Classification of Fungi

• Fungi include eukaryotic microorganisms. They might appear as moulds, yeasts, or a hybrid of the two.
• Some fungi have the potential to lead to systemic, cutaneous, subcutaneous, autoimmune, or allergy disorders.
• Yeasts are tiny fungi made up of single cells that divide by budding. In contrast, hyphae, which are lengthy filaments that develop through apical extension, are seen in moulds.
• All fungi, irrespective of size or shape, are heterotrophic, meaning they digest their food outside by discharging hydrolytic enzymes throughout their surroundings (absorptive nutrition).
• Fungi also possess the ability to manufacture lysine via the L-adipic acid biosynthetic route, a chitinous cell wall, plasma membranes carrying the sterol ergosterol, 80S rRNA, and microtubules composed of tubulin, among other characteristics.

Classification of Fungi

• As with the classification of bacteria, the classification of fungi is largely designed for practical application, although it is also related to phylogenetic aspects.
• Binomial nomenclature is used, having both a generic and a particular name (e.g., Aspergillus niger).
• Species are grouped into genera, families (with the suffix -aceae), orders (with the suffix -ales), and classes (with the suffix -ales) (suffix-mycetes).
• The real slime moulds (Myxomycetes), lower fungi (Phycomycetes), and higher fungi are all included in the classification of mycota, or fungus and moulds (Eumycetes).
• In 1979, Alexopolous and Mims created a classification system for fungi. The fungus that contains slime moulds is located in the super kingdom Eukaryota, which also comprises four other kingdoms, inside the kingdom of mycetae.
• They categorise the mycetae kingdom into three groups, namely:

1. Gymnomycota
2. Mastigomycota and
3. Amastigomycota

Subdivisions, classes, sub-classes, and orders make up the division.

Division I, Gymnomycota

• There are phagotrophic organisms in there that lack cell walls.
• There are two subdivisions within this division.
• Acrasiogymnomycotina and Plasmodiogynomycotina are these.

Subdivision 1. Acrasiogymnomycotina

It consists only of the class Acrasiomycetes.

Class 1. Acrasiomycetes

• Just one kind of flagellated cell is present. There are two subclasses in the class.
• Dictyosteliomycetidae and Acrasiomycetidae.

Subdivision 2. Plasmodiogymnomycotina

It is divided into two classes:

Class 1 Protosteliomycetes

Class 2 Mycomycetes

It includes the actual slime mould and has three subclasses, specifically:

Sub class 1. Ceratiomyxomycomycetidae

Order – Ceratiomyxales

Sub Class 2. Mycogasteomycetidae

There are four orders in it.

Order

• Liceales
• Echinosteleales
• Trichlales
• Physarales

Sub Class 3. Stemonitomycetidae

Order 1. Stemonitales

Division II Mastigomycota

• includes filamentous, coemocytic, unicellular, and able to absorb nutrients fungus.
• It has two subcategories:

Sub division 1 Haplomastigomycotina

consists of fungus with uniflagellate or biflagellate zoospores.

Class 1 Chytridiomycetes: Fungi that produce zoospores and have an inserted whiplash flagellum on the back of the spore.

Class 2 Hyphochytridiomycetes: Motile cells with a single tinsel flagellum inserted at the front end are classified as Class 2 Hyphochytridiomycetes.

Class 3 Plasmodiophoromycetes are parasitic fungi that produce whiplash-shaped flagella on both ends of their biflagellate motile cells.

Sub division 2. Diplomastigomycotima 

Zoospores are produce through biflagellate ooagamous sexual reproduction.

Class 1 Oomycetes

There are four orders in it:

Order 1 Lagenidiales

Order 2 Saprolegnailes

Order 3: Leptomitales

Order 4. Peronosporales

Division III Amastigomycota

Mycelium that is aseptate or septate, with motile cells absent, and fungi with absorptive feeding.

There are four subsections under this:

Sub division 1: Zygomycotina

Class 1 Zygomycetes includes six orders.

Class 2 Trichomycetes comprises five orders.

Sub division 2 Ascomycotina

Usually septate mycelial fungi that produce haploid ascospores in sac-like cells known as 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

On several kinds of basidia, septate mycelium develops basidiospores exogenously.

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 contains imperfect fungus whose sexual maturity is unknown. It has just one form class.

Blastomycetidae, Coelomycetidae, and Hyphomycetidae are the three form subclasses of the form class Deuteromycetes.

On the Basis of Spore Production

The kingdom Mycota are divided into the following groups based on how the vegetative thallus is organized; the morphology of the reproductive structures; how spores are produced; and the specific life cycle that is involved.

