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