Introduction to Mycology

Mycology

• The study of fungi, including their structure, biochemistry, impact on humans, and the diseases they cause.

Fungal Characteristics

• Eukaryotic organisms
• Cell walls contain chitin, unlike bacteria (peptidoglycan) and plants (cellulose).
• Can be single-celled (yeasts) or multicellular (molds).
• Reproduce sexually or asexually.
• Heterotrophic, meaning they obtain nutrients from other organisms.

Fungal Nutrition

• Saprophytic: Fungi that break down dead organic matter for food.
• Symbiotic: Fungi living in mutually beneficial relationships with other organisms.
• Parasitic: Fungi that live in or on other organisms and cause harm.

Mycorrhizae

• Symbiotic relationship between fungi and plant roots.
• Fungi help plants absorb water and nutrients from the soil.
• Plants provide fungi with essential nutrients.

Dimorphism

• Some fungi can exist as both yeast and mold forms depending on environmental conditions.
• Yeast form at 37°C (body temperature).
• Mold form at room temperature.
• Example: Sporothrix schenckii

Reproduction

• Asexual:
  • Spore release into the air.
  • Fragmentation of hyphae.
• Sexual:
  • Production of sexual spores.

Fungal Cultures

• Optimum Growth Temperatures:
  • Saprophytes (molds): 20-30°C.
  • Parasitic fungi (yeasts): 30-37°C.
• Standard Incubation Temperature: 30°C.
• pH: 4-6 (average 5.0).
• Osmotic Pressure: Fungi are more resistant than bacteria.
• Nutrition: Fungi require less nitrogen and moisture than bacteria.
• Oxygen:
  • Molds are aerobic.
  • Yeasts are facultative anaerobes (can live with or without oxygen).
• Metabolism: Fungi can metabolize complex carbohydrates like lignin in wood.

Harmful Effects of Fungi

• Allergies: Mold spores can trigger allergic rhinitis.
• Mycoses: Direct fungal infections in animals (including humans).

Fungal Staining

• 10% Potassium Hydroxide: Used to clear skin and nail samples to visualize fungal elements.
• Lactophenol Cotton Blue:
  • Lactic acid preserves fungal structures.
  • Phenol is a killing agent.
  • Cotton blue stains fungal structures blue.

Fluorescence Test

• Wood's lamp (UV light):
  • Emitting wavelength 320-450 nm (peak 365 nm).
  • Infected hair and skin may fluoresce when examined in the dark.

Culture Media

• Sabouraud Dextrose Agar (SDA):
  • Primary isolation media for most fungi.
  • Low pH inhibits bacterial growth.
  • Disadvantages: May not support growth of all yeast phases, high susceptibility to contamination.
• Potato Dextrose Agar (PDA):
  • Composed of potato infusion and dextrose, promoting fungal growth.
  • Cheaper than SDA but less optimal.
• Mycosel (SDA-CC):
  • Contains antimicrobial supplements:
    • Cycloheximide: Inhibits saprophytic fungi.
    • Chloramphenicol: Inhibits Gram-bacterial growth.
  • Can substitute Chloramphenicol with Gentamicin or Tetracycline.
• Brain Heart Infusion Media (BHI):
  • Recommended for yeast phase of dimorphic fungi at 35-37°C.
  • Made from pig heart and calf brain, enriched with nutrients to promote growth of fastidious fungi.

Fungal Reproduction: Asexual

• Yeast:
  • Binary fission.
  • Budding.
  • Pseudohyphae formation.
• Molds:
  • Fragmentation of Hyphae: Breaking off pieces of hyphae to form new fungi.
  • Asexual Spores: Different types of spores produced by molds.

Fungal Spores: Asexual

• Chlamydospore:
  • Thick-walled, resistant spores formed by Candida species in adverse environments.
  • Rounded, enlarged hyphal segments.
• Blastospore/Blastoconidia:
  • Budding yeast spores.
• Arthrospore:
  • Rectangular spores formed by fragmentation of hyphae.
  • Found in Coccidioides immitis.
• Macroconidia and Microconidia:
  • Macroconidia: Large, multicellular, septated spores.
  • Microconidia: Small, single-celled, round or oval spores.
• Conidiospore:
  • Spores produced in a chain at the tip of a conidiophore.
  • Formed by phialides (flask-shaped projections).
• Sporangiospore:
  • Asexual spores contained within a sac-like structure called a sporangium.
  • Found in nonseptated hyphae of Zygomycetes (e.g., Rhizopus, Mucor, Absidia).

Dermatophytes

• Fungi that require keratin for growth.
• Cause superficial infections of skin, hair, and nails.
• Genera:
  • Microsporum: Hair and skin only.
  • Trichophyton: Hair, skin, and nails.
  • Epidermophyton: Skin and nails only.

Dermatophyte Test Medium

• Used to recover dermatophytes from samples (hair, skin, nails).
• Detects the production of alkaline metabolites by dermatophytes, causing a color change in the medium.

Fungal Reproduction: Sexual

• Three Phases:
  • Plasmogamy: Fusion of haploid fungal cells, resulting in a dikaryotic cell with two nuclei.
  • Karyogamy: Fusion of the two nuclei to form a diploid nucleus.
  • Meiosis: Diploid nucleus undergoes meiosis to produce haploid nuclei and spores.

Fungal Sexual Spores

• Ascospores:
  • Produced in a sac-like structure called an ascus.
  • Typically occur in clusters of four or eight.
• Zygospores:
  • Resulting from the fusion of two identical cells arising from fused hyphae.
• Basidiospores:
  • Contained in a club-shaped structure called a basidium.
  • Commonly found in mushrooms.

Fungal Phyla

• Ascomycetes:
  • Reproduce sexually by forming ascospores.
  • Reproduce asexually by forming conidiospores, microconidia, and macroconidia.
• Zygomycota:
  • Aseptate hyphae.
  • Asexual spores: Sporangiospores.
  • Sexual spores: Zygospores.
  • Clinically important members: Rhizopus, Mucor, Absidia.
• Basidiomycetes:
  • Reproduce sexually by forming basidiospores.
  • Reproduce asexually by hyphal fragmentation.

Imperfect Fungi (Deuteromycetes)

• Fungi that do not exhibit a sexual stage.
• Example: Penicillium.