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. Potato dextrose agar (PDA) is another selective culture media which is used for the isolation of fungi from both clinical and environmental samples. 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.
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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. Dimorphic or diphasic fungi are fungi with two growth forms, i.e., they can exist either as yeasts or mould depending on the dispositions of their environment. The phenomenon in which fungi exhibit two growth forms is known as dimorphism. Fungi 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. Fungal species such as Candida found on the human body as normal flora are generally called mycoflora. 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.
The term dimorphism is used to describe a fungus with two growth forms i.e., the yeast and mould forms. Some fungi exist as yeast in tissues of humans or animals but outside the body, they can also exist as mould in the environment. Such fungi are called dimorphic or diphasic fungi. Examples of dimorphic fungi include Histoplasma capsulatum (which cases histoplasmosis in humans), Blastomyces dermatitidis (which causes blastomycosis in humans), Paracoccidioides species such as P. brasiliensis (which causes paracoccidioidomycosis in humans), Candida albicans (which causes candidiasis) and Sporothrix species such as S. schenckii (which causes sporotrichosis in humans). Dimorphic fungi are mostly responsible for systemic (endemic) mycoses. Systemic mycosis can also be called invasive mycosis because of their all-encompassing nature in vivo and general distribution in the body. 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 nutrient depletion). 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.
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