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.
Classification of Fungi
- 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.
- They divide the kingdom mycetae into three divisions namely:
- Mastigomycota and
- The division is subdivided into subdivision, classes, sub-classes, and orders.
Division I Gymnomycota
- It includes phagotrophic organism devoid of cell walls.
- This division comprises two subdivisions.
- These are Acrasiogymnomycotina and Plasmodiogynomycotina.
Subdivision 1. Acrasiogymnomycotina
It includes a single class Acrasiomycetes.
Class 1. Acrasiomycetes
- Lacks flagellated cells except for one species. The class comprises two subclasses.
- Acrasiomycetidae and Dictyosteliomycetidae.
Subdivision 2. Plasmodiogymnomycotina
It is divided into two classes:
Class 1 Protosteliomycetes
Class 2 Mycomycetes
It includes the true slime mold and comprises three sub class namely:
Sub class 1. Ceratiomyxomycomycetidae
Order – Ceratiomyxales
Sub Class 2. Mycogasteomycetidae
It comprise four orders.
Sub Class 3. Stemonitomycetidae
Order 1. Stemonitales
Division II Mastigomycota
- Includes fungi with absorptive nutrition, unicellular or filamentous, mycelium coemocytic.
- It comprises two sub divisions:
Sub division 1 Haplomastigomycotina
- Includes fungi with uni-or, bi-flagellate zoospores.
Class 1 Chytridiomycetes– Fungi producing zoospores furnished with a single whiplash flagellum inserted at the posterior end.
Class 2 Hyphochytridiomycetes- Motile cells with a single tinsel flagellum inserted at the anterior end.
Class 3 Plasmodiophoromycetes- Parasitic fungi producing biflagellate motile cells with both the flagella of whiplash type inserted at the anterior end.
Sub division 2. Diplomastigomycotima Sexual reproduction ooagamous, zoospores biflagellate.
Class 1 Oomycetes
– It comprises four orders:
Order 1 Lagenidiales
Order 2 Saprolegnailes
Order 3. Leptomitales
Order 4. Peronosporales
Division III Amastigomycota
Fungi with absorptive nutrition, motile cells lacking, mycelium aseptate or septate.
This includes four sub divisions:
Sub division 1 Zygomycotina
Class 1 Zygomycetes – it includes six orders.
Class 2 Trichomycetes – it comprises five orders.
Sub division 2 Ascomycotina
Fungi usually with a septate mycelium producing haploid ascospores in sac like cells called asci.
Class 1 Ascomycetes- divided into five sub classes:
Sub class 1. Hemiascomycetidae- comprising three orders.
Sub class 2. Plectomycetidae- Five orders
Sub class 3. Hymenoascomycetidae – Ten orders
Sub class 4 Laboulbeniomycetidae – Two orders
Sub class 5 Lowloascomycetidae – five orders
Sub division 3. Basidiomycotina
Septate mycelium, produces basidiospores, exogenously on various types of basidia.
Class 1 Basidiomycetes: it is split into 3 sub clases:
Sub class 1 Holobasidiomycetidae
Sub class 2 Phragmobasidiomycetidae
Sub class 3 Teliomycetidae
Sub division 4. Deuteromycotina
It includes imperfect fungi in which sexual stage is unknown. It comprises a single form class.
Form Class Deuteromycetes with three form sub classes namely Blastomycetidae, Coelomycetidae and Hyphomycetidae.
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.
- 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.
- 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 meioses to produce haploid biflagellate zoospores.
- Example; Phytophthora infestans(causes potato blight)
- 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 absence of motile sexual or asexual cells.
- 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
- 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.
- 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.
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.
Classification of Medically Important Fungi
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.
Superficial mycoses (or tineas) mostly occur in the tropics and are restricted to the outer surface of the hair and skin. Examples are:
- Piedraia hortae, a filamentous member of the Ascomycota which causes black piedra, a disease of the hair shaft characterised by brown/black nodules on the scalp hair (actually the ascostromata of the fungus).
- Trichosporon cutaneum, a yeast belonging to the Basidiomycota that is common in soil, water samples, plants, mammals and birds, as well as being a member of the normal flora of mouth, skin and nails. It causes white piedra, a superficial infection of the skin, and scalp and pubic hair (although it is emerging as an opportunistic pathogen of the immunocompromised).
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.
These three genera are Microsporum, Trichophyton and Epidermophyton (all filamentous Ascomycota) and they are often labelled ‘dermatophytes’ (with the disease being called ‘dermatophytosis’) although, of course, they are not plants, so they can’t be any sort of ‘-phyte’ and a better term would be dermatomycosis. These fungi all have the ability to degrade keratin and grow as non-invasive saprotrophs on skin and its appendages, but their growth causes irritation and inflammation of underlying epithelial cells, this being an allergic reaction that may result in death of these cells.
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.
- Madurella mycetomatis and M. grisea (filamentous, Ascomycota) cause human mycetoma (common name: madura foot), which is a localised infection causing locally invasive tumour-like abscesses, accompanied by chronic inflammation, resulting in swelling, distortion and ulceration of the infected body part. The fungus is introduced through mild wounds in the skin and may grow for several years in the cutaneous and subcutaneous tissues, extending to connective tissues and bones. Mycetomas are usually resistant to chemotherapy, leaving surgery, even amputation, as the only resolution.
- Sporothrix schenckii (thermally dimorphic, Ascomycota) causes sporotrichosis. Sporothrix is the anamorph and Ophiostoma stenoceras the teleomorph. The fungus occurs in soil worldwide although the disease is localised, with Peru having the highest prevalence of Sporothrix schenckii infections. Also called ‘rose handler’s disease’, sporotrichosis starts by entry of the fungus through minor skin injury and can then spread through the lymphatic system. The fungus is dimorphic, forming septate vegetative hyphae, conidiophores and conidia at 25°C, while at 37°C oval to cigar-shaped budding yeast cells are produced. As the yeast form is distributed by the lymphatic system, disseminated sporotrichosis can result in infections of the lungs and bones and joints, endophthalmitis (inflammation of the internal layers of the eye), meningitis and invasive sinusitis.
Systemic mycoses are infections that affect the whole body. We divide these into mycoses due to primary (usually dimorphic) virulent pathogens, and those due to opportunistic pathogens.
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.
Classification Based on Virulence
Primary pathogens can establish infections in normal hosts. Opportunistic pathogens cause disease in individuals with compromised host defense mechanisms.
- Deep mycoses are caused by primary pathogenic and opportunistic fungal pathogens.
- The primary pathogenic fungi are able to establish infection in a normal host; whereas, opportunistic pathogens require a compromised host in order to establish infection (e.g., cancer, organ transplantation, surgery, and AIDS).
- The primary deep pathogens usually gain access to the host via the respiratory tract. Opportunistic fungi causing deep mycosis invade via the respiratory tract, alimentary tract, or intravascular devices.
- The primary systemic fungal pathogens include Coccidioides immitis, Histoplasma capsulatum, Blastomyces dermatitidis, and Paracoccidioides brasiliensis.
- The opportunistic fungal pathogens include Cryptococcus neoformans, Candida, Aspergillusspp., Penicillium marneffei, the Zygomycetes, Trichosporon beigelii, and Fusarium spp.
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- Sastry A.S. & Bhat S.K. (2016). Essentials of Medical Microbiology. New Delhi : Jaypee Brothers Medical Publishers.
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- San-Blas G: Paracoccidioides brasiliensis: cell wall glucans, pathogenicity, and dimorphism. p. 235. In McGinnis MR (ed): Current Topics in Medical Mycology. Vol.1. Springer-Verlag, New York, 1985.