medical mycology

GENERAL MYCOLOGY

Fungi are diverse groups of saprophytic and parasitic eukaryotic organisms. Human fungal diseases (mycoses) are classified by the location where the infection occurs. Some fungi secrete a variety of metabolic products that are highly toxic when ingested, thus fungi can produce poisonings as well as infections.

General structure of fungi

Fungi are eukaryotic cells their structure corresponds essentially to that of the plant cell but it is devoid of chlorophyll. Fungal cells are composed of:

1. Cell wall:

Composition:

* Chitin:

The cell wall is composed mainly of chitin which is a polymer of N-acetyl glucosamine. It is synthesized by enzyme chitin synthase which is present in membrane bound pockets called chitosomes.

* Cellulose.

* Polysaccharides as mannans and glucans.

* Glycoproteins that contain large amount of sulphr containing amino acids and disulphide bonds which are more prevalent in hyphae than in yeast. Reduction of these disulphide bonds is associated with transformation of hyphae to yeast form.

Functions:

* It maintains the shape of the fungus.

* Osmotic strength.

* Protection.

* Antigenic determination.

2. The cytoplasmic membrane:

The fungal cytoplasmic membrane contains ergosterol rather than cholesterol present in mammalian membranes. Prokaryotes (except mycoplasma) do not contain sterols in their membranes.

3. Cytosol:

It is complex structure and contains many membrane bound organelles such as nucleus, mitochondria, vacuoles, glycogen and volutin granules, endoplasmic reticulum…etc.

4. Nucleus:

Fungi differ from the bacteria in containing a true nucleus which is demarcated from the cytoplasm of the cell by a nuclear membrane. The nucleus contains; as a rule; a nucleolus and always several chromosomes.

5. Capsule:

It helps adherence and clumping. Also it protects against phagocytosis.

Differences between bacteria and fungi.

Classification of Fungi

I- According to morphology:

1) Yeasts:

• Unicellular round or oval fungi 4-15 um in size.
• Reproduce by budding (blastoconidia formation).
• Sometimes buds do not separate off and extend to form pseudohyphae that show constrictions at the attachment sites. This form is known as yeast like fungi e.g. albans.
• They form moist mucoid, creamy or waxy colonies.

2) Moulds:

• Multicellular filamentous, "fluffy" colonies consisting of branching tubular structures called hyphae. Collection of intertwined hyphae called mycelium.
• Hyphae may be septate (with transverse walls); these septa are typically perforated or nonseptate (coenocytic which is hollow multinuclear structure). E.g. zygomycetes have nonseptate hyphae.
• Moulds identified primarily by microscopic morphology with particular attention to size, separation and branching pattern of hyphae as well as appearance of reproductive structures. On laboratory media moulds form:

-   Vegetative hyphae act like roots, penetrating the supporting medium and absorbing nutrients.

-   Aerial hyphae project above the surface of the mycelium and bear the reproductive structures of the mould that often spread through the air. They give the mould colony a characteristic velvety like appearance.

3) Dimorphic Fungi:

• Grow as moulds at environmental temperatures (25°C), forming reproductive spores that if inhaled by a host grow as yeasts at body temperature (37°C).
• Staining of tissues shows the yeast forms and cultures at 25°C show moulds with characteristic colony appearance and microscopic morphology.

II- According to mode of habitat:

Fungi are described according to their habitat and way of living as:

1) Saprophytic:

Saprophytic fungi live on inanimate material; in the soil, in water, in dust, in the air, on clothes or on dead bodies.

2) Parasitic:

Parasitic fungi live on or in the body of living creatures. They are subdivided into:

a- true pathogenic fungi:

Those cause disease in man or animals or plants.

b- Commensal fungi (opportunistic fungal pathogens):

Live on or in the body without exerting any harmful effect but many cause disease if the body resistance is lowered by any means (I.e. opportunistic infection).

III- According to the method of reproduction:

All fungi can reproduce asexually and many of them can also reproduce sexually.

Anamorph: is the asexual phase in fungus life cycle. This phase is commonly detected in clinical specimens.

Telomorph: is the sexual phase in fungus life cycle. This phase is detected only under specialized conditions e.g. in laboratory.

Medically important fungi are classified into four phyla:

• Ascomycota: Sexual reproduction in a sack called an ascus with the production of ascopspores.
• Basidiomycota: Sexual reproduction in a sack called a basidium with the production of basidiospores.
• Zygomycota: sexual reproduction by gametes and asexual reproduction with the formation of zygospores.
• Mitosporic Fungi (Fungi Imperfecti - deuteromycetes) - no recognizable form of sexual reproduction. It includes most pathogenic fungi.

IV- According to the level of tissue involvement of fungal infections: