INTRODUCTION TO MYCOLOGY
MYCOLOGY is the branch of microbiology that deals with the study of Fungi and Fungal diseases.
GENERAL CHARACTERISTICS OF FUNGI
⇒ All fungi are Eukaryotic protists.
⇒ They may be Multicellular (Moulds) or Unicellular (Yeasts).
⇒ They are chemotropic organisms i.e. obtaining their nutrients from chemicals in nature.
⇒ Fungi are obligate or facultative aerobes.
⇒ Water, soil, and decaying organic debris are the natural habitat of fungi.
⇒ Their cell walls contain Chitin, Mannan, and other polysaccharides.
⇒ They possess true nuclei with nuclear membrane and paired chromosomes.
⇒ They reproduce Sexually, Asexually or by both processes.
⇒ They are non-motile, cells show various degree of specialization.
⇒ They grow as branching filaments called Hyphae, usually 2-10 microns in width, which may be septate or non-septate.
CLASSIFICATION OF FUNGI
Fungi are classified as follows:-
A.) MORPHOLOGICAL CLASSIFICATION OF FUNGI
⇒ From a diagnostic point of view, fungi may be classified depending on cell morphology into four groups:
- Yeast-like fungi
- Dimorphic fungi
⇒ Yeasts – Characteristics:-
- Yeast is a Unicellular fungus that has a single nucleus and reproduces either asexually by budding [bud = blastospore (blastoconidium)] or sexually by true spore formation.
- Each bud that separates can grow into new yeast.
- Macroscopically appears as creamy mucoid colonies on the culture media
- Microscopically appears as oval to round (3-15 microns in diameter) in tissues and in culture.
- The most important pathogenic yeast is Cryptococcus neoformans.
⇒ Yeast-like fungi – characteristics:-
- Unicellular fungi that reproduce by budding or by fission.
- Macroscopically appears as pasty colonies on the culture media.
- Microscopically appears as spherical or oval structure; filamentous structures may be seen due to the chains of elongated budding cells joined end to end (pseudohyphae) in tissues and in culture.
- For e.g. – Candida albicans.
⇒ Moulds – Characteristics:-
- Multicellular fungi that reproduce by asexual means (spore formation); some exhibit sexual reproduction.
- Moulds are composed of Hyphae which may have cross-walls or septa or may lack septa (coenocytic).
- Macroscopically appears as cottony/ woolly/ velvety/ granular growth on the culture media.
- Microscopically appears as thread-like filamentous Hyphae (2-10 microns) seen in tissues and in culture.
- For e.g. – Aspergillus fumigatus, Penicillium notatum
⇒ Dimorphic fungi – characteristics:-
- Many fungi especially those that cause disease in human and animal are dimorphic i.e. they have two forms.
- Dimorphic fungi can change from the yeast form in the animals to the moulds or mycelium form in the external environment in response to change in various environmental factors.
- The shift is called as Y M shift.
- They exist as yeasts in the host tissue and in the cultures at 37°
- Grows as hyphal (mycelial) form in the soil and in the cultures at 22-25°.
- For e.g. – Blastomyces dermatitidis
B.) TAXONOMICAL / SYSTEMIC CLASSIFICATION OF FUNGI
Fungi are placed in phylum Thallophyta. This classification of fungi is based on the sexual spore formation. There are four classes of fungi as follows :
- Deuteromycetes/Hyphomycetes/Fungi imperfecti
DIVISIONS OF FUNGI
DIVISION COMMON NAME APPROXIMATE NO. OF SPECIES
Zygomycota Zygomycetes 600
Ascomycota Sac fungi 35,000
Basidiomycota Club fungi 30,000
Deuteromycota Fungi imperfecti 30,000
- The division Zygomycota contains the fungi called Zygomycetes.
- These are lower fungi that have non-septate Hyphae and produce endogenous asexual spores, called Sporangiospores, contained within swollen sac-like structures called Sporangia.
- The Hyphae of Zygomycetes known as Coenocytic, with many haploid nucleoids.
- Zygomycetes also produce sexual spores known as oospores in some fungi and Zygospores in others.
- Zygospores are tough thick walled zygotes called Zygospores that can remain dormant when the environment is too harsh for the growth of the fungus.
- They usually reproduce asexually but if food becomes starve or environmental condition unfavorable it begins sexual reproduction.
- The Zygomycetes also contributes to human welfare for e.g. – Rhizopus is used in Indonesia to produce food; another Zygomycetes is used with soybean to make a curd called Sufu.
- The division Ascomycota contains the fungi called Ascomycetes are commonly known as sac fungi.
- They have septate Hyphae ad form exogenous asexual spores called conidia.
- It forms sexual spores (Ascospores) are present within a sac or Ascus.
- Asexual reproduction is common in the Ascomycetes and takes place by means of Conidiospores.
- It includes both yeasts and filamentous fungi. Many yeast genera are classified specifically within Ascomycetes because of their sexual reproduction.
- Many species are quite familiar and economically important for example – most of the Red, Brown and blue-green moulds that cause food spoilage are Ascomycetes.
- The division Basidiomycota contains the Basidiomycetes commonly known as the club fungi.
- It forms sexual spores called Basidiospores on a basidium or base. A basidium is produced at the tip of Hyphae and normally in club shape.
- The Basidiomycetes affects humans in many ways; many mushrooms are used as food throughout the world.
- For e.g. – Basidiomycetes includes Smuts, Jelly fungi, Mushrooms & Bird net fungi.
- When a fungus lacks the sexual phase (perfect stage) or if this phase has not been observed it is placed within the division Deuteromycota, commonly called as Deuteromycetes or Fungi imperfecti.
- Most Deuteromycetes reproduce by means of Conidia.
- Most fungi imperfecti are terrestrial; with only a few being reported from freshwater and marine habitats.
- Many Deuteromycetes directly affects human welfare and causing numerous diseases such as Ringworm.
- The chemical activities of many fungi are important to industries such as some species of Penicillium used in the synthesis of antibiotics.
C.) PATHOLOGICAL CLASSIFICATION OF FUNGI:
⇒ Infection caused by the fungus is known as Mycoses.
⇒ Based on the pathogenic potential, fungi may be considered as :
- Primarily pathogenic – Those fungi which are able to cause infections in healthy individuals; for e.g., thermally dimorphic fungi.
- Opportunistic pathogens – Those fungi that are able to cause infection in patients who are Immuno-compromised due to some other infections or diseases or who are receiving immunosuppressive drugs etc.
⇒ Pathogenic fungi may cause:
- Actual infection of tissues (Mycoses) – Fungal infections or Mycoses in humans can be classified according to the tissues involved into –
- Superficial Mycoses
- Subcutaneous Mycoses
- Systemic or Visceral Mycoses
- Opportunistic Mycoses
- Mycotoxicoses – These are diseases due to toxic metabolic products released by fungi. For e.g. – aflatoxicosis due to consumption of grains containing aflatoxins secreted by Aspergillus flavus contaminating the groundnuts, corn, and peas. The fungus does not necessarily have to be present in the tissues to exert its pathogenic effect since its toxic metabolites are present. There is no invasion of tissues by the fungus.
- Hypersensitivity (Allergic reactions) – A type I and/or type III hypersensitivity reaction is provoked by inhalation of fungal spores. For e.g. – Allergic Bronchopulmonary Aspergillosis (due to spores of Aspergillus fumigatus) and allergic fungal rhinosinusitis. There is no invasion of tissues by the fungus.