Fungal Morphology & Mycoses Overview

Fungi Overview

Key Features

Kingdom: Fungi
Eukaryotic

Rigid cell walls that comprise chitin and glucan.*

Cell membranes contain the sterol erogsterol, which is the target of some antifungal drugs.
Fungi exist in a variety of forms:

– Unicellular fungi are often referred to as yeast. – Multicellular forms are often referred to as filamentous or mold forms. – Dimorphic: many species exist as both mold and yeast forms, depending on environmental conditions such as temperature, carbon dioxide concentration, and pH.

Fungi use a variety of strategies for reproduction.

– Can reproduce sexually or asexually, and that some species are capable of both. – Mechanisms of reproduction also vary, and include the production of spores and budding.

Mycoses are fungal diseases.

Most fungi are opportunistic pathogens,* rather than primary pathogens, because they lack the ability to breach the host's protective barriers.

Important exceptions cause primary respiratory infections that can disseminate within the host.

– Includes some ascomycetes and endemic dimorphic fungi that

Mycotoxins are toxins produced by fungi

– They cause mycotoxicoses. – Not all mycotoxins are harmful to humans – we use mycotoxins that are toxic to bacteria to create antibiotics, such as penicillin!

Typical fungal cell

Outer cell wall comprises chitin and glucan; chitin provides rigidity.
Cell membrane comprises erogsterol in place of the cholesterol found in our cell membranes.
Nucleus, mitochondria, Golgi, and endoplasmic reticulum.

Fungi morphology

Fungi take on a range of shapes, from ovoid to filamentous.

A given population of fungi may appear as a mixture of morphological types.
The filamentous forms grow via elongation, while the unicellular forms replicate via budding or fission.
The filamentous form can invade host tissues.

Filamentous forms: Hyphae & Pseudohyphae Hyphae appear as smooth branching filaments:

May be septated or coenocytic (non-septated).

– Septa may have pores for materials to move within the filament.

Mycelium is a mass of hyphae

– These can be visible to the eye, and are commonly referred to as "mold."

Pseudohyphae look similar to hyphae, but comprise chains of cells with constriction at the septal junctions. – A mass of pseudohyphae is sometimes referred to as a pseudomycelium. – The differences between hyphae and pseudohyphae can be difficult to distinguish.

Unicellular forms: Yeast

Appear as ovoid or round.
They replicate via budding or fission, and form discrete, separate cells.

Notice that we've identified individual cells within the same sample as the pseudohyphae.

Cell (aka, yeast)-to-pseudophyhae transition

Individual fungal cell develops an outgrowth, which is called the "germ tube."
The germ tube ultimately becomes a chain of cells that remain attached to one another at septa.
As mentioned earlier, some fungi species undergo this transition in response to environmental conditions.

Fungal Morphology & Mycoses Overview

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