1. Introduction to Mycology

INTRODUCTION TO MYCOLOGY

  • Mycology: the study of fungi, including their taxonomy, environmental impact, and genetic and biochemical properties.

  • Medical Mycology: focuses on the study of fungi in relation to human disease, encompassing both single-celled yeasts and filamentous molds as agents of disease.

    • Fungal agents include:

      • True Pathogens

      • Opportunistic Pathogens

EPIDEMIOLOGY

  • Humans are generally resistant to fungal infections, except for dimorphic fungi.

  • Major predisposing factors for infections:

    • Immunocompromised host

    • Use of immunosuppressive agents or having serious underlying diseases

    • Complex surgical procedures

    • Antibacterial therapy

  • Mycoses are generally NOT communicable; they are acquired through:

    • Inhalation of spores

    • Direct contact with spores

    • Inoculation by trauma into the skin

FUNGI: GENERAL CHARACTERISTICS

  • Fungi are eukaryotic organisms.

  • Cell Wall (CW) Characteristics:

    • Protect cells from osmotic shock.

    • Determine cell shapes and contain antigenic components, primarily complex carbohydrates such as chitin, glucans, and mannose-proteins.

    • The CW glucan (absent in humans) is the target for antifungal echinocandins like caspofungin.

  • Dominant membrane sterol: Ergosterol (not cholesterol), targeted by imidazoles, triazoles, and polyenes antifungals.

  • Fungi show a lack of susceptibility to antibacterial antibiotics.

Growth Characteristics

  • Obligate aerobes; most molds are aerobic.

  • Most yeasts are facultative anaerobes.

  • Fungi are more resistant to osmotic pressure than bacteria and can grow in low moisture conditions.

COMPARISON: FUNGI VS. BACTERIA

Feature

Fungi

Bacteria

Nucleus

Defined nuclear membrane

No membrane

Mitochondria

Present

Absent

Cell wall content

Glucans; mannans; chitin

Peptidoglycan

Cell membrane

Contains sterols; no respiratory enzymes

Sterols absent; contains respiratory enzymes

Antibiotic susceptibility

Resistant to penicillins, tetracycline, chloramphenicol

Resistant to griseofulvin and amphotericin B

Dimorphism

Mycelial or yeast forms

None

Cultivation

Optimal at pH < 6.0

Optimal at pH 7.2-7.6

Spores

Asexual reproduction and survival

(Not specified in content)

GENERAL GROWTH FORM OF FUNGI

Yeasts

  • Single vegetative cells; usually spherical or ellipsoidal (3-15um).

  • Optimal growth between 35°C to 37°C.

  • Colonies appear moist, creamy, opaque, or pasty (1-3 mm), resembling bacterial colonies without aerial hyphae.

  • Identification is based on biochemical testing and molecular methods; most reproduce asexually by budding (few divide by binary fission).

Molds

  • Multicellular, filamentous organisms.

  • Colonies appear fluffy, cottony, woolly, or powdery.

  • Optimal growth temperature is around 25°C.

  • Identification involves growth rate, macroscopic and microscopic appearance.

DIMORPHISM AND POLYMORPHISM

  • Dimorphism: some fungi can exist in two forms based on growth conditions:

    • Yeast or tissue state (37°C with CO2).

    • Mold phase (room temperature 22°-25°C).

  • Thermally dimorphic fungi include:

    • Blastomyces dermatitidis

    • Coccidioides immitis

    • Histoplasma capsulatum

  • Polymorphic fungi exhibit both yeast and mold forms in the same culture, e.g., Exophiala spp.