Introduction

There are many different organisms in the fungal lineage which include mushrooms, rusts, smuts, puffballs, truffles, morels, moulds and yeasts, as well as many lesser known organisms. fungi are eukaryotic and heterotrophic organisms which posess a chitinous cell wall. Sexual and asexual reproduction of the fungi is commonly via spores, often produced on specialized structures or in fruiting bodies. Some species have lost the ability to form reproductive structures, and propagate solely by vegetative growth.

Fungi are abundant in soil, vegetation, water and on decaying material and wood. Fungi have a eukaryotic cell structure and are able to grow as branching or filamentous forms (mycelia) or as single cells (yeasts).

The study of fungi is called mycology. Fungi are fundamental for life on earth in their roles as symbionts and by playing a role in xenobiotics, a critical step in the global carbon cycle. Many fungi produce toxins, antibiotics, and other secondary metabolites, some of which cause diseases of animals (as well as plants).

Although there are over 50,000 identified species of fungi, only a few hundred are known to be pathogenic, causing infections known as mycoses (mycology being the study of fungi). Fungi are nonphotosynthetic organisms (lack chlorophyll) which are restricted to a parasitic or saprophytic existence. They are abundant in soil, vegetation, water and on decaying vegetation and wood. Fungi have a eukaryotic cell structure and are able to grow as branching or filamentous forms (mycelia) or as single cells (yeasts).

Fungal Structure

Types of Fungi

Aspergillus cleistothecia showing Hulle cells which is the sexual form called ascospores - Copyright Professor Andrew N. Rycroft, BSc, PHD, C. Biol.F.I.Biol., FRCPath

Any fragment of hyphae can grow to form a new colony (if it is not too badly damaged). However, the main method of fungal reproduction is through spores which are produced in very large numbers. There are different types of spores: vegetative, asexual and sexual.

Vegetative spores can be divided into arthrospores and chlamydospores. Arthrospores are formed when septate hyphae disarticulate into separate cells. Arthrospores are usually formed by dermatophytes. Chlamydospoes are formed by segmentation of mycelium. They are thick walled, enlarged, resting spores which contain reserve food material and can remain viable after the rest of the mycelium has died. Budding from the ends or sides of the parent cell forms blastospores, for example, in Candidosis infections.

Asexual spores are either sporangiospores or conidiospores. Sporangiospores are formed when the cytoplasm cleaves within a sporangium (multinucleate segment of hyphae), producing either naked zoospores which have one or more flagella and are liberated through a hole in the sporangial wall or producing aplanospores. Aplanospores are non-motile and walled and are released when the sporangial wall breaks down. Conidiospores, such as Aspergillus can be formed directly from the mycelium, produced within a specialised fructification or can be formed externally by the abstriction of a conidiophore. Different types of conidia can be produced which differ in cell size and number and are called either microconidia (small and unicellular) or macroconidia (large and multicellular).

The sexual spores are divided into four categories depending upon the spores produced and their morphology. Zygomycetes are primitive fungi with a non-septate mycelium. The sexual spores are oospores or zygospores (the asexual spores are produced in a sporangium). Ascomycetes are higher fungi. They have a septate mycelium and the sexual spores are called ascospores (the asexual spores are conidia). Basidiomycetes are also higher fungi with a septate mycelium which has clamp connections. The sexual spores are basidiospores (the asexual spores are produced externally). Deuteromycetes are an artifical group of fungi which have an unknown sexual state.

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

Cutaneous: including dermatophytoses

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

Systemic: Also known as deep, endemic or primary

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

Cutaneous and superficial fungal infections are normally self-limiting, with recovery associated with a resistance to re-infection. As hosts often develop delayed-type hypersensitivity, this resistance appears to be cell-mediated.

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Special accreditation for the use of lecture notes, images and input from:

Professor Andrew N. Rycroft, BSc, PHD, C. Biol.F.I.Biol., FRCPath

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