Importance of medical fungi
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Classification of fungi is done so that fungi with similar characteristics and structures can be grouped together and thus making identification easier. Fungi are classified on the basis of their ability to reproduce sexually, asexually, or both. Ascospores, basidiospores, oospores, and zygospores, as well as any specialized structures associated with their development, are the basis of sexual structures. These criteria reflect phylogenetic relationships because they are based upon structures that form following meiosis. The term holomorph is used to describe the whole fungus consisting of sexual and asexual structures (Samuel Baron, 1992)
The study of fungi is called mycology. Fungi are described as eukaryotic organisms that bear spores, have absorptive nutrition, lack chlorophyll and reproduce sexually or asexually. Most fungi are terrestrial organisms although there are some that are freshwater or marine. Many fungi are pathogenic and infect plants as well as animals however, some fungi also form beneficial relationships with other organisms e.g. majority of vascular plants form plant root associations (mycorrhizae) with fungi. Endophytic fungi are found in upper parts of plants where they affect plant reproduction and palatability to herbivores (Prescott, Harley and Klein, 2008)
All genera of fungi belong to one of 3 broad groups which are yeasts, moulds and other. Yeasts reproduce substantially by budding or fission and moulds by elongation of tips. The other group consists of genera that are not typically fungi but are studies along with fungi for various reasons. All fungal genera of medical importance can be classified into 5 sexual groups which are Ascomycetes, Basidiomycetes, Zygomycetes, Chytridiomycetes or Chytrids and Fungi Imperfecta (Doctor Fungus USA, 2007). The Fungi Imperfecta does not observe sexual reproduction therefore only the other four of the genera which observe sexual reproduction will be discussed in detail in this essay. The four correspond to the phyla Ascomycota, Basidiomycota, Zygomycota and Chytridiomycota respectively. These four phyla are truly sexual because they are characterised by production of sexual spores namely ascospores, basidiospores, ascospores and oospores respectively. The first three groups infect humans whereas the fourth group causes disease in plants and lower animals.
Fungi can reproduce both sexually and asexually. The sexual form is known as teleomorph and the asexual form is known as anamorph (Koneman et al., 2006)
The Ascomycota consist of ascomycetes or sac fungi which form meiotic spores called ascospores enclosed in a special sac-like structure called an ascus. They include morels, some mushrooms and truffles, as well as single-celled yeasts and many species that have only been observed undergoing asexual reproduction. The products of meiosis are retained within the ascus and thus most ascomycetes are used to illustrate principles of genetics and heredity (Deacon JW, 2005 and Kamistein D, 2002).Examples of some famous and infamous fungi within this are Saccharomyces cerevisiae the yeast of commercial importance for baking and brewing, Penicillium chrysogenum producer of penicillin, Aspergillus flavus producer of aflatoxin, contaminant of nuts and stored grain, Candida albicans cause of thrush, diaper rash and vaginitis (Tree of life web project, 2009). Ascomycota are morphologically diverse and include organisms from unicellular yeasts to complex cup fungi and consist of 2000 identified genera and 30,000 species (SparkNotes Editors, 2009). Ascomycetes have a characteristic reproductive structure called the ascus. Many ascomycetes are yeasts which alternate between haploid and diploid states. The haploid and diploid cells undergo mitosis when nutrients are plentiful and produce haploid and diploid daughter cells which leave a scar on the mother cell as it separates. When no more scarred cellwall remains on the mother cell it dies (Prescott, Harley and Klein, 2008). When nutrients are limited yeast cells undergo meiosis producing four haploid cells that remain enclosed in the ascus and when sufficient amounts of nutrients are received two haploid cells will fuse to form a diploid cell (Prescott, Harley and Klein, 2008).
