Fungi - Overview

Introduction

Fungi are widely distributed throughout the environment and are eukaryotic non-photosynthetic heterotrophs that produce exoenzymes and obtain nutrients by absorption. Moulds and yeast represent the two main morphological types of fungi with multicellular moulds forming branching filaments called hyphae and unicellular yeasts having an oval or spherical appearence. Fungi grow aerobically and most are strict aerobes. Fungal reproduction can either be sexual or asexual and in some species both types can occur. Fungi are able to tolerate high osmotic pressures and acidic environments as low as pH 5. Most fungal species of veterinary importance are referred to as deuteromycetes although some pathogenic fungi do exist in other phyla.

Types of Fungi

Within the kindgom of Fungi there are four phyla; Ascomycota (ascomycetes), Basidiomycota (basidiomycetes) and Zygomycota (zygomycetes). These can be ditinguished by their sexual forms or telomorphs. Fungi imperfecti (deuteromycetes) do not have a sexual form and therefore represent a heterogenous fourth phyla.

Fungal species can be called saprophytic, parasitic or mutualistic. Mutualistic fungi have obligatory associations with microorganisms without which the fungi cannot complete it's own life cycle. Mutualistic fungi are non-pathogenic. For more detailed information on types of fungi, please see Types of Fungi.

Fungal Structure

Hyphal cell walls formed by moulds provide the cell with a rigid structure that provides a high degree of protection against osmotic pressure. These hyphal cell walls are mainly composed of chitin and other polysaccharides. The cell wall of yeasts contains protein complexes with polysaccharides and in some species also lipids. In both species the cell wall is lined by a bilayered cell membrane. Yeasts and mould both have defined nuclei, nuclear membranes, mitochondria and networks of microtubules. For more information on fungal structure, please see Fungal Structure.

Growth and Reproduction

Fungal spores are able to germinate where environmental conditions are favourable. Fungal spores are the result of either sexual or asexual reproduction. When germination occurs the spores swell and the metabolic activity increases allowing the production of tubular projections which develop into branched hyphae. Hyphal walls are thin but the cells are able to develop lateral branches at localised areas leading to a branched hyphal structure. These lateral branches result in the formation of a mycelium, an interlaced branched network of hyphae.

Yeasts mainly reproduce via asexual division by budding in which daughter cells separate from parent cells via the formation of a cross-wall budding point. For more detailed information on fungi reproduction please see Fungal Reproduction.

Fungal Disease

Fungal disease usually manifests using at least one of three mechanisms; tissue invasion (mycosis), toxin production (mycotoxicosis) and induction of hypersensitivity.

Mycoses
If the mycosis is restricted to superficial tissues, this can be classified as either dermatomycoses or dermatophytoses. Dermatomycoses are usually opportunistic infections of the skin or muco-cutaneous junctions. Often this is caused by Candida species or Malassezia pachydermatis. Dermatophytoses are clinically more important than dermatomycoses as they are zoonotic and are often associated with the invasion and destruction of keratinised structures such as Microsporum species and Trichophyton species. See link for more detailed information on dermatophytosis.

Subcutaneous infections often result following penetration of a foreign body through the superficial layers of the dermis. Systemic mycoses often originate in the respiratory or digestive tracts from opportunistic infections. Factors such as antimicrobial therapy, viral infection or other immuno-suppressors increase the predisposing rick factors to fungal infection.

Mycotoxicosis
Mycotoxicoses represent a group of diseases that result from the ingestion of fungal toxins that have been pre-formed in stored food or standing crops.

Hypersensitivity
Hypersensitivity reactions to to fungal infections are rare but are associated with chronic pulmonary disease in cattle and horses.

Diagnosis of Fungal Infection

The main method used to determine the species of mycological infection is by culture. Any culturing procedures should be carried out in a biohazard cabinet due to the zoonotic effect of spores aerosols. Clinical signs and history may be sufficient to make a presumptive diagnosis, particularly when considering dermatophytoses. Any specimens taken for laboratory analysis can include hair and skin scrapings, biopsy or post-mortem (if the disease is a systemic mycosis).

Direct examination of wet preparations via light microscope represents another method for diagnosis. Species that can be detected via this method include ringworm arthrospores around infected hairs, Cryptococcus neoformans within cerebrospinal fluid and spores taken from a colony cultured previously. In some cases an adhesive tape technique can be used to produce a slide for microscopic examination. This is performed by placing adhesive tape on the superficial site of infection and removing material attached to the tape.

Where it is appropriate to use a culturing technique, fungi are usually isolated on a Sabouraud dextrose agar (pH 5.5) which inhibits the growth of most bacteria. In order to differentiate fungal species that may be implicated in mycotic disease a number of features are observed and utilised. These include the presence of absence of septa (internal cell walls), either colourless or pigmented hyaline structures or specific hyphal structures. The colonial characteristics may also be observed including size, appearance, colour and elevation.

Yeasts are mainly differentiated based on colonial appearance and the size and shape of individual cells. There are also some biochemical tests that can be used for differentiation including ELISA.