4. Fungi

the basic facts

Questions

What are mycoses and what types occur?

In addition to infections, how else can fungi cause disease?

Medically important fungi are eukaryotic organisms and include the yeasts (unicellular organisms), moulds (filamentous fungi) and dimorphic fungi (moulds that transform into yeasts at body temperature). They are widely distributed in the environment and can survive in extreme conditions where nutrients are limited. Most fungi are saprophytes (living off dead organic matter) in soil and water. Certain fungi are also of great commercial value in the production of bread, alcohol and antibiotics.

Yeasts (e.g. Candida albicans) are the simplest of the fungi. They are unicellular, spherical in shape and reproduce by budding (Fig. 3.4.1A). In some yeasts, including the medically important genus Candida, the buds elongate to form filaments (pseudohyphae) (Fig. 3.4.1B).

Fig. 3.4.1 Culture plates and microscopy of yeasts (A,B; Candida spp.) and moulds (C,D; Aspergillus spp., filamentous fungi).

Moulds are composed of numerous microscopic branching, filamentous hyphae (e.g. Aspergillus fumigatus; Fig. 3.4.1C,D), known collectively as mycelia; these are involved in gaining nutrients and reproduction. The reproductive mycelia produce spores, termed conidia, either asexually or by sexual reproduction from opposite mating strains. Spores are disseminated in the atmosphere, enabling fungi to colonize new environments.

Certain pathogenic fungi are dimorphic (e.g. Histoplasma capsulatum), being a yeast form when invading tissues at 37°C but a mould when living in the environment (room temperature).

Diseases caused by fungi

The study of fungi is called mycology, and the diseases they cause are termed mycoses. Mycoses are classified depending on the degree of tissue involvement and mode of entry into the host:

superficial: localized to the epidermis, hair and nails but can extend deeper into keratinized tissue

subcutaneous: confined to the dermis, subcutaneous tissue or adjacent structures

systemic: deep infections of the internal organs caused by:

– primary pathogenic fungi, which infect previously healthy persons

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