An Introduction to Fungi

Question 1

Classification and structure of fungi

Answer 1

Classification is principally based on morphology into three main categories; moulds, yeasts and mushrooms. The fungal kingdom is divided into basidiomycetes, ascomycetes and zygomycetes and can all cause human disease.

Question 2

Dermatophytic fungal infections

Answer 2

1. Ringworm/tinea…capitis(head), cruris (genitals), facei (face), manum (hands), barta (mouth), pedis (feet), corporis (abdo), ungium (nails).
2. Dermatophytosis; causatives include epidermophyton, microsporum and trichophyton spp. They are moulds with a predilection to degrade keratin as a nutrient source.
3. Pityriasis versicolor; causatives include malassezia spp. They are yeasts that also form hyphae in infected skin.

Question 3

Systemic fungal infectons

Answer 3

1. Candida spp; infect deep organs of patients with immune dysfunction. The candida spp is commensal to GI tract and some environmental. They are all yeasts,
2. Aspergillosis spp; They are ubiquitous in the environment , infection occurs by inhalation. They are moulds.
3. Cryptococcosus may cause pulmonary cryptococcosis, meningitis or disseminated infection. Yeast and capsule.

Question 4

Properties of Fungi

Answer 4

• Eukaryotes; membrane bound cell organelles
• cell wall contains glucans, mannons and chitins (polysaccharides)
• Cell membrane contains ergosterol
• Rigid cell wall/non-motile
• Chemcheterotrophs (require organic compounds for carbon and energy sources)
• Lack chlorophyll so are incapable of photosynthesis
• Gram positive
• Reproduction may be sexual or asexual or both
• Food stored in lipids and glycogen

Question 5

Host factors that contribute to pathogenicity of fungal infections.

Answer 5

• Favourable micro-environment (warm, moist)
• Broad spectrum antibacterial agents reduce competition for epithelial colonisation sites in the gut.
• Immunosuppresion

Question 6

Diagnostic methods for detecting fungi

Answer 6

1. Direct detection via histopathology or CT.
2. Detection of circulating fungal antigens
3. Detection of circulating antibodies to fungi
4. PCR for fungal DNA
5. Culture of fungus from normal sterile site

Question 7

Mechanism of action of anti-fungal drugs

Answer 7

Target; drug class; route of administration

1. Sterols; azoles; topical, oral, IV
2. Cell wall; echinocandins; IV
3. Cell membrane; polyenes; susp, topical, IV, lozenge, pastille, ointment
4. DNA synthesis; flucytosine; IV

Question 8

Problems with antifungal drugs

Answer 8

Narrow spectrum of activity, static or cidal, IV vs oral, Toxicity (polyenes - liver damage), resistance, cost