YEAST AND MOULDS

Yeast and mould are the two basic morphological forms in which fungal organisms are known to grow or develop into. And this does not include mushrooms (Figure 1), which are usually macroscopic in nature and can be seen by the naked eyes. Mushrooms are of immense economic importance, and some mushrooms are edible and can be consumed by man as a source of protein, fat, carbohydrate, vitamins and minerals. Mushrooms and their economic importance as well as mushroom cultivation are described later in this section. Yeasts are single-celled or unicellular fungi, and they are usually round to oval in shape. Yeast is the single-celled growth form of some fungi. They reproduce sexually (by spore formation) and asexually (by budding).

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Moulds are filamentous-branching forms of fungi that also bear conidia or fungal spores. Moulds are multicellular fungal cells and they appear as threadlike structures. Mould is the filamentous form of fungi that exist as hyphae or mycelia colonies. They are multicellular forms of fungi. Moulds grow as filamentous, branching strands of hyphae. Fungi are ubiquitously distributed in the universe, and they directly or indirectly affect our daily lives even though we may be unaware about some of their activities. For example, the change in the colour of a loaf of bread kept in the refrigerator or in the open after some days is due to fungal activity (moulds in particular). More so, moulds adversely affect the aesthetics of buildings, art works and other materials in the environment. About 100,000 species of fungi have been described and classified by mycologists, and only a small fraction of these actually cause disease in humans and animals. Majority of fungi are used industrially for the production of several goods and products used by man, plants and animals because of their immense economic importance in nature. Table 1. elucidates some key terminology associated with the study of fungi.

Figure 1. Schematic illustration of a mushroom. Mushrooms grow in the natural environment on tree trunks, in association with other vegetations. They also grow in the soil on their own but they can also be cultivated for edible and other economic purposes. Photo courtesy: https://www.microbiologyclass.com
 

Table 1. EXPLANATION OF SOME FUNGAL TERMS 

TermMeaning
YeastYeast is the single-celled growth form of some fungi. They reproduce sexually (by spore formation) and asexually (by budding).
HyphaeHyphae is the long branching filaments or structures of fungi. It is a network of filaments (with cross-walls) formed by multicellular fungi (i.e., moulds). Some fungal hyphae known as coenocytic hyphae are without cross-walls and contain more than one nucleus (i.e., they are multi-nucleated). Fungal hyphae without cross-walls are generally called coenocytic hyphae or non-septate hyphae while those with cross-walls are known as septate hyphae.  
MouldMould is the filamentous form of fungi that exist as hyphae or mycelia colonies. They are multicellular forms of fungi. Moulds grow as filamentous, branching strands of hyphae.
Thallus (plural: thalli)Thallus is the vegetative body of fungi. It can also be known as fungal colony or fungal structure. It is the visible structures of fungi. Fungal thallus structure or organization comprises fungal hyphae (septate and non-septate), mycelium and fungal spores or conidia.  
Conidia (singular: conidium)Conidia are the asexual reproductive spores of fungi. They are generally known as fungal spores in some cases. However, the term “spores” is usually used to describe the sexual forms of fungi in terms of their reproduction. Thus the term “conidium” is strictly used for the asexual reproductive forms of fungi. In fungi, spores are formed sexually by meiosis and asexually by mitosis.
SporangiosporesSporangiospores are asexual spores produced by zygomycetes. They are borne or enclosed in the sporangia (singular: sporangium) of a fungal thallus.
DermatophytesDermatophytes are fungal cells that cause skin infections or diseases. They are a group of fungal organisms that can degrade the keratin layers or tissues of animals and humans. Dermatophytes mainly affect the nails, skin and hair, causing a range of infections generally known as dermatophytosis (for example, ringworm). Typical examples of dermatophytes are Microsporum species, Epidermophyton species and Trichophyton species.  
Dematiaceous fungiDematiaceous fungi are fungal organisms with cell walls that contain the skin pigment, melanin. Such fungi leave a characteristic black colouration on the skin after an infection or disease.
Septum (plural: septa)Septa are the cross-walls that form in a fungal hypha. They help to differentiate coenocytic hyphae from septate hyphae.
MacroconidiaMacroconidia are large conidia or spores of fungi.
MicroconidiaMicroconidia are small conidia or spores of fungi.
PseudohyphaePseudohyphae are chains of an elongated bud. They are usually formed from fungal species whose buds fail to separate from the parent cell, but instead continue to extend to form an elongated budding yeast cells known as pseudohyphae. Pseudohyphae generally have cell walls that are less-rigid than those of hyphae.
Perfect fungiPerfect fungi are fungi that carryout sexual reproduction. They are generally known as teleomorphs. Teleomorphs are fungal organisms with sexual characteristics.
Imperfect fungiImperfect fungi or fungi imperfecti are fungi that reproduce by asexual reproduction. They are generally known as anamorphs. Anamorphs are fungal organisms with asexual characteristics. 
MyceliumMycelium is a mass or network of hyphae. They are generally a collection of intertwined or knotted hyphae that penetrates the supporting medium or substrate on which the fungus is growing. 
MushroomMushrooms are fruiting fungal bodies that are macroscopic in nature. They are visible; and are known to grow in moist environments. Some mushrooms are edible while others are poisonous in nature.
PhialidesPhialides are non-septate, colourless or pigmented conidia formed from vegetative hyphae. They are the terminal ends of conidiophores. Phialides are usually formed by Phialophora and Trichoderma species.
Metulla (singular: metulae)Metulla are short extensions or cell branches which bear one or more phialides in the conidiophores.
Columella (singular: columellae)Columella are axial or central, unicellular or multicellular structures formed within the fruiting body of some fungi. It is an extension of the sporangiophore into the cavity of the sporangium. Columella is seen in Mucor species and myxomycetes fungi.
SterigmataSterigmata are four small protuberances or finger-like protrusions which are formed at the terminal ends of some vegetative cells; and from which daughter cells or spores emanate by budding.
AscosporesAscospores are the sexual spores of Ascomycetes.
Ascus (plural: asci)Ascus is the sac-like fungal structures containing 4 or 8 ascospores. They are mainly formed by the Ascomycetes.
DimorphismThe 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 but as mould in the environment. Such fungi are called dimorphic or diphasic fungi. Examples of dimorphic fungi include Histoplasma capsulatum, Blastomyces dermatitidis, Paracoccidioides species, Candida albicans and Sporothrix species. Dimorphic fungi are mostly responsible for systemic (endemic) mycoses; and 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 nutrients). In plants, dimorphic fungi changes from mould form to yeast form 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.

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.       

Dix, N.J.  and Webster, J.  (1995). Fungal Ecology. Chapman and Hall, London.

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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