Introduction of Medically Important Fungi and Yeast

Beneficial effect of fungi

1 .Fungi helps in Decomposition of organic wastes and carbon recycling.

2 .Fungi helps in Biosynthetic factories. The fermentation property is applied in alcohols, fats, citric, oxalic and gluconic acids.

3 .Fungi as an important source of antibiotics, like Penicillin.

4 .Fungi as a model organism for biochemical and genetic studies. Eg: Neurospora crass

5. Some fungi are used as a source of food i.e. edible mushroom.

6. Yeasts provide nutritional supplements like vitamins and cofactors.

7 . Penicillium is used for flavoring Roquefort and Camembert cheeses.

8. Ergot produced by Claviceps purpurea produce ergot which contains medically important alkaloids that help in uterine contractions, controlling bleeding and treating migraine.

9. Help in malaria control as fungi are used to trap mosquito larvae in paddy fields .

Harmful effects of fungi

1 .Fungi causes damage of food , paper, clothes, etc

2. Fungi causes plants, animal and human diseases, including allergies.

3. Poisonous mushrooms produce toxins and cause food infection or intoxication

4.Fungi also causes spoilage of agricultural products such as vegetables and cereals in the godown.

5. magnetic tapes, disks, glass lenses, marble statues, bones, and wax are also damaged by fungi.

General properties of fungiI

1. Fungi are eukaryotic which contain membrane bound cell organelles containing nuclei, mitochondria, Golgi apparatus, endoplasmic reticulum, lysosomes etc and they also exhibit mitosis.

2. There is presence of ergosterols in the cell- membranes and possesses 80S ribosomes

3. There is the presence of chitin in the cell wall.

4. They lack chlorophyll i.e heteromorphic mode of nutrition.

5. Fungi are osmotrophic i.e. they obtain their nutrients by absorption.

6. They acquire nutrients as saprophytes (live off of decaying matter) or as parasites (live off of living matter).

7. All fungi need water and oxygen and there are no obligate anaerobes.

8. Usually, reproduce asexually and/or sexually by producing spores in some cases.

9. Fungi grow either by budding or by hyphal tip elongation.

10. In fungi, food storage is generally in the form of lipids and glycogen.

Classification of fungi

Classification of fungi on the basis of following;

Based on Sexual reproduction

1. Zygomycetes
2. Ascomycetes
3. Basidiomycetes
4. Deuteromycetes (Fungi imperfecti)

Based on Morphology

1. Molds
2. Yeasts
3. Dimorphic fungi

Morphology of fungi

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Fungi exist in two major forms; the filamentous (hyphal) and single-celled budding forms (yeast). But, for the classification,sake they are studied as molds, yeasts, and dimorphic fungi. All fungi commonly have eukaryotic morphology. They have a cell wall which is composed of chitin, which may be consist of mannans, glucans and other polysaccharides in addition to polypeptides. Some lower fungi also contain cellulose in their cell wall. Some fungi like Cryptococcus and yeast form of Histoplasma capsulatum have polysaccharide capsules that help them to invade phagocytosis. There is plasma membrane inside the cell wall ; a typical bi-layered membrane, in addition to the presence of sterols. Fungal cell membranes contain ergosterol in contrast to cholesterol which is found in mammalian cells.The cytoplasm of fungi consists of various organelles like mitochondria, Golgi apparatus, ribosomes, endoplasmic reticulum, lysosomes, microtubules and a membrane-enclosed nucleus. The nucleus consists of paired chromosomes.

Molds

The basic structural units of the molds are tube-like projection known as hyphae. As the hyphae grow they become intertwined to form a loose network,called mycelium. In the hyphae,vegetative hyphae comprise the body of the fungus and is responsible for the nutrient absorption and water exchange whereas aerial hyphae ,which is also known as reproductive hyphae give rise to the fruiting bodies from which spores originate. Three types of hyphae exist in the medically important fungi.They are coenocytic hyphae,the dark and pigmented septate hyphae of the dematiaceous fungi and the septate non-pigmented hyphae of the hyaline molds.Mycelium is of three type, they are :

i. Vegetative mycelium is that mycelium that penetrates the surface of the medium and absorbs nutrients.

ii. Aerial mycelium is that mycelium that grows above the agar surface.

iii.Fertile mycelium is aerial hyphae that produce reproductive structures such as conidia or sporangia.

