Fungi: Characteristics and Classification

Introduction

• 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.

                                                                         Image source: paramedicsworld

Characteristics

• 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.

Classification

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.

                                                                            Image source:examplesof

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.

References: 

i) https://www.onlinebiologynotes.com/fungi-characteristics-classification/

ii) https://www.peoi.org/Courses/Coursesen/bot/bot14.html

Fungi: Characteristics and Classification