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Classification of Fungi

Fungi are eukaryotic microorganisms. They can occur as yeasts, molds, or as a combination of both forms.

Some fungi are capable of causing superficial, cutaneous, subcutaneous, systemic or allergic diseases.

Yeasts are microscopic fungi consisting of solitary cells that reproduce by budding. Molds, in contrast, occur in long filaments known as hyphae, which grow by apical extension.

Regardless of their shape or size, fungi are all heterotrophic and digest their food externally by releasing hydrolytic enzymes into their immediate surroundings (absorptive nutrition).

Other characteristics of fungi are the ability to synthesize lysine by the L-α-adipic acid biosynthetic pathway and possession of a chitinous cell wall, plasma membranes containing the sterol ergosterol, 80S rRNA, and microtubules composed of tubulin.

The classification of fungi, like that of bacteria, is designed mainly for practical application but it also bears some relation to phylogenetic considerations.

The nomenclature is binomial, with a generic and a specific name (eg: Aspergillus niger).

Species are collected in genera, genera in families (suffix –aceae), families in orders (suffix-ales), and orders in classes (suffix-mycetes).

The division of mycota, or fungi and moulds, includes the true slime moulds (Myxomycetes), the lower fungi (Phycomycetes), and the higher fungi (Eumycetes).

Alexopolous and Mims proposed fungal classification in 1979. They place the fungi including the slime molds in the kingdom mycetae of the super kingdom Eukaryota which, in addition, includes four other kingdoms.

1. Classification On the Basis of Spore Production

On the basis of the organisation of the vegetative thallus, the morphology of reproductive structures, the way of spores production and particular life cycle involved the kingdom mycota is classified into following divisions.

1. Phycomycetes

It includes the simplest type of fungi. It is also called as Algae-Fungi because most of the characteristics of them are similar to algae like Vaucheria.

They have simple thallus which is unicellular or coenocytic or aseptate filaments.

They reproduce asexually by the formation of zoospores or non-motile spores.

Sexual reproduction is isogamous or heterogamous which takes place by gametangial contact.

The diploid phase is represented by zygote.

Phycomycetes has been classified into subclasses: oomycetes and zygomycetes.

2. Oomycetes

Oomycetes range from a primitive unicellular thallus to a profusely branched filamentous mycelium.

Many members of them are terrestrial and obligate parasites.

Asexually they reproduce by biflagellate zoospores.

Sexual reproduction is oogamy that involves the fusion of male and female gametes to form oospore.

Oospore undergoes meiosis to produce haploid biflagellate zoospores.

Example; Phytophthora infestans(causes potato blight)

3. Zygomycetes

The group is named zygomycetes because a diploid resting spore called the zygospore is formed during the life cycle.

They are mostly saprophytic, some others are parasites on plants and animals.

The vegetative body is mycelium which is well developed, profusely branched and coenocytic.

The asexual reproduction takes place by sporangiospores, aplanospores or by conidia.

Sexual reproduction occurs by conjugation of gametangia resulting in the formation of zygospore.

Examples; Rhizopus, Mucor etc

4. Ascomycetes

The species of ascomycetes are called the sac fungi because they produce sexual pores within the sac-like vascus.

General Characteristics

Ascomycetes are mostly terrestrial occurring as saprophytes or parasites.

They have well-developed, branched, septate mycelium except yeast. Yeast is a unicellular fungus.

Asexually they reproduce by non-motile spores, conidia, oidia or chlamydospores.

Sexual reproduction takes place by the fusion of gametangia of opposite mating types.

There is absence of motile cells.

Examples, Saccharomyces cerevisiae, Penicillium, Aspergillus etc.

5. Basidiomycetes

The members of basidiomycetes are saprophytic or parasitic. The group is named basidiomycetes as they produce the basidiospores at the club-shaped basidium during sexual reproduction.

Mycelium is highly developed, profusely branched and septate.

The mycelia are differentiated into two mating types; (+ve) and (-ve).

There are two kinds of mycelium; primary mycelium and secondary mycelium.

Asexual reproduction takes place by fragmentation, budding, oidia, conidia or chlamydospore.

The dikaryotic cell is formed during sexual reproduction.

The absence of motile cell throughout the life cycle.

Basidiomycetes are the most advanced fungi as their fructifications are often large and prominent.

Examples; Mushrooms, Puccinia, Ustilago etc.

6. Deuteromycetes (The Imperfect Fungi)

Deuteromycetes compromises more than 17000 species of the diverse habits and habitats. It is considered as an artificial class of fungi.

The fungi are saprophytes as well as parasites.Parasitic fungi cause serious diseases to plants, animals including human beings.

Some of them are unicellular while others are multicellular.

They reproduce asexually by conidia along with some other types of spores.

The sexual reproduction is entirely absent.

The asexual stage or imperfect stage in Deuteromycetes is well defined. But the sexual or perfect stage is absent in life cycle, therefore, they are called ‘Fungi Imperfecti’.

Example; Alternaria, Fusarium, Helminthosporium etc.

2. Classification of Medically Important Fungi

1. Classification Based on Site

Mycoses are classified as superficial, cutaneous, subcutaneous, or systemic (deep) infections depending on the type and degree of tissue involvement and the host response to the pathogen.

1. Superficial mycoses(or tineas) mostly occur in the tropics and are restricted to the outer surface of the hair and skin.
2. Cutaneous mycoses. There are three genera of fungi that commonly cause disease in the non-living tissues of skin, hair, or nails/claws of people and animals, by growing in a zone just above where the protein keratin is deposited.
3. Subcutaneous mycoses are generally caused by fungi that are normally saprotrophic inhabitants of soil, particularly in tropical and subtropical areas of Africa, India and South America, which become infective by being introduced through wounds in the skin. Most infections involve people who normally walk barefoot.
4. Systemic mycosesare infections that affect the whole body. We divide these into mycoses due to primary (usually dimorphic) virulent pathogens, and those due to opportunistic pathogens. 

3. Classification Based on Route of Acquisition

Infecting fungi may be either exogenous or endogenous.

When classified according to the route of acquisition, a fungal infection may be designated as exogenous or endogenous in origin.

If classified as exogenous, an infecting organism may be transmitted by airborne, cutaneous, or percutaneous routes.

An endogenously-acquired fungal infection may be acquired from colonization or reactivation of a fungus from latent infection.

4. Classification Based on Virulence

Primary pathogens can establish infections in normal hosts. Opportunistic pathogens cause disease in individuals with compromised host defense mechanisms.