Phycomycetes

• It contains the most basic kind of fungus. Due to the fact that the majority of their traits resemble those of algae like Vaucheria, they are also known as Algae-Fungi.
• They have a unicellular, coenocytic, or aseptate filamentous simple thallus.
• By producing zoospores, or non-motile spores, they reproduce asexually.
• Isogamous or heterogamous sexual reproduction occurs through gametangial touch.
• The zygote represents the diploid phase.
• There are two categories of phycomycetes: oomycetes and zygomycetes.

Oomycetes

• Oomycetes can have a simple unicellular thallus or a filamentous mycelium with several branches.
• They include several obligate terrestrial parasites.
• They create biflagellate zoospores to reproduce asexually.
• Oogamy, or sexual reproduction, is the fusing of male and female gametes to create an oospore.
• Meioses are performed on the oospore to create haploid biflagellate zoospores.
• Phytophthora infestans, for instance (causes potato blight).

Zygomycetes

• During the life cycle, a diploid resting spore known as the zygospore forms. The group is known as zygomycetes.
• Most of them are saprophytes, whereas a few others are parasites of both plants and animals.
• Mycelium, which is highly developed, abundantly branching, and coenocytic, is the vegetative body.
• lack of sexual or asexual cells that can move.
• Asexual reproduction can occur by conidia, aplanospores, or sporangiospores.
• Gametangia are conjugated during sexual reproduction, culminating in the creation of zygospores.
• Examples include Rhizopus and Mucor.

Ascomycetes

• Due to the fact that they produce sexual pores within the sac-like vascus, the ascomycete species are known as “sac fungus.”

General Characteristics

• The majority of ascomycetes are terrestrial saprophytes or parasites.
• Except for yeast, they have well matured, branching, septate mycelium. A yeast organism just has one cell.
• They can reproduce asexually via chlamydospores, conidia, oidia, or non-motile spores.
• The fusing of gametangia from opposing mating types results in sexual reproduction.
• There are no motile cells present.
• Aspergillus, Penicillium, and Saccharomyces cerevisiae are a few examples.

Basidiomycetes

• Basidiomycetes are either saprophytic or parasitic organisms. Because they generate basidiospores in the club-shaped basidium during sexual reproduction, the group is known as the basidiomycetes.
• Highly developed, heavily branching, and septate mycelium.
• The mycelia are divided into two forms of mating: (+ve) and (-ve).
• Mycelium comes in two varieties: main mycelium and secondary mycelium.
• By fragmentation, budding, oidia, conidia, or chlamydospore, asexual reproduction occurs.
• During sexual reproduction, the dikaryotic cell is created.
• a motile cell’s absence throughout the life cycle.
• Due to their often massive and noticeable fructifications, basidiomycetes are the most developed fungi.
• Examples include mushrooms, porcinis, astragalus, etc.

Deuteromycetes (The Imperfect Fungi)

• More than 17000 different species of Deuteromycetes exist, with a variety of lifestyles and environments. It is regarded as a type of synthetic fungus.
• The fungi are both parasites and saprophytes.
• Serious illnesses in plants, animals, and people are brought on by parasitic fungi.
• While some of them have just one cell, others have many.
• Asexual conidia, along with several other spore types, are how they reproduce.
• There is no sexual reproduction whatsoever.
• In Deuteromycetes, the asexual or imperfect stage is well characterised. However, because their life cycle lacks a sexual or ideal stage, they are known as “Fungi Imperfecti.”
• Alternaria, Fusarium, Helminthosporium, etc. are a few examples.

