Fungi - part 3
Categorization and identification of fungi

While many may think that classification and identification of different organisms is the same thing, actually they are not only singular, but correlated procedures. Classification denotes separating organism according to their characteristics of a hierarchy. This process depends on the extent to which the organisms are related to each other. On the other hand, the procedure of identification involves grouping different isolated organisms within a specific categorization format. It is not an easy task to classify microorganisms and the parasite Pneumocystis carinii is a case in point. For many years, this parasite was considered to be a protozoan. This parasite is responsible for developing pneumonia in humans and its victims are primarily people who are incapable of developing a normal immune response or immunocompromised. In fact, pneumonia is the most widespread life-threatening disease in people enduring AIDS. Pneumocystis carinii is a single cell flagellate (producing filiform runners or branches resembling runners) being that does not possess any cell wall. Nevertheless, latest molecular genetic researches have demonstrated that compared to other protozoa, Pneumocystis carinii is actually more intimately related to the fungi. Scientists have come to this conclusion depending on the arrangement of the organism's ribosomal RNA molecules - an aspect that is usually made use of in molecular taxonomy.

Categorization of fungi

It may be noted here that the classification of moulds or mycelial fungi is basically done in consistent with the organism's macro and micro morphology (form and structure). However, comparatively the yeasts have a simpler formation and hence, they show a restricted variety of morphologies. As a result, it is quite difficult to categorize yeasts and therefore, they are classified partially depending on their responses in biological experiments. In fact, there is no unanimously agreed method to classify fungi. According to a number of methods to categorize fungi, the sharing out of fungus is known as phyla. In addition, some methods of classification have actually realigned the fungi categorized in the Phycomycetes. Moreover, constituents of the fungi belonging to the Myxomycota and Mastigomycotina have been rearranged into the Protoctista kingdom that also comprises protozoa and nucleated algae. Many experts even do not recognize these varieties as genuine fungi. On the other hand, fungi belonging to Zygomycotina have been maintained as a cluster in the fungal territory and are known as the Zygomycota.

The slime moulds, known as the Myxomycotina, are distinguished by an ameba-like vegetative phase. Conversely, when the conditions are suitable, the amoeboid cells gather together and make a distinction to develop reproductive formations that appear similar to other fungi. Therefore, developmental biologists examine members of the cluster of fungi comprising Physarum polycephalum and Dictostelium discoideum in an exhaustive manner. In fact, slime moulds are generally free-living beings present in places such as leaf waste and soils. Nonetheless, a number of slime mould species are also found as parasites. Most often the parasites are found associated with higher plants, other fungi as well as algae, inclusive of the marine algae. The parasite called Polymyxa graminis, which does not cause any symptom, is usually found connected to the roots of cereal crops and is also able to function as vector for viral diseases.

The Mastigomycotina is a type of fungi that forms or produces zoospores or asexual spores. This type of fungi is able to form branched cell chains that bind to the surface of their host by means of a structure resembling a root and is called rhizoid. Many species of these fungi are basically soil saprophytes where they perform a vital function of decomposers. On the other hand, the Mastigomycotina is also found in fresh water environment, and may also be related to water that is contaminated with sewage. While some of the species of these types of fungi are present in nature as parasites of fish and insects, while there are other species that are found as parasites surviving on plants and algae. The downy mildews are obligate parasites that are not capable of developing in normal laboratory cultures. This group of obligate parasites comprises the notable plant pathogens like Phytophthora infestans and they are responsible for causing potato blight (any of a range of highly destructive fungus diseases of the potato).

On the other hand, the Zygomycotina are very familiar soil saprophytes and a number of species of this type of fungi are found in the animal excreta. Fungi species belonging to the genus Entomophthora are parasites that survive on houseflies and aphids. In addition, the Zygomycotina also comprise a very significant cluster of fungi that are able to develop symbiotic relations with higher plants, which are called mycorrhizas. These formations entail a very close relation between a fungus and the root system of the plant it is associated with. For instance, orchids possess mycorrhizal relationship with fungi in their root system. In this kind of association, while the fungus obtains its organic nourishments from the plant it is associated with and, in return, is supplied with the mineral nourishments that the mycorrhiza obtains from the adjacent soil. It may be mentioned here that it is almost impossible to develop any fungus that forms mycorrhizal relations with higher plants in a stimulated or artificial culture.

The genus Ascomycotina comprises yeasts similar to those belonging to the genus Saccharomyces. The type of yeasts belonging to this genus includes Saccharomyces cerevisiae that forms the foundation of the baking as well as brewing industries. Hence, these yeasts are very important from the commercial point of view. Normally, yeasts are mostly related to fruits, but they are also present in freshwater as well as marine surroundings. In fact, the mycelial Ascomycotina are very widespread soil saprophytes and may also be found in animal excreta. On the other hand, fungi belonging to the genus Tuber are capable of forming mycorrhizal relations with the root systems of different trees. The fruiting parts of this type of fungi are reaped as truffles (any of several subterranean, edible and ascomycetous fungi of this genus) that are extremely valued as gastronomic delicacies. It is interesting to note that in France pigs are specially trained to locate truffles by the smell emitted by these edible fungal parts. However, not all varieties of Ascomycotina are beneficial or safe for consumption. The ascomycete fungi are responsible for causing mildews of roses, while Ceratocystis ulmi causes the disease known as Dutch elm that destroys the elm trees in England. Similarly, the dermatophyte fungi are responsible for causing diseases like ringworm and athlete's foot. All these types of fungi, including the dermatophyte, are grouped as members of the Ascomycotina family.

