MORPHOLOGY OF FUNGI (Mold, Yeast and Dimorphic Fungi)

 MORPHOLOGY OF FUNGI

·       The body or Vegetative structure of a Fungus is called a Thallus (Plural – Thalli).

·     Thallus varies in complexity and size, ranging from the single-cell microscopic yeasts to multicellular molds.

·   The fungal cell usually is encased in a cell wall of Chitin. Chitin is a strong but flexible nitrogen containing polysaccharide consisting of N-acetylglucosamine (NAG) residues.

Yeast

·   A yeast is a unicellular, non-filamentous fungus that has a single Nucleus and reproduces either Asexually by Budding and Transverse division or Sexually through Spore formation.

·      Generally, yeast cells are larger than bacteria, vary considerably in size, and are commonly Spherical to Oval shaped.

·       Yeasts are frequently found as a white powdery coating on fruits and leaves.

·     Yeasts are capable of Facultative anaerobic growth. Yeasts can use oxygen or an organic compound as the final electron acceptor; this is a valuable attribute because it allows these fungi to survive in various environments.

·      If given access to Oxygen, yeasts perform Aerobic respiration to metabolize Carbohydrates to Carbon-di-oxide and Water. If oxygen is denied, they ferment Carbohydrates and produce Ethanol and Carbon-di-oxide. This fermentation is used in the brewing, wine-making, and baking industries.

Mold

·       The body of a fungus is called a Thallus.

·       The Thallus of most multicellular fungi consists of a Mycelium.

·       Mycelium is a loosely organized mass of threadlike structures called Hyphae (Singular: hypha).

a)     The portion of a Hyphae that obtains Nutrients is called the Vegetative hyphae.

b)     The portion of a Hyphae concerned with reproduction is the Reproductive or Aerial hyphae.

·      From the Mycelium, Aerial hyphae reach up into the air above the surface, and Spores called Conidia are formed on their tips.

ü  Conidia are Asexual spores of fungi.

ü  They are often pigmented black, green, red, yellow, or brown.

ü  Conidia give the mycelium a dusty appearance.

ü  Conidia function to disperse the fungus to new habitats

·     The hyphal cells of most fungi have one or two nuclei and many hyphal cells are separated by cross-walls called Septa (Singular: septum).

ü  The Hyphae contain cross-walls called Septa (Singular: septum), which divide them into distinct, uninucleate cell-like units. These hyphae are called Septate hyphae.

ü  In a few fungi, the Hyphae contain no septa and appear as long, continuous cells with many nuclei. These are called Coenocytic hyphae.

·      Pores in septa allow both Cytoplasm and Nuclei to pass between cells.  Some fungi have septa with so many pores that they are sieve like, and a few lack septa entirely.

·     Certain fungi with a single septal pore have an organelle called a Woronin body. When a hyphal cell ages or is damaged, the Woronin body moves to and blocks the pore so that materials from the damaged cell cannot enter a healthy cell.

·    Fungi can produce Spores both Sexually and Asexually, and Spores can have one or several nuclei. Typically, aquatic fungi produce motile spores with flagella, and terrestrial fungi produce spores with thick protective walls.

Dimorphic Fungi

·    Some fungi, most notably the pathogenic species, exhibit Dimorphism - two forms of growth (grow either as a mold or as a yeast).

ü  Blastomyces dermatitidis – Blastomycosis

ü  Candida albicans – Candidiasis

ü  Histoplasma capsulatum – Histoplasmosis

ü  Sporothrix schenckii – Sporotrichosis

ü  Paracoccidioides brasiliensis - Paracoccidioidomycosis

·       The mold like forms produce Vegetative and Aerial hyphae; the yeast like forms reproduce by Budding.

·       Dimorphism in pathogenic fungi is Temperature-dependent. At 37 °C, the fungus is yeast like, and at 25 °C, it is mold like.

·       Dimorphic fungi can change from the Yeast (Y) form in the animal to the mold or Mycelial form (M) in the external environment in response to changes in various environmental factors (Nutrients, CO₂ tension, Oxidation-reduction potentials and Temperature). This shift is called the “YM shift”.

·       In plant-associated fungi the opposite type of dimorphism exists. The mycelial form occurs in the plant and the yeast form in the external environment.