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Mycology: Morphology, Reproduction, Distribution, Importance, Beneficial and Harmful Effects, Medically Important Fungi

Mycology: General properties of Fungi, Morphology, Reproduction, Distribution, Importance, Beneficial Effects, Harmful Effects, Terminologies, Medically important fungi

Mycology

      Fungi are eukaryotic microorganisms with the absorptive mode of nutrition, which do not contain chlorophyll and reproduce by both asexual and sexual methods.
      The study of fungi is called Mycology and the scientist who studies fungi are called mycologists.
      About 90,000 fungal species have already been described; however, some estimates of total number suggest that 1.5 million species may exist.

General properties of Fungi

      They are eukaryotic; cells contain membrane bound cell organelles including nuclei, mitochondria, Golgi apparatus, endoplasmic reticulum, lysosomes etc.
      Have ergosterols in their membranes and possesses 80s ribosomes.
      Have a rigid cell wall and are therefore non-motile, a feature that separates them from animals.  All fungi possess cell wall made of chitin.
      Are chemoheterotrophs (require organic compounds for both carbon and energy sources) and fungi lack chlorophyll and are therefore not autotrophic.
      Fungi are osmotrophic; they obtain their nutrients by absorption.
      They obtain nutrients as saprophytes (live off of decaying matter) or as parasites (live off of living matter).
      All fungi require water and oxygen and there are no obligate anaerobes.
      Typically reproduce asexually and/or sexually by producing spores.
      They grow either reproductively by budding or non-reproductively by hyphal tip elongation.
      Food storage is generally in the form of lipids and glycogen.

Morphology of fungi

      Fungi exist in two fundamental forms; the filamentous (hyphal) and single celled budding forms (yeast). But, for the classification purpose, they are studied as molds, yeasts, yeast like and dimorphic fungi.
      All fungi have typical eukaryotic morphology. They have a rigid cell wall composed of chitin, which may be layered with mannans, glucans and other polysaccharides in association with polypeptides. Some lower fungi possess cellulose in their cell wall. Some fungi such as Cryptococcus and yeast form of Histoplasma capsulatum possess polysaccharide capsules that help them to evade phagocytosis.
      Fungal membranes possess ergosterol in contrast to cholesterol found in mammalian cells. The cytoplasm consists of various organelles such as mitochondria, Golgi apparatus, ribosomes, endoplasmic reticulum, lysosomes, microtubules and a membrane enclosed nucleus.

Reproduction of fungi

      Reproduction in fungi can be either asexual or sexual.
      Asexual reproduction is accomplished in several ways.
      Asexual reproduction in yeast takes place by fission (a parent cell can divide into two daughter cells by central constriction and formation of a new cell wall) or budding (somatic vegetative cells may bud to produce new organisms. 
      The most common method of asexual reproduction in filamentous fungi is spore production. Asexual spore formation occurs in an individual fungus through mitosis and subsequent cell division.
There are several types of asexual spores:
 a. Arthroconidia or arthrospores:  A hypha can fragment (by the separation of hyphae through the splitting of the cell wall or septum) to form cells that behave as spores. These cells are called arthroconidia or arthrospores.
b. Chlamydospores: If the cells are surrounded by a thick wall before separation, they are called chlamydospores.
 c. Sporangiospores: If the spores develop within a sac at a hyphal tip, they are called sporangiospores.
 d. Conidiospores: If the spores are not enclosed in a sac but produced at the tips or sides of the hypha, they are termed conidiospores.
 e. Blastospores Spores produced from a vegetative mother cell by budding are called blastospores.
Sexual reproduction in fungi involves the union of compatible nuclei. Some fungal species are self-fertilizing and produce sexually compatible gametes on the same mycelium (homothallic). Other species require out crossing between different but sexually compatible mycelia (heterothallic). Depending on the species, sexual fusion may occur between haploid gametes, gamete-producing bodies called gametangia or hyphae. This sexual reproduction yields spores.
  a. Ascospores: These single celled spores are produced in a sac called ascus. There are usually eight ascospores in each ascus.
 b. Basidiospores: These single cell spores are produced on a club shaped structure called basidium.
 c. Zygospores: Zygospores are large, thick walled spores formed when the tips of two sexually compatible hyphae of certain fungi fuse together.
 d. Oospores: These are formed within a special female structure called oogonium. Fertilization of the eggs by male gamete formed in an anthridium give rise to oospores.
Fungal spores are important for several reasons. The size, shape, color, and number of spores are useful in the identification of fungal species. The spores are often small and light; they can remain suspended in the air for long periods. Thus they frequently aid in fungal dissemination, a significant factor that explains the wide distribution of many fungi.

