Fungal Lectures Block 1.2

Fungal Lectures (Block 1.2)

• The study of fungi is called mycology
• Fungi are eukaryotic organisms
• Normally harmless to humans, except when a patient is immunocompromised or has severe immunodeficiency
• Fungi can be pathogenic in these conditions, eg. HIV patient, or taking immunosuppressant medication long term
• Fungi are generally classified as obligate aerobes and some are facultative anaerobes
• None are obligate anaerobes
• All fungi require an organic source of carbon for metabolism
• The natural habitat for most fungi is the environment, with Candida albicans being an important exception as part of the normal human flora

Fungal Morphology

• Yeasts: Single cells that reproduce by budding
• Moulds: Grow as long filaments called hyphae, forming a mycelium
  • Some hyphae have transverse walls (septate hyphae)
  • Others do not have transverse walls (non-septate hyphae)
• Dimorphic Fungi: Medically important fungi exhibiting two morphologies depending on temperature
  • Exist as molds in the environment (25°C)
  • Exist as yeasts in human tissues (body temperature 37°C)

Fungal Classification Examples

• Moulds: Aspergillus, A. fumigatus, Mucor spp., Rhizopus spp., Dermatophytes
• Yeasts: Candida, C. albicans, Cryptococcus, C. neoformans, C. gattii, Malassezia, M. furfur
• Dimorphic Fungi: Histoplasma capsulatum, Coccidioides immitis, Sporothrix schenkii

Clinical Classification of Fungal Infections

• Superficial mycoses: Affect the keratinized part of the skin
• Cutaneous mycoses: Affect the deeper layers of the skin (e.g., hair, nails)
• Subcutaneous mycoses: Affect the subcutaneous layer of the skin, often following trauma
• Systemic (deep) mycoses: Affect internal organs, common in immunocompromised patients
• Fungal allergies: Result from fungal spores causing conditions like asthma or urticaria
• Mycotoxicosis: Caused by potent fungal toxins (like aflatoxin), which are often toxic to the liver

Immune Response to Fungi

• Acute suppuration with neutrophils in the exudate
• Formation of granulomas
• Delayed hypersensitivity skin test response to fungal antigens

Laboratory Diagnosis

• Microscopic examination of specimens
• Culture of the organism
• DNA probe tests, which involve treating the specimen with KOH to dissolve tissue and leave the alkali-resistant fungi intact

Fungal Culture

• Fungi are frequently cultured on Sabouraud agar, which inhibits bacterial growth, facilitating fungal isolation and identification.

References

• Review of Medical Microbiology and Immunology (Warren Levinson, 13th Ed.)
• Jawetz, Melnick, & Adelberg's Medical Microbiology (26th Ed.)
• Lippincott's Illustrated Reviews: Microbiology (2nd Ed.)