Types of Fungi

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There are two principle types of fungi; the moulds and the yeasts. The moulds are further divided into filamentous or dimorphic fungi.

Filamentous fungal hyphae are the branching tubular structures of moulds which become entwined as the fungi grows to form a mycelium. Hyphae grow by apical elongation as new wall material is laid down constantly at the tips of the terminal and lateral branches. Septate hyphae are divided by cross walls into septa. Each septum has a small central pore (dolipore) maintaining protoplasmic continuity throughout the hyphae, allowing rapid nutrient transport and nuclear migration. Dead or damaged parts of septate hyphae are cut off by a plug blocking the septal pore. Non-septate hyphae only form cross walls to cut off dead or damaged parts of the hyphae and to delimit reproductive organs. There are aerial (above the surface) and vegetative (surface) hyphae. The aerial hyphae produce spores which are reproductive fungal cells and vegetative mycelium have an assimilative function. There are two main types of spore produced, conidiospores and sporangiospores. These produce asexual spores called conidia which are relatively resistant to physical and chemical agents.

Dimorphic fungi develop as a mycelium or into another form depending on environmental conditions. There is usually a yeast like parasitic phase and a mycelial saprophytic phase. Dimorphic fungi show phenotypic duality and more than one physiological factor may be involved in dimorphic changes.

The yeasts range from 3µm to 5µm and are oval or spherical cells. Pseudohyphae are produced by some yeasts which are chains of irregular yeast cells. Dimorphic fungi change from the mycelial form into yeasts in certain conditions, e.g. 37°C, or when inside animal tissue. Yeasts reproduce mainly by budding of the oval, unicellular or round thallus. The daughter cells may remain attached to the parent yeast cells for several generations forming chains or colonies. To identify yeasts, the ability to assimilate sugar and nitrogen and the fermentation of various sugar compounds is studied.

Major pathogenic fungi

Superficial: These infections affect only the skin, hair, nails etc.

  • Generally not dangerous
  • Examples- Hortaea werneckii, causing tinea nigra, and Piedraia hortae, causing black piedra

Cutaneous: including dermatophytoses

  • Only affect superficial keratinized tissue- skin, hair and nails
  • Again generally not dangerous
  • Examples- Candida albicans, causing candidiasis of the skin and nails and Microsporum species

Subcutaneous: Usually cause chronic localised infections of the skin and underlying dermis

  • Normally reside in soil and vegetation, entering the skin through cuts and grazes
  • Can occasionally affect the deep tissues, e.g. bones and muscles
  • Examples- Sporothrix schenkii, causing sporotrichosis, Madurella mycetomatis causing mycetoma

Systemic: Also known as deep, endemic or primary

  • Usually serious
  • Sometimes fatal
  • Examples- Histoplasma capsulatum causing histoplasmosis, Blastomyces dermatitidiscausing blastomycosis

Opportunistic: Ubiquitous fungi that affect only susceptible hosts; immunocompetent individuals are normally resistant

  • Examples- Aspergillus fumigatus, causing aspergilliosis, Cryptococcus neoformans, causing cryptococcosis

Test Yourself with the Fungi Flashcards

Fungi Flashcards