Classification of Medically Important Fungi

Classification Based on Site

• Mycoses are classified as superficial, cutaneous, subcutaneous, or systemic (deep) infections based on the type and amount of tissue participation as well as the host’s reaction to the pathogen.
• The majority of superficial mycoses, also known as tineas, are tropical in origin and only affect the skin’s and hair’s outermost layers. Examples include
• Piedraia hortae, a filamentous Ascomycota fungus, is the causal agent of black piedra, a disease of the hair shaft characterised by dark or black nodules on the scalp hair (actually the ascostromata of the fungus).
• The Basidiomycota yeast Trichosporon cutaneum is frequently seen in soil, water samples, plants, animals, and birds. It is also a common component of the oral, cutaneous, and nail flora. It leads to white piedra, a superficial skin infection, as well as pubic and scalp hair (but it is developing as an opportunistic infection in immunocompromised individuals).
• skin-colored mycoses. Three genera of fungus frequently cause illness in the dead tissues of human and animal skin, hair, or nails/claws by growing in an area slightly above the keratinization site.
• These three genera, Microsporum, Trichophyton, and Epidermophyton, are all filamentous Ascomycota, and they are frequently referred to as “dermatophytes” (the disease is known as “dermatophytosis”). However, because they are not plants, they are obviously unable to be any kind of “-phyte,” and dermatomycosis would be a more appropriate term. These fungi can all break down keratin and spread as non-invasive saprotrophs on skin and its appendages. However, as they expand, these fungi irritate and inflame the epithelial cells beneath the skin, which can lead to an allergic reaction and cell death.
• In most cases, fungi that are often saprotrophic soil dwellers, especially in tropical and subtropical regions of Africa, India, and South America, produce subcutaneous mycoses. These fungi spread infection through skin incisions. The majority of infections affect those who typically go barefoot.
• Human mycetoma, often known as “madura foot,” is a localised infection that results in locally invasive tumour-like abscesses and persistent inflammation, swelling, deformation, and ulceration of the infected body part. It is caused by Madurella mycetomatis and M. grisea (filamentous, Ascomycota). The fungus enters the body through small skin wounds and can spread to connective tissues and bones after years of growth in the cutaneous and subcutaneous tissues. Chemotherapy is typically ineffective against mycetomas, leaving surgery or perhaps amputation as the main treatment option.
• The sporotrichosis-causing Sporothrix schenckii is an Ascomycota (thermally dimorphic) species. The teleomorph is Ophiostoma stenoceras, while the anamorph is Sporothrix. Peru has the highest frequency of Sporothrix schenckii infections, despite the fact that the disease is localised and the fungus is present in soil all over the world. Sporotrichosis, often known as “rose handler’s sickness,” is a fungal infection that enters the body through a small skin wound and spreads through the lymphatic system. The fungus is dimorphic, producing oval to cigar-shaped budding yeast cells at 37°C and septate vegetative hyphae, conidiophores, and conidia at 25°C. Disseminated sporotrichosis may result in infections of the lungs, bones, and joints; endophthalmitis (inflammation of the inside layers of the eye); meningitis; and invasive sinusitis because the yeast form is spread through the lymphatic system.
• Infections known as systemic mycoses impact the entire body. These are separated into mycoses caused by opportunistic infections and those caused by main (often dimorphic) pathogenic pathogens.

Classification Based on Route of Acquisition

• Fungi that cause infection might be endogenous or external.
• A fungal infection may be classified as foreign or endogenous in origin depending on how it was acquired.
• An infectious agent that is exogenous may be spread by airborne, cutaneous, or percutaneous means.
• A fungus may colonise a person or reactivate from a dormant infection to cause an endogenously acquired fungal infection.

Classification Based on Virulence

• Primary pathogens have the ability to infect healthy hosts. In those with weak host defensive mechanisms, opportunistic microorganisms cause illness.
• Primary pathogenic and opportunistic fungal pathogens are the culprits behind deep mycoses.
• While opportunistic infections require a damaged host to begin infection, primary pathogenic fungi may infect a normal host without one (e.g., cancer, organ transplantation, surgery, and AIDS).
• The main deep pathogens often enter the body through the respiratory system. The respiratory tract, digestive system, or intravascular devices are all entry points for opportunistic fungi that cause deep mycosis.
• Coccidioides immitis, Histoplasma capsulatum, Blastomyces dermatitidis, and Paracoccidioides brasiliensis are the main systemic fungal infections.
• The opportunistic fungal pathogens include Trichosporon beigelii, Fusarium spp., Candida, Aspergillus spp., Penicillium marneffei, the Zygomycetes, and Cryptococcus neoformans.

References

• https://www.ncbi.nlm.nih.gov/books/NBK8125/
• Murphy JW, Friedman H, Bendinelli M (eds): Fungal Infections and Immune Responses. Plenum Press, New York, 1993.
• Engelkirk, P. G., Duben-Engelkirk, J. L., & Burton, G. R. W. (2011). Burton’s microbiology for the health sciences. Philadelphia: Wolters Kluwer Health/Lippincott Williams & Wilkins.
• Sastry A.S. & Bhat S.K. (2016). Essentials of Medical Microbiology. New Delhi : Jaypee Brothers Medical Publishers.
• Trivedi P.C., Pandey S, and Bhadauria S. (2010). Textbook of Microbiology. Pointer Publishers; First edition
• 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.
• https://www.kullabs.com/classes/subjects/units/lessons/notes/note-detail/1526
• https://mycology.adelaide.edu.au/docs/fungus3-book.pdf
• http://www.onlinebiologynotes.com/fungi-characteristics-classification/
• http://www.mmc.gov.bd/downloadable%20file/Common%20fungal%20dieases%20akram.pdf
• http://www.mmc.gov.bd/downloadable%20file/Common%20fungal%20dieases%20akram.pdf
• https://www.ncbi.nlm.nih.gov/books/NBK7902/
• http://www.davidmoore.org.uk/Assets/Clinical_groupings.htm