Sexual reproduction in these occurs by ascus formation with each ascus containing eight haploid ascospores. However, in more complex ascomycetes special ascogenous hyphae develop and into these pairs of nuclei migrate. One nucleus of each pair is from a male mycelium (antheridium) and other from a female cell (ascogonium). As ascogenous hyphae grow the paired nuclei divide so that there is one pair of nuclei in each cell. When the hyphae mature, nuclear fission occurs at the tips in the mother cells. Meiosis occurs in the diploid zygote nucleus and the resulting four haploid nuclei divide mitotically producing a row of eight nuclei in each developing ascus. These nuclei wall off from each other and thousands of asci are packed together in a cup shaped fruiting body, the ascocarp. When the ascospores mature they are released and upon reaching a suitable environment they germinate and start the cycle all over again (Prescott, Harley and Klein, 2008)
Asexual reproduction in these is termed a blastic process, which involves the blowing out of the hyphal tip wall and can involve all wall layers, or there can be a new cell wall synthesized which is extruded from within the old wall (MicrobiologyBytes, 2009)
Members of the Basidiomycota consist of the basidiomycetes or club fungi and produce meiospores called basidiospores on club-like stalks called basidia. Most common mushrooms, the rust fungus and smut fungi which are major pathogens of grains belong to this group (Deacon JW, 2005 and Kamistein D, 2002). The most common example of species in this phylum is the pathogenic yeast Cryptococcus neoformans is an important human and animal pathogen and produces a disease called cryptococcosis which is a systemic infection of the lungs and central nervous system (Prescott, Harley and Klein, 2008). Basidiomycetes are characterised by the most complex and large structures found in the fungi. They very rarely produce asexual spores and most of their life cycle is spent as vegetative mycelium, exploiting complex substrates. Sexual reproduction starts when compatible mycelium fuse in presence of two mating type of nuclei (MicrobiologyBytes, 2009).This creates a dikaryon in which each cell in the thallus contains two haploid nuclei resulting from a mating event is another characteristic feature of Basidiomycota. The dikaryon has single copies of the two mating type of nuclei held within every hyphal compartment for long periods of time. “Maintenence of the dikaryon requires elaborate septum formation (figure 1 below) during growth and nuclear division” (MicrobiologyBytes, 2009)
Sexual spore formation starts with formation of a fruit body the primordium and is triggered by environmental conditions. The primordium expands and divides to form large fruit bodies of mushrooms and toadstools leaving the mycelium within it as a dikaryon and diploid formation only occurs within the modified hyphal tip the basidium. Within the basidium meiosis takes place and the four products are extruded from the tip on to the sterigmata that is below and this event usually occurs across a large area of basidia called a hymenium which is usually formed over an extensive sterile layer of tissue like the mushroom gill. This is shown in figure 2 on following page.
Asexual reproduction in Basidiomycota happens either by budding or asexual spore formation. Budding occurs when the outgrowth of parent cells is separated into a new cell. Any cell can bud. Asexual spore formation occurs in conidiophores. The septae of terminal cells cause division of a random number of nuclei into cells. The cell wall thickens into a protective coat causing the spores to break off and these are then dispersed (SparkNotes Editors, 2009)
These are known as zygomycetes and reproduce sexually with meiospores called zygospores and asexually with sporangiospores. Rhizopus stolonifer is a common species that belongs to this group. Medically relevant genera include Mucor, Rhizomucor, and Rhizopus (Deacon JW, 2005 and Kamistein D, 2002). Humans usually get infected in the upper respiratory tract through inhalation of airborne spores but can also get primary gastrointestinal infections from ingestion of contaminated foodstuffs (Koneman et al., 2006). These include moulds invading food products. The distinguishing characteristic of these is the production of zygospores during sexual reproduction and the lack of hyphal cell walls except in reproductive structures. The mycelia of Zygomycota are divided into three types of hyphae. The rhizoids function in food absorption and reach below the surface. Sporangiophores are above the ground and bear the sporangia which are spore producing. Groups of the sporangiophores and rhizoids are connected by stolons above the ground. Call walls that separate individual cells are only present in reproductive structures to allow the cytoplasm and nuclei to move between cells (SparkNotes Editors, 2009).
Sexual reproduction requires compatible strains of opposite mating types and when these two come close to each other hormones are produced causing their hyphae to form prometangia which are projections which then form mature gametangia. When the gametangia fuse the nuclei of the two gametes also fuse forming a zygote which develops a thick coat becoming a dormant zygospore. When meiosis occurs at the time of germination, the zygospore splits open producing a hypha which bears asexual sporangium and starts a cycle for asexual reproduction (Prescott, Harley and Klein, 2008).
Asexual reproduction varies greatly among the different species within this phylum. Spores may form by separation and thickening of hyphal cells but may also be produced in specialised organs whose structure widely varies (SparkNotes, 2009). The bread mold, Rhizopus stolonifer grows on a moist surface of carbohydrate rich food stuffs where its hyphae rapidly cover the surface. The hyphae also called rhizoids extend into the food surface and absorb nutrients. Other hyphe called the stolons become erect and arch into the substranum forming new rhizoids. Others still remain erect and produce asexual sporangia at their tips which are filled with black spores giving it the characteristic colour. Each spore when get the liberty can germinate to start a new mycelium (Prescott, Harley and Klein, 2008).
These grow and survive in a wide range of habitats. These fungi are predominantly asexual and discharge naked zoospores from sporangial openings. The rate at which fully developed zoospores become motile varies and ultimately when they locate a suitable substrate they withdraw their flagellum producing a wall around the zoospore and developing into a thallus. Sexual reproduction is more commonly found among members of the most the Class Monoblepharidomycetes among the Chytridiomycota where motile sperms fuse with non-motile oospores. Chytriomyces hyalinus produces resting sporangia zygotes as the result of rhizoidal fusion. Electron microscopy has demonstrated the migration of nuclei through the rhizoids of contributing thalli and fusion of nuclei in the zygote (Botany & Mycology, 2009).
All of the three phyla undergo both sexual and asexual reproduction. The reproduction processes are quite similar in all three of them and the main difference being the type of spores produced because each phylum has distinct types of spores that they produce. The spores are the main sexual structures in each of the phylum.
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