Fungi have two distinct macroscopic appearances.
- Moulds
- Yeast
Moulds
- Molds (referred to at times as moulds) are entangled filamentous structures.
- The thallus is the growing vegetative form.
-The basic unit is the hypha (plural: hyphae), a filament made up of chains of cylindrical cells that may be septated, or divided into individual cells connected by pores, or not septated (coenocytic).
•Zygomycetes are medically important fungi that are usually non-septate or coenocytic. Growth occurs from the tip of the hypha. The mycelium represents masses of hyphae. Hyphae may grow on, into or above the substrate. Aerial hyphae or mycelia may extend into the air. The vegetative hyphae may give rise to reproductive structures.
Yeasts
•The second major fungal morphology is the yeast. Yeasts produce smooth individual colonies when grown on artificial media that appear similar to bacteria and consist of single cells that typically multiply by budding or fusion. Yeast may form pseudohyphae; they are distinguished from true hyphae by being separated by cell walls without pores and by the fact that the terminal cell is shorter than the proximal cell.
Dimorphism
- Some fungi exist as either molds or yeast, depending on environmental circumstances.
- Thermal dimorphism is most common, growing as a mold at room temperature (22°C) and as a yeast at higher temperatures (35°C).
Medically Important Dimorphic Fungi
- Blastomyces, Coccidioides, Histoplasma, Paracoccidioides, Penicillium, and Sporothrix
- Coccidioides does not have a true “yeast” stage but rather produces a spherule when grown at higher temperatures under special conditions.
- Candida albicans may also be dimorphic, growing as a yeast under most conditions but forming a “germ-tube” when exposed to serum.
Asexual Reproduction
- Most fungal reproduction is asexual in which cells with haploid nuclei divide by mitosis.
- This is called the “imperfect state.” Budding, fragmentation, and sporulation are common mechanisms of asexual fungal reproduction
Sexual Reproduction
- Under stressful conditions, many fungi also have a sexual or “perfect state.”
- In such a case cells with haploid nuclei fuse to become diploid and then undergo meiosis
“Fungi Imperfecti” or the “Deuteromycetes”
•In some fungi, no sexual state has been observed.
Anamorph
The morphological state of asexual reproduction is termed the “anamorph.”
Teleomorph
the sexual reproductive stage (morph)
Holomorph
The “holomorph” represents both the anamorphic and teleomorphic stages of any given fungus.
Homothallic
•mating types on the same hypha
Heterothallic
•mating occurring between hyphae
Conidia
•Most fungal reproduction is asexual or anamorphic. The spores produced from this are haploid and are called conidia
Methods for Releasing Conidia
•Fungi use several methods to release conidia into the environment, including budding, fragmentation and release of multiple spores at once, called sporulation.
-Aspergillus species are examples of this. These species have a specialized structure resembling the aspergillum used to sprinkle holy water in ceremonies of the Catholic church, hence their name.
Cell Membrane
- Fungi have both an inner cell membrane and an outer wall. The cell membrane consists of a phospholipid bilayer intercalated with ergosterol, a sterol component. The cell wall contains a variety of structural elements consisting of saccharide chains. These include chitin (N-acetyl glucosamine polymer), β-glucans (either in β1,3 or β1,6 linkages) and mannoproteins.
- The cell membrane and cell wall elements are targets for antifungals.
- Mannoproteins make up the outer portion of the cell wall and are important for recognition of fungi by the innate immune system.
Fungal Ecology
- Most medically important fungi reside in the environment.
- Many have been associated with conditions that have a high organic nitrogen content. Infections due to these fungi, including those due to Aspergillus, Zygomycetes, Pneumocystis, Cryptococcus, and all the endemic mycoses, are acquired from environmental sources.
- Dermatophytes reside on human hosts but can also persist in environmental sites.
- On the other hand, Candida species are part of the normal gastrointestinal and vaginal flora. Infection occurs when local or systemic host immunity is unable to control their growth.
