Unit 3: Medical mycology
Immunology and Mycology notes · PTU syllabus (BMLS407-18)
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Unit summary
Fungi cause infections from ringworm to life-threatening meningitis. This unit covers medical mycology, superficial and deep mycoses, the classification of important fungi, common contaminants, culture media, direct microscopy, processing of clinical samples, identification of yeasts and moulds, and preservation of fungal cultures.
After this unit you can
- Classify fungi and mycoses
- Process clinical samples for fungal diagnosis
- Use direct microscopy and culture media
- Identify yeasts and moulds and preserve cultures
PTU syllabus topics
- Introduction to medical mycology
- superficial and deep mycoses
- taxonomy and classification of medically important fungi
- morphological and cultural characteristics of common fungal contaminants
- culture media used in mycology
- direct microscopy
- processing of clinical samples (skin, nail, hair, pus, sputum, CSF)
- isolation and identification technique for yeasts and moulds
- preservation of fungal cultures
Superficial
Outer skin and hair: pityriasis versicolor
Cutaneous
Skin, hair, nails: dermatophytes (ringworm)
Subcutaneous
Below skin: mycetoma, sporotrichosis
Systemic
Deep organs: histoplasmosis
Opportunistic
Weak immunity: Candida, Aspergillus, Cryptococcus
Topic 1
Introduction to medical mycology
- Fungi: eukaryotic, with chitin cell walls and ergosterol membranes; grow as yeasts (single cells, budding), moulds (hyphae forming a mycelium; septate or aseptate), or dimorphic forms.
Zygomycetes (Mucorales)
Aseptate hyphae — Mucor, Rhizopus
Ascomycetes
Septate — Aspergillus, Candida (teleomorphs)
Basidiomycetes
Cryptococcus
Deuteromycetes (fungi imperfecti)
No known sexual stage — dermatophytes and others
Topic 2
Superficial and deep mycoses
Superficial
Pityriasis versicolor, tinea nigra
Malassezia furfur
Cutaneous (dermatophytoses)
Tinea capitis, corporis, cruris, pedis, unguium (ringworm)
Trichophyton, Microsporum, Epidermophyton
Subcutaneous
Mycetoma (Madura foot), sporotrichosis, chromoblastomycosis
Madurella, Sporothrix
Systemic (deep)
Histoplasmosis, blastomycosis
Dimorphic fungi
Opportunistic
Candidiasis, cryptococcal meningitis, aspergillosis, mucormycosis
Candida, Cryptococcus, Aspergillus, Rhizopus
- Mucormycosis rose sharply in India in 2021 among COVID-19 patients with uncontrolled diabetes and steroid use.
Topic 3
Common fungal contaminants
- Laboratory contaminants: Aspergillus, Penicillium (brush-like conidiophores), Rhizopus, Mucor, Cladosporium, Alternaria. Interpret cautiously — repeated isolation from sterile sites suggests true infection.
Topic 4
Culture media in mycology
Sabouraud dextrose agar (SDA)
Peptone, 4% glucose, pH 5.6
General fungal culture
SDA with chloramphenicol and cycloheximide
Inhibits bacteria and saprophytic moulds
Dermatophytes (cycloheximide inhibits Cryptococcus and some Candida)
Corn meal agar
Low nutrient
Chlamydospores of Candida albicans
CHROMagar Candida
Chromogenic
Species of Candida by colony colour
Niger seed (birdseed) agar
Phenol oxidase substrate
Brown colonies of Cryptococcus
Brain heart infusion agar
Rich
Dimorphic fungi at 37 °C
- Incubate at 25–30 °C (and 37 °C for yeasts and dimorphic fungi) for up to 4 weeks.
Topic 5
Direct microscopy
- KOH mount (10–40%)
- Dissolves keratin in skin, hair and nails; shows hyphae
- India ink
- Capsulated Cryptococcus in CSF
- Gram stain
- Yeasts and pseudohyphae (Gram-positive)
- Lactophenol cotton blue (LPCB)
- Stains fungal structures from culture
- Calcofluor white
- Fluorescent stain for chitin
- Hair invasion: ectothrix and endothrix patterns; Wood's lamp shows green fluorescence in some Microsporum infections.
Topic 6
Processing clinical samples
Skin
Scrape the active edge after cleaning with 70% alcohol
KOH mount; SDA with antibiotics
Nail
Clip and scrape subungual debris
KOH (longer clearing); culture
Hair
Pluck infected hairs
KOH; culture
Pus and tissue
Aspirate or biopsy; look for granules (mycetoma)
Crush granules; KOH; culture
Sputum and BAL
Early morning sputum
KOH or calcofluor; culture
CSF
Lumbar puncture
India ink, cryptococcal antigen (latex), culture
Topic 7
Identification of yeasts and moulds
- Germ tube test
- C. albicans forms germ tubes in serum at 37 °C within 2 hours
- Chlamydospores
- On corn meal agar — C. albicans
- Sugar assimilation and fermentation
- Species identification
- Urease
- Positive for Cryptococcus
- Automated systems
- VITEK 2, MALDI-TOF
- Moulds: identified by colony appearance (front and reverse, texture, rate of growth) and microscopy of LPCB mounts — septate or aseptate hyphae, conidia arrangement. Slide culture preserves the natural arrangement of spores. Dimorphism shown by growth as yeast at 37 °C and mould at 25 °C.
Topic 8
Preservation of fungal cultures
- Subculture on SDA slants every few months; storage under sterile mineral oil, in sterile distilled water (Castellani method), freezing in glycerol at −80 °C, or lyophilisation for long-term storage.
Key terms
- Mycosis
- Fungal infection
- Dermatophyte
- Fungus infecting keratinised tissue
- Dimorphic fungus
- Fungus growing as yeast at 37 °C and mould at 25 °C
- Germ tube
- Short filament from a yeast cell, identifying C. albicans
- Sabouraud dextrose agar
- Acidic glucose-rich medium for fungi
Quick revision
- Yeasts, moulds, dimorphic fungi; classification.
- Superficial, cutaneous, subcutaneous, systemic, opportunistic mycoses.
- Contaminants; SDA and other media.
- KOH, India ink, LPCB, calcofluor; sample processing.
- Germ tube, chlamydospores, slide culture, dimorphism; preservation methods.
Important exam questions
Practice questions written to the PTU exam pattern for this unit's syllabus: short answers (Section A style) and long answers (Sections B and C style).
Short-answer questions
- Q1.Name the three genera of dermatophytes.
- Q2.What is the use of KOH in mycology?
- Q3.Which stain shows Cryptococcus in CSF?
- Q4.Why is cycloheximide added to SDA?
- Q5.What is the germ tube test?
- Q6.Name two methods of preserving fungal cultures.
Long-answer questions
- Q1.Classify mycoses with examples.
- Q2.Describe culture media used in mycology.
- Q3.Describe the processing of clinical samples for fungal diagnosis.
- Q4.Describe the identification of yeasts and moulds.
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