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Fungal Morphology & Mycoses Overview

Fungal Morphology & Mycoses Overview

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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.

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