigid than those of hyphae.|
|Perfect fungi||Perfect fungi are fungi that carryout sexual reproduction. They are generally known as teleomorphs. Teleomorphs are fungal organisms with sexual characteristics.|
|Imperfect fungi||Imperfect fungi or fungi imperfecti are fungi that reproduce by asexual reproduction. They are generally known as anamorphs. Anamorphs are fungal organisms with asexual characteristics.|
|Mycelium||Mycelium is a mass or network of hyphae. They are generally a collection of intertwined or knotted hyphae that penetrates the supporting medium or substrate on which the fungus is growing.|
|Mushroom||Mushrooms are fruiting fungal bodies that are macroscopic in nature. They are visible; and are known to grow in moist environments. Some mushrooms are edible while others are poisonous in nature.|
|Phialides||Phialides are non-septate, colourless or pigmented conidia formed form vegetative hyphae. They are the terminal ends or regions of conidiophores. Phialides are usually formed by Phialophora species and Trichoderma species.|
|Metulla (singular: metulae)||Metulla are short extensions or cell branches which bear one or more phialides in the conidiophores.|
|Columella (singular: columellae)||Columella are axial or central, unicellular or multicellular structures formed within the fruiting body of some fungi. It is an extension of the sporangiophore into the cavity of the sporangium. Columella is seen in Mucor species and myxomycetes fungi.|
|Sterigmata||Sterigmata are four small protuberances or finger-like protrusions which are formed at the terminal ends of some vegetative cells; and from which daughter cells or spores emanate by budding.|
|Ascospores||Ascospores are the sexual spores of Ascomycetes.|
|Ascus (plural: asci)||Ascus is the sac-like fungal structures containing 4 or 8 ascospores. They are mainly formed by the Ascomycetes.|
|Dimorphism||The term dimorphism is used to describe a fungus with two growth forms i.e., the yeast & mould forms. Some fungi exist as yeast in tissues but as mould in the environment. Such fungi are called dimorphic or diphasic fungi. Examples of dimorphic fungi include Histoplasma capsulatum, Blastomyces dermatitidis, Paracoccidioides species, Candida albicans and Sporothrix species. Dimorphic fungi are mostly responsible for systemic (endemic) mycoses; and they are geographically limited to certain areas of the world. Dimorphism is usually a strategy used by fungi to dodge harsh environmental conditions (such as changes in temperature and nutrients). In plants, dimorphic fungi changes from mould form to yeast i.e., the mould form exist in the plant while the yeast form occurs in the outside environment. But the reverse is the case for human or animal dimorphic fungi where the dimorphic fungi changes from the yeast form (which occur inside the body) to the mould form in the environment.|
1.5 Dimorphism and other forms of fungi
Majority of fungal organisms cause disease in plants, animals and humans but many fungi serve beneficial purposes to mankind and his environment. Fungi are used in a number of industrial processes that involves fermentation. Some notable drugs (for example, penicillin), foods (for example, bread and cheese) and drinks (for example, alcoholic and non-alcoholic beverages) are typical products of the fermentative action or activities of fungi. Some fungi can exist as both yeast (especially in the parasitic phase) and moulds (especially in the saprophytic phase) in different conditions; and such fungi are generally known as dimorphic or diphasic fungi. Dimorphism is the phenomenon or condition in which a particular fungus exists in two morphological or growth forms viz yeast and mould forms. It is an important pathogenicity mechanism employed by a number of fungal pathogens, especially those that are responsible for causing systemic and opportunistic mycoses in humans. These fungal pathogens that exhibit dimorphism include but not limited to Candida albicans, Histoplasma capsulatum, Paracoccidioides brasiliensis, Blastomyces dermatitidis, Sporothrix schenckii, and Penicillium marneffei. Other forms of fungi include mushrooms, puffballs and mildews. Mushroom as aforementioned, is another group or class of fungi but they are macro-fungal cells that possess many economic and health benefits to humans.
Mushrooms are the macroscopic reproductive or fruiting bodies of fungi. They are visible and can be seen around our homes, in bushes, on stored or baked foods and in farms. Mushroom farming is big time business in many parts of the world, and some mushrooms (for example, Agaricus bisporus) are important source of food for humans. Other edible mushrooms include Lentinula species, Pleurotus species, Volvariella species and Ganoderma species. Mushrooms possess medicinal value aside their edible and nutritional benefits. Generally, mushrooms are important source of proteins, low fats, vitamins, carbohydrates, minerals and fibers for humans. Slime moulds were formerly classified as a form of fungi because they exhibit some fungal characteristics (especially the formation of fruiting bodies) but this is no longer the case as the organism is more protists-like than fungi. Slime moulds (both the cellular and plasmodial forms as will be seen later in this Chapter) are motile amoebic organisms and they differ from fungi phylogenetically even though they may resemble fungal organisms in some way. Unlike bacteria and some prokaryotes that have a peptidoglycan cell wall, fungi have a chitinized rigid cell wall (i.e., cell wall that contains chitin).
Fungal cell wall also contains cellulose, lipids, polysaccharides and other complex organic molecules. Fungi are much larger than prokaryotic cells (bacteria in particular), and their cell membrane is very rich in sterols (for example, ergosterol). Cholesterol and not ergosterol is the sterol that makes up the cell membrane of mammalian and human cells. Most fungal drugs (for example, amphotericin B) are selectively toxic because they target the ergosterol component of the fungal cell membrane since the mammalian cell membrane is mainly made up of cholesterol. Fungi produce sexual and asexual spores that help in their dispersal and reproduction in the natural environment. In bacteria, spore production is strictly for survival purposes especially when environmental condition is harsh for existence but spores of fungi are used for reproduction and dispersal purposes. Fungi cause a variety of infections in human population as well as in plants and animals. Infections caused by pathogenic fungi in human population as shall be seen later in this book is generally known as mycoses.
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