The genus Basidiomycotina comprises a number of fungi that survive developing mutually beneficial alliances with plants. While a few of the species are responsible for causing diseases, the majority of them are saprophytes and are usually found growing in composts, soil, animal excreta and leaf waste. Fungi belonging to the genus Agaricus are capable of forming fairy rings or rings originating from mycelial growth. Several of these fungi also form mycorrhizal associations with higher plants, especially trees. In fact, the species of fungi called Merulius lacrymans is responsible for dry rot in timber. Basidiomycotina belonging to the group Teliomycetes comprise fungi that cause smuts or plant rusts. However, they are often important from the economic point of view, as they normally have an effect on cereal crops. As the name implies, fungi belonging to the category of Gasteromycetes comprise a group of fungi that are edible.

Fungi Imperfecti or the Deuteromycotina inevitably comprise an assortment of saprophytic as well as parasitic fungi. Many fungi, such as those similar to the genera Aspergillus, Penicillium and Cladosporium, are notable fungi that work to spoil foods. Aspergillus flavus as well as associated species of fungus are responsible for the production of alfatoxin and the presence of this poisonous substance in foods is very worrisome, as alfatoxin belongs to the group of most potent carcinogens discovered thus far. On the other hand, the fungal species known as Aspergillus fumigatus is liable for causing the disease aspergillosis in humans. Some types of this disease may result in grave and sometimes even fatal contagion in people who are immunocompromised or have a poor immune system. Especially, transplant patients are most vulnerable to this disease. Nevertheless, the species Aspergillus niger is of immense economic value, as it is made use of in the industrial production of citric acid. Likewise, members of the genus Penicillium are vital in manufacturing antibiotics. While Penicillium chrysogenum is utilized in the industrial production of anti-bacterial antibiotics belonging to the penicillin family, another species Penicillium griseofulvum is made use of to manufacture the anti-fungal agent medication called griseofulvin.

Identification of moulds

The process of identifying a mycelial fungus entails the study of its macro as well as micro morphology (cell structure and formation). These aspects differ on the basis of the medium on which the microorganism grows as well as the temperature applied to cultivate a mould. Therefore, it is frequently essential to grow a fungus on an assortment of media with a view to obtain its entire classification. It is essential to carefully examine the dyed surface of the mould colonies. While these aspects (the colored textures) may vary from one area of the mould colony to another, they may also be different on the exterior and the beneath of the colony. At the same time, careful note should be taken of the special formations, such as the fruiting bodies, sclerotia and others.

Prior to unsettling the colonial development of moulds with a view to make microscopic research, the complete culture may be scrutinized by making use of a low-power aim in order to examine the fruiting bodies situated in the original, natural or existing place or position (in situ). Subsequently, lactophenol or lactophenol cotton blue build ups ought to be made from the mould colony. A separate mount needs to be prepared from every area of the colony that displays a dissimilar macro-morphology or structure and form of the mould. In order to observe all the structural aspects related to a specific fungus, it is essential to have a number of mounts from all diverse areas of the mould colony. It is essential to have a precise examination to obtain a successful detection of fungus developing in a simulated or artificial culture. It is frequently necessary to make use of specific culture procedures like slide or cellophane cultures in case a fungus possesses frail spore formations. Doing so will enable one to properly examine the micro morphology of the particular fungus without any trouble.

Sometimes it may be tricky to differentiate the fruiting structures of a fungus, such as perithecia, pycnidia, sclerotia and cleistothecia from one another. While it is easy to differentiate or identify a number of sclerotia in some fungi owing to their asymmetrical form, while it is difficult to make a distinction in other fungi. The most excellent method to make out these formations is to crush them to find out what emerges from them. However, at times it is not easy and almost impossible to crush sclerotia. It may be mentioned here that when sclerotia is crushed, it give away a number of oil globules. On the other hand, when pycnidia are crushed, it releases a large number of conidia. Similarly, cleistothecia and perithecia release asci and ascospores when squashed. In fact, both asci and ascospores appear to overflow from the fruiting structures of the fungi.

Identification of yeasts

As discussed earlier, the yeasts have a comparatively uncomplicated formation, but it is indeed an irony that this makes it even more difficult to identify them in comparison to other types of fungi. Classification of yeasts is basically done on the basis of some degree of dissimilarities in their cell structure or cytological differences as well as depending on their biochemical features. The cytological aspects utilized while identifying the yeasts comprise issues such as the size of the cell, form of the cell as well as whether it possesses a capsule or not. A number of yeasts have the ability to produce pseudohyphae, while a marginal number of them have the ability to produce genuine septate mycelium that are similar to those present in moulds. At the same time, a number of yeasts are also competent for sexual reproduction to turn out ascospores (a spore formed within an ascus). On the other hand, very few types of yeast reproduce sexually to generate basidiospores (a spore that is borne by a basidium).

Cryptococcus neoformans, the yeast that is enclosed in a capsule and is responsible for causing meningitis (inflammation of the meninges), especially among patients enduring AIDS, belongs to the Basdiomycotina family of yeasts. Candida albicans is a significant commensal organism that co-exists with other organisms, but, as far as humans are concerned, it is a very opportunist pathogen. It is able to identify this yeast from scientific specimens owing to its capability make a mycelial (the mass of hyphae that form the vegetative element of a fungus) microbe tube when it is nurtured for a duration of one to two hours in serum at 37°C. In fact, the biochemical examinations entail monitoring the aptitude of the isolate to absorb a number of chemicals, especially sugar and nitrogen sources. In addition, the isolate's ability to ferment dissimilar sugars is also observed during the biochemical tests. Presently, the classification process of yeasts has been regularized and one may avail a variety of strips from the market that can be immunized to examine absorption of carbon as well as nitrogen. A classification summary of yeasts may be prepared on the basis of using the arrangement of substrate (surface on which the yeast grows or is attached). In addition, the characteristics of the yeast may also be got hold of by referring a manual on the subject or making use of a database in a computer.



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