Fungal Biology and Pathology Study Notes

Fungi

Overview of Fungi

• Mycology is the study of fungi.
• Mycologist: A person who studies fungi.
• Fungi are considered one of the major microorganisms and are classified as eukaryotic organisms.
  • Possess a complete set of organelles.
  • Contain a nucleus.

Comparison: Fungi vs. Bacteria

• Features:
  • Fungi:
  • Nucleus: Eukaryotic
  • Organelles: Present and membrane-bound
  • Cell Wall Components: Chitin
  • Bacteria:
  • Nucleus: Prokaryotic
  • Organelles: Few are absent and not membrane-bound
  • Cell Wall Components: Peptidoglycan

Types of Fungi

1. Survival Strategies of Fungi

• Saprophytic Fungi:
  • Main food source: Dead and decaying organic matter.
  • Known as “Garbage disposers” or “Recyclers.”
• Parasitic Fungi:
  • Cannot make their own food; live on or within animals and plants.

2. Structure of Fungi

• Cell Wall Composition: Chitin, made up of multiple layers of polysaccharides.
• Spores:
  • Essential for both reproduction and survival.
  • Function like seeds; can germinate in favorable conditions (moist, warm) to create new fungi.

3. Fungi Classification

• Unicellular Fungi:

  • Composed of a single cell (e.g., yeasts).
  • Can perform all necessary activities independently.
  • Contain membrane-bound organelles and a chitin cell wall.
  • Asexual reproduction through budding.
  • Pseudohyphae: Chains of easily disrupted fungal cells, marked by constrictions.

• Multicellular Fungi:

  • Composed of multiple cells (e.g., molds).
  • Surrounded by a chitin cell wall.
  • Hyphae: Elongated, tubular filaments that contain organelles and a nucleus; perform extracellular digestion by secreting enzymes.
  • Mycelium: A branched network of hyphae.
  • Capable of sexual or asexual reproduction via spores.

4. Hyphae Types

• Septate Hyphae:
  • Contain septa (cross walls) dividing the hyphae into cells; each division has one nucleus and a set of organelles.
• Aseptate Hyphae:
  • Lack septum; protoplasm is continuous, containing multiple nuclei and organelles.

5. Types of Hyphae According to Growth

• Aerial Hyphae: Extend above the surface.
• Vegetative Hyphae: Grow beneath the surface.

6. Fungal Reproduction

• Asexual Reproduction:
  • Asexual spores/conidia produced through cell division, mitosis, or budding (seen in yeasts).
• Sexual Reproduction:
  • Sexual spores produced by the fusion of two gametes (seen in molds).

Fungi Life Cycle

• Asexual Reproduction:
  • Involves one parent (parent one) releasing spores.
  • Haploid Cells: Resulting cells carry half the genetic material of the parent.
  • Fuses to form diploid cells through processes:
  • Plasmogamy: Fusion of cytoplasm.
  • Karyogamy: Fusion of nuclei, forming a diploid nucleus.
  • Diploid: Full set of genetic material from both parents.

Types of Yeasts

• Yeasts:
  • Eukaryotic, unicellular organisms lacking mycelia.
  • Reproduce by budding (asexual reproduction).
  • Habitat: Found in wine, beer, soil, water, and skin of fruits and vegetables.
  • Common examples:
  • Saccharomyces cerevisiae: Used in baking and fermentation.
  • Candida albicans: An opportunistic pathogen.

Yeast Reproduction Diagram (Asexual)

1. Formation of haploid cells in mycelium.
2. Fusion of two haploid cells from different mycelia.
3. Fusion of nuclei from two haploid cells forms a diploid.
4. Meiosis results in new haploid cells.

Classification of Fungi

Major Modes of Transmission

• Saccharomyces cerevisiae: Known as “Baker’s yeast,” ferments sugar to alcohol, serves as a leavening agent.
• Candida albicans: Harmless in skin's microbiota, can flourish under favorable conditions, leading to yeast infections.

Types of Multicellular Fungi

1. Molds:
   • Multicellular and grow in form of hyphae.
   • Can reproduce sexually or asexually through spore formation.
   • Example: Penicillium:
   • Source of penicillin, an antibiotic.
2. Dimorphic Fungi:
   • Can exist as yeasts or molds depending on temperature:
   • 37°C: Yeast form.
   • 25°C: Mold form.
   • Examples: Sporothrix schenckii, Cryptococcus neoformans.
3. Fleshy Fungi:
   • Visible to the naked eye (e.g., mushrooms) and consist of mycelium in soil.

Modes of Transmission of Fungal Infections

1. Direct skin contact with infected host.
2. Indirect contact with contaminated fomites.
3. Inhalation of fungal spores leading to respiratory infections.

General Mechanism of Causing Human Disease

• Fungi do not secrete toxins; damage comes from direct invasion and displacement of tissue, causing inflammation.

Types of Fungal Diseases

1. Superficial Mycoses

• Affects the outermost areas of the human body:
  • Examples: Hair, fingernails, toenails, epidermis.
  • Common fungi: Malassezia furfur (Pityriasis versicolor) and dermatophytes (Microsporum, Trichophyton, Epidermophyton).
  • Typical presentation: Circular, raised skin lesions.
  • Transmission: Direct contact or fomites.
  • Management: Topical antifungal agents; hygiene practices.

2. Deep Mycoses

A. Subcutaneous Mycoses

• More severe; involve tissues beneath the skin.
• Examples:
  • Sporotrichosis (caused by Sporothrix schenkii): Characterized by a small painless bump progressing to an ulcer.
  • Mycetoma (caused by Madurella mycetomatis): Firm, painless masses under the skin.
• Transmission: Direct contact with fungi or fomites.
• Management: Antifungal therapy and protective clothing.

B. Systemic Mycoses

• Transmission:
  • Inhalation of spores or indirect contact.
• Management:
  • Systemic antifungal therapy; for immunocompromised, N95 masks recommended.
  • Common opportunistic pathogens include:
  1. Pneumocystis pneumonia: Symptoms include fever, cough, dyspnea.
  2. Aspergillosis (Aspergillus fumigatus): Symptoms include fever, cough.
  3. Cryptococcosis (Cryptococcus neoformans): Starts with inhalation, can lead to meningoencephalitis.
  4. Candidiasis (Candida albicans): Can occur after skin damage or antibiotic use; identified by thick, white discharge.