Fungi: Characteristics and Classification


  • Study of fungus (yeast and moulds) is called mycology.
  • Derived from Greek word where “mykos” meaning fungus and “logus” means study.
  • Known earlier than bacteria as the causative agent for various human diseases.
  • Favus and thrush are found described as early as 1839.
  • Only scant attention was given towards fungi as compared to other pathogens.
  • Thus, study of fungus though began earlier could not get much popular.
  • One reason may be due to benign nature of the common mycotic diseases.
  • Another reason is the use of techniques in the field of mycology is more those used by botanists than by bacteriologists.
  • Though fungal infections are common, they are more fatal and serious.
  • Developed countries are able to control bacterial diseases, however, fungal infections are also the matter of concern.
  • Also cause many fatalities as compared to diseases caused by bacterial and other pathogens.
  • Mostly saprophytic in nature and found in soil.
  • Some human mycotic infections are opportunistic also.
  • Various misuse and overuse of different types of drugs have increased opportunistic fungal infections at present.

General Features & Classification Of Fungi | Mycology Notes

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  • Eukaryotic Protista different from bacteria.
  • Also different than various other prokaryotes in several ways.
  • Contains rigid cell wall made from chitin, mannan and other polysaccharides.
  • Sterols contents in cytoplasmic membrane.
  • True nucleus with nuclear membrane and paired chromosomes present.
  • Division occurs asexually, sexually or by both ways.
  • Single unicellular and many multicellular in nature.
  • Various degrees of specialistion shown by the cells of fungi.


Fungi are classified in several ways on different basis. Some of basis of classification are:

a) On the basis of morphology

  • Depending on cell morphology, fungi are categorized into four groups.
  • This is from diagnostic point of view.

i) Yeasts

  • Single celled (unicellular) fungi.
  • Budding is the reproducing means (bud = blastospore [blastoconidium])
  • Macroscopic colonies-pasty colonies in culture.
  • Resembling like that of bacterial colonies.
  • Oval to round with 3-15 micronmeter in diameter.
  • Spherical or oval in forms in tissues and in culture.
  • Hyphae-like (filamentous) structures not seen in tissues or in culture.
  • Examples: Cryptococcus neoformans.

ii) Yeast-like fungi

  • Unicellular fungi.
  • Budding and fission acts as means of reproduction.
  • Pasty colonies in culture like that of bacterial colonies.
  • In tissues and culture, exists as spherical or oval forms.
  • Pseudo-hyphae (filamentous structures) may also be seen.
  • Examples: Candida albicans.

20 Examples of Fungi - Phycomycetes, Ascomycetes, Basidiomycetes and  Deuteromycetes

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iii) Moulds or filamentous fungi

  • Made up of several hyphae.
  • May have cross walls or septa or may be devoid of septa.
  • Septa having hyphae are multicellular and septa absence hyphae are coenocytic.
  • Mostly exhibit asexual means of reproduction through spore formation.
  • Whereas some shows sexual reproduction also.
  • Cottony, woolly, velvety, granular surface texture.
  • From the reverse, observable pigmentation.
  • Thread-like filamentous hyphae of 2-10 micron in tissues and in culture.
  • May be aseptate or septate on the basis of presence or absence of cross-walls.
  • Zygomycetes are aseptate whereas Aspergillus fumigatus show septate hyphae.
  • Vegetative or aerial type of mycelium.
  • Any shape may be shown by hyphae like racquet, nodular, pectinate, spiral, root-like, chandelier-like, etc.
  • Example: Aspergillus fumigatus.

iv) Thermally dimorphic fungi

  • In culture and environment grow as filamentous forms.
  • Temperature required is 22-25 degree.
  • In culture at 37 degree and in tissues, grows as yeast.
  • Example: Histoplasma capsulatum.

b) Systemic classification

Based on the formation of sexual spore, systemic classification of fungi is done into four classes.

i) Phycomycetes

  • Non-septate hyphae having lower fungi.
  • Form endogenous asexual spores called sporangiospores.
  • Spores are present in sporangia, a swollen sac-like structure.
  • Sexual spores also produced called oospores in some fungi.
  • Some produces zygospores also as sexual spores.
  • Other three classes so called higher fungi have septate hyphae and form exogenous asexual spores called conidia.

ii) Ascomycetes

  • Form sexual spores called ascospores within a structure.
  • Sac-like structure is called ascus.
  • Include both yeast and filamentous fungi.

iii) Basidiomycetes

  • Sexual spores called basidiospores are formed.
  • Spores are formed in structure called basidium or base .

iv) Fungi imperfecti

  • Also called deuteromycetes or hyphomycetes.
  • A provisional group consisting of fungi whose sexual phases have not been identified.
  • Medical importance having fungi mostly belong to this group.

c) Pathogenic classification

Fungi are categorized in following way based on their pathogenic potentiality.

i) Primarily pathogenic

  • Cause infections in healthy individuals.
  • Example: thermally dimorphic fungi

ii) Opportunistic pathogens

  • Infections in patient caused by various mechanisms.
  • Generally, those having defects in immune mechanisms by various reasons are affected.
  • Immunosuppressive drugs or individuals having diabetes mellitus or debilitating conditions or those suffering from HIV are more affected.

Pathogenic fungi may cause:

a) Actual infection of tissues(mycoses)

  • Superficial, cutaneous, sub cutaneous and deep or visceral mycoses are some types.
  • For damage or pathogenic effects, fungus should be present in the tissues.

b) Mycotoxicoses

  • Toxic metabolic products from pathogens also cause diseases.
  • They are released by fungi also.
  • Aflatoxicosis is related to aflatoxins secreted by Aspergillus flavus.
  • Found in contaminated groundnuts, corn and peas.
  • Ergotism is caused after consuming rye contaminated with Claviceps purpurea.
  • In these cases, fungus presence is not necessary within the tissues to exert pathogenic effects.
  • Their metabolites can effect easily though there is no invasion of tissue by fungus.

c) Hypersensitivity (allergic reaction)

  • Inhaling fungal spores evoke Type I and Type III hypersensitivity reaction.
  • Spores of Aspergillus fumigatus are more noticeable.
  • Causes allergic bronchopulmonary aspergillosis and allergic fungal rhinosinusitis.
  • Here also there is no invasion of fungus in the tissues.
  • Just their presence is sufficient to evoke the hypersensitivity reaction.




Fungi: Characteristics and Classification