Distribution of fungi

      Fungi are primarily terrestrial organisms, although a few are freshwater or marine.
      They are saprophytic as well as parasitic.
       Many are pathogenic and infect plants and animals. Fungi also form an association with other organisms. The association of fungi with the root of the higher plant is called mycorrhizae and association between fungi and algae is called lichen.

Importance of fungi

      Fungi are important to humans in both beneficial and harmful ways.
       They degrade complex organic materials in the environment to simple organic compounds and inorganic molecules.
      In this way, carbon, nitrogen, phosphorus, and other critical constituents of dead organisms are released and made available for living organisms.
      Besides this, fungi are the major cause of plant disease.
      Over 5,000 species of fungi attack economically valuable crops and garden plants and also many wild plants.
      Similarly, many diseases of animals and humans are caused by fungi.
      Some of the fungi have wide industrial importance. Fungi play important role in making of bread, wine, beer, cheese, soy sauce, and commercial production of many organic acids, certain drugs (eg. cortisone, cyclosporine) and many antibiotics (eg Penicillin).
      In addition, fungi are important research tools in the study of fundamental biological processes. Cytologists, geneticists, biochemists, biophysicists, and microbiologists use fungi in research.

Beneficial Effects of Fungi:

      Decomposition - nutrient and carbon recycling.
      Biosynthetic factories: The fermentation property is used for the industrial production of alcohols, fats, citric, oxalic and gluconic acids.
      Important sources of antibiotics, such as Penicillin.
      Model organisms for biochemical and genetic studies. Eg: Neurospora crassa
      Saccharomyces cerviciae is extensively used in recombinant DNA technology, which includes the Hepatitis B Vaccine.
      Some fungi are edible (mushrooms).
      Yeasts provide nutritional supplements such as vitamins.
      Penicillium is used to flavor Roquefort and Camembert cheeses.
      Ergot produced by Claviceps purpurea contains medically important alkaloids that help in inducing uterine contractions, controlling bleeding and treating migraine.
      Fungi (Leptolegnia caudate and Aphanomyces laevis) are used to trap mosquito larvae in paddy fields and thus help in malaria control.

Harmful Effects of Fungi:

      Destruction of food, lumber, paper, and cloth.
      Animal and human diseases, including allergies.
      Toxins produced by poisonous mushrooms and within food (Mycetism and Mycotoxicosis).
      Plant diseases.
      Spoilage of agriculture products such as vegetables and cereals in the godown.
      Damage the products like magnetic tapes and disks, glass lenses, marble statues, bones, and wax.