Example of molds include;

- Aspergillus Epidemophyton

- Rhizopus Trichophyton

- Rhizomucor Microsporum ,etc

Yeasts

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Yeasts are unicellular organisms which are round to oval and range in size from 2 -60 micrometers. They reproduce by budding; result in blastospore (blastoconidia) formation. Sometimes as the cells buds the buds fail to detach and elongate forming elongated hyphae like filament called pseudohyphae. This property is seen in Candida albicans. The same species also possess the ability to produce true hypha, which is seen as germ tube. The difference between these two is that there is a constriction in pseudohyphae at the point of budding, whereas the case of germ tube there is no constriction. yeast such as Cryptococcus and the yeast form of Blastomyces dermatitides produce polysaccharide capsule. Capsules can be observed by negative staining methods i.e using India ink or Nigrosin. The capsule itself can be stained by Meyer Mucicarmine stain. Some of the yeasts are pigmented . As Rhodotorula sp produces pink colonies due to carotenoid pigments whereas some yeasts such as Phaeoannellomyces werneckii and Piedraia hortae are dematiaceous, that produce colonies. True yeasts don't produce pseudohyphae. Yeast-like fungi may be basidiomycetes, for example, Cryptococcus neoformans or ascomycetes such as Candida albicans.

Reproduction in fungi

Fungi reproduce by asexual, sexual and parasexual means.

Asexual reproduction

Asexual propagules are termed as spores or conidia depending on the basis of mode of production and asexual spores are produced the following mitosis whereas sexual spores by meiosis .The asexual spores of zygomycetes; known as sporangiospores within a sac-like structure called sporangia. The sporangiospores result from the mitotic cleavage of cytoplasm in the sporangium and the sporangia are produced on a special type of hyphae called sporangiophore. This endogenous process from where spore formed within a sac is known as sporogenesis. Conidia produce either by budding off conidiogenous hyphae or by differentiation of preformed hyphae. These develop by following mitosis of a parent nucleus and are formed in any manner. This exogenous process is popularly known as conidiogenesis i.e a process that occurs both in yeasts and molds. Conidia are produced on specialized structures called conidiophore. Here the conidium is originated from part of parents . In the mode of development, the conidium is differentiated by a septum before it is differentiation .Hence the conidium results from the conversion of an entire parent cell into the conidium. The cell that gives rise to a conidium is known as a conidiogenous cell.

Sexual Reproduction

Sexual propagules are produced by the fusion of two nuclei and then generally undergo meiosis. The first step in sexual methods of reproduction involves plasmogamy (cytoplasmic fusion of two cells) and the second step is karyogamy (fusion of two compatible nuclei), which result in the production of diploid or zygote nucleus. This is then followed by genetic recombination and meiosis. The formed four haploid spores are known to be sexual spores, e.g. zygospores, ascospores and basidiospores. Zygospores, which are the sexual spores of zygomycetes are round,oval, thick walled reproductive structures that result from the union of two gametangia. Ascomycetes produce sexual spores called ascospores in a special sac-like cell which is known as ascus. In basidiomycetes, the basidiospores are released, which is the terminal cell of hyphae.

Parasexual reproduction

Parasexual reproduction was first seen in Aspergillus and is known to occur in basidiomycetes, ascomycetes, and Deuteromycetes. This process involves genetic recombination without the need of specific sexual spores,

REFERENCE

Cheesbrough, M. Medical Laboratory Manual for Tropical Countries. Vol. Vol 2. ELBS London, 2007.

Tille, P. Diagnostic Microbiology. 13th. Elsevier, 2014.

D, Grenwood, Slack RCB and Peutherer J. Medical Microbiology. Dunclude Livingstone: ELBS, 2001.