FUNGAL MORPHOLOGY (THALLUS)

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Fungi unlike other groups of microorganisms possess or have unique structures and morphological features which distinguish them from other microbial cells including bacteria, viruses, algae and protozoa. The structure or morphology of fungi is generally known as thallus. The phrase thallus (meaning fungi body) also applies to other organisms such as algae, lichens and some plants. However, thallus generally refers to the entire vegetative body of a multicellular organism that is usually non-motile in nature. Typical example of such an organism is fungi. The thallus of a fungus is usually known as mycelium in some cases. In summary, thallus is the vegetative body of a fungus. Morphologically, fungi exist in several structural forms including mycelium, pseudomycelium, yeast or mould; and these structural forms make up the thallus of a fungus.

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MYCELIUM (PLURAL: MYCELIA)

Mycelium is the long branching structures of fungal hyphae (Figure 1). Mycelia are specialized structures used by fungal cells to absorb nutrients from their environment or supporting medium. They are known to penetrate the substrate on which they are growing on to acquire nutrients by absorption.

Figure 1. A sketch of mycelium. Photo courtesy: https://www.microbiologyclass.com

Two types of mycelia are exhibited by fungi. They are vegetative mycelia and aerial mycelia. Vegetative mycelia act as the root of a fungal organism. They are known to penetrate the substrate on which the fungus is growing. Vegetative mycelia help the fungus to absorb nutrient and water from the substrate for the growing fungal cell. Another type of mycelia which do not penetrate nutrient substrate but grow above it is the aerial mycelia. Aerial mycelia are important for asexual reproduction in imperfect fungi. Aerial mycelia bear or carry conidia or fungal spores; and they serve as important reproductive structures in fungi. 

YEAST CELL

Yeasts are the unicellular forms of fungi. They usually reproduce by budding, an asexual reproduction pattern. Budding is a type of asexual reproduction in which the daughter cell emanates or develops from the parent cell. In budding, the daughter cell develops entirely from the mother or parent cell as a localized outgrowth. Structurally, yeast is oval or spherical in form. They grow selectively on Sabouraud dextrose agar (SDA) plate, producing a smooth, white fluffy colony that may later turn to a brownish gray colour (Figure 2).  

Figure 2. Schematic illustration of yeast cells growing on Sabouraud Dextrose Agar (SDA) medium. Yeast colonies are usually soft, opaque, and cream colored, and with a diameter of about 1-3 mm in size. They generally assume the shape of groundnuts. Notice the groundnut-shaped colonies (arrows) which are characteristic of fungal growth on solid culture media. Photo courtesy: https://www.microbiologyclass.com

HYPHA (SINGULAR: HYPHAE)

Hyphae is the tube-like extension of a fungal cell. They are usually rigid or thick in form. A mass of hyphae is required for the formation of fungal mycelia. Hyphae formation by a fungal cell can be septate i.e., with cross-walls (Figure 3) or non-septate i.e., without cross-walls (Figure 4).

Figure 3. A sketch showing septate hyphae. Septate hyphae have cross-walls or compartments that contain nuclei. Photo courtesy: https://www.microbiologyclass.com
Figure 4. A sketch of non-septate or coenocytic hyphae. Non-septate hyphae do not have cross-walls. They can also be called coenocytic hyphae. Coenocytic hyphae have multiple nuclei; and thus they are said to be multi-nucleated in nature. Photo courtesy: https://www.microbiologyclass.com

As aforementioned, fungi produce two types of hyphae, which are aerial hyphae and vegetative hyphae. Aerial hyphae are branching structures of fungi that do not penetrate the supporting medium but instead they project above the surface of the mycelia. They usually bear the reproductive structures of the filamentous fungi (i.e., moulds). On the other hand, vegetative hyphae can also be called the substrate hyphae. They are the hyphae that penetrate the supporting medium on which the fungus is growing. Substrate or vegetative hyphae absorb nutrients from the supporting medium necessary for fungal growth. Generally, fungal mycelia are a mass of hyphae.      

PSEUDOMYCELIUM

Pseudomycelium or Pseudohypha is the less-rigid hyphae formed by some fungi (Figure 5). They are different from the true-hyphae (i.e., septate and non-septate hyphae) which are known to be very rigid. Pseudomycelium is usually formed by some species of yeasts such as Candida species; and these organisms produce or form a multicellular chain of yeast-like cells known as pseudomycelium during their reproduction.

Figure 5. Schematic illustration of pseudomycelium formation. Pseudomycelium isa cellular association occurring in various higher bacteria and yeasts – in which cells cling together in chains resembling small true mycelia. Photo courtesy: https://www.microbiologyclass.com

 

Further reading

Anaissie E.J, McGinnis M.R, Pfaller M.A (2009). Clinical Mycology. 2nd ed. Philadelphia, PA: Churchill Livingstone Elsevier. London.

Baumgardner D.J (2012). Soil-related bacterial and fungal infections. J Am Board Fam Med, 25:734-744.

Calderone R.A and Cihlar R.L (eds). Fungal Pathogenesis: Principles and Clinical Applications. New York: Marcel Dekker; 2002.

Champoux J.J, Neidhardt F.C, Drew W.L and Plorde J.J (2004). Sherris Medical Microbiology: An Introduction to Infectious Diseases. 4th edition. McGraw Hill Companies Inc, USA.       

Gladwin M and Trattler B (2006). Clinical Microbiology Made Ridiculously Simple. 3rd edition. MedMaster, Inc., Miami, USA.

Larone D.H (2011). Medically Important Fungi: A Guide to Identification. Fifth edition. American Society of Microbiology Press, USA.

Madigan M.T., Martinko J.M., Dunlap P.V and Clark D.P (2009). Brock Biology of Microorganisms, 12th edition. Pearson Benjamin Cummings Inc, USA.

Stephenson S.L (2010). The Kingdom Fungi: The Biology of Mushrooms, Molds and Lichens. First edition. Timber Press.

Sullivan D.J and Moran G.P (2014). Human Pathogenic Fungi: Molecular Biology and Pathogenic Mechanisms. Second edition. American Society of Microbiology Press, USA.

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