Terminologies used in Mycology

Some of the terminology used in mycology is as follows.
 A) Fungal element: Fungi include unicellular yeast, multicellular filamentous fungi called molds, macroscopic puffballs, and mushrooms. The body or vegetative structure of a fungus is called thallus. It varies in complexity and size, ranging from the single cell microscopic yeast to multicellular molds, macroscopic puffballs, and mushrooms. Fungi are eukaryotic microorganisms. The fungal cell consists of cell wall made up of chitin, cytoplasmic membrane, and other cell organelles (eg nucleus, ribosome, mitochondria etc.
B) Yeast: Yeast is a unicellular fungus that has a single nucleus and reproduces either asexually by budding and transverse binary fission or sexually through the formation of spores (Ascospores). Each bud that separates can grow into new yeast. Generally, yeast cells are larger than bacteria, vary considerably in size and are commonly spherical or egg shaped.  They do not have flagella but do possess most of the other eukaryotic organelles. eg Saccharomyces spp, Candida spp, Cryptococcus spp.
C) Molds: Molds are multicellular filamentous fungi.
D) Hyphae: Hyphae is the unit structure of filamentous fungi. Hyphae are composed of an outer cell wall and an inner lumen which contains the cytosol and cell organelles. The hyphae of some fungi have cross walls called septa and called septate hyphae. Hyphae of other fungi do not have septa and are called aseptate or coenocytic hyphae.
E) Mycelium: Mycelium is the tangled mass of the fungal hyphae of filamentous fungi.
F) Pseudomycelium or pseudohyphae or sprout mycelium: A mycelium like structure consisting of chains of cells formed by sequential budding is called pseudo-mycelium.
G) Germ tube: A short, hyphae like structure which develops from certain types of spore on germination or in yeast (eg Candida albicans) undergoing yeast to mycelium transition. A germ tube usually develops into hyphae.
H) Rhizoids: Rhizopus (bread molds) is fungi that grow in bread, vegetable, fruits, and other food products and cause food spoilage. These molds produce clusters of root like holdfasts which are called rhizoids. Morphologically, Rhizopus has nonseptate, cottony mycelia with sporangiophores arising at the nodes where the rhizoids form.
I) Dimorphic fungi: Many fungi, especially those that cause diseases in humans and animals, are dimorphic, i.e. they have two forms. Dimorphic fungi can change from (1) the yeast (Y) form in the animal to (2) the mold or mycelial form (M) in the external environment in response to changes in various environmental factors (nutrients, CO2 tension, oxidation-reduction potentials, temperature). This shift is called the YM shift.
J) Mycoses: Diseases caused by fungi are called mycoses.

Medically important fungi

      Among 90,000 species of fungi that have been described, fewer than 500 have been associated with human disease, and about 100 are capable of causing infection in normal individuals.
      Medical mycology is the discipline that deals with the fungi that cause human disease.
      It should be noted that the importance of opportunistic fungal pathogens is increasing because of the expansion of the immunocompromised patient population.

Classification of medically important fungi

      Taxonomic classification of fungi is complex and has little value in clinical mycology laboratory.
      Among a large number of fungi identified, only few may cause disease in human.
      In a medical setting, medically important fungi are categorized on the basis of basic morphology ( eg yeast, mycelial fungi, and dimorphic fungi) and disease they cause (e.g. superficial mycoses, cutaneous mycoses, subcutaneous mycoses and opportunistic mycosis).

Pathogenesis of fungal disease

      Most fungi are saprophytic or parasitic to plants and are adapted to their natural environment. Infection in humans is a chance event, occurring only when conditions are favorable.
      Except for few fungi such as the dimorphic fungi that cause systemic mycoses and dermatophytes, which are primary pathogens, the rest are only opportunistic pathogens.
      The human body is a hostile environment and offers great resistance to fungal invasion. Most fungi are saprophytic and their enzymatic pathways function more efficiently at the redox potential of non-living substrates than at the relatively more reduced state of living metabolizing tissue. Some fungi such as Candida and Malassezia have adapted to the human environment and exist as commensals.
      The complex interplay between fungal virulence factors and host defense factors will determine if a fungal infection will cause a disease. Infection depends on inoculum size and the general immunity of the host.

Virulence factor of medically important fungi

      Ability to adhere to host cells by way of cell wall glycoproteins
      Production capsules allowing them to resist phagocytosis.
      Ability to acquire iron from red blood cells as in Candida albicans
      Ability to damage host by secreting enzymes such as keratinase, elastase, collagenase
      Ability to resist killing by phagocytes as in dimorphic fungi
      Ability to secrete mycotoxins
      Having a unique enzymatic capacity
      Exhibiting thermal dimorphism
      Ability to block the cell-mediated immune defenses of the host.

Host defense mechanism

      Physical barriers, such as skin and mucous membranes
      The fatty acid content of the skin
      The pH of the skin, mucosal surfaces, and body fluids
      Normal flora
      Chemical barriers, such as secretions, serum factors
      Most fungi are mesophilic and cannot grow at 37oC.
      Natural Effector Cells (polymorphonuclear leucocytes) and the Professional Phagocytes (monocytes and macrophages)

Predisposing factors

      Prolonged antibiotic therapy
      Underlying disease (HIV infection, cancer, diabetes, etc.)
      Age
      Surgical procedures
      Immunosuppressive drugs
      Irradiation therapy
      Drug addiction
      Transplants
      Occupation

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