L47: Medically Important Fungi

Question 1

Fungi structure

Answer 1

1. Eukaryotes
2. Cell wall (60% glucan, 30% protein, 10% chitin)
3. Ergosterol cell membrane

Question 2

Classification of fungi

Answer 2

1. Yeasts (Candida albicans, parapsilosis, tropicalis, krusei, Cryptococcus neoformans)
   - unicellular
   - budding
2. Moulds (Aspergillus fumigatus, Dermatophytes)
   - multicellular
   - Mycelium (hyphae + spore (asexual/sexual reproduction))
3. Dimorphic fungi (Penicillium marneffei, in HK)
   - Yeast at body temp
   - Mould at room temp

Question 3

Risk factors for fungal infection

Answer 3

Neutropenia: Aspergillus

HIV infection: C. neoformans, P. marneffei

Question 4

Clinical disease caused by fungi

Answer 4

Superficial

1. Candidiasis
   - oral, vaginal thrush
2. Dermatophytosis
   - Tinea pedis, capitum, unguium

Systemic
In immunocompetent hosts:
1. Allergic reaction to spore

In immunocompromised hosts:

2. Refractory superficial infections: mucocutaneous candidiasis
3. Systemic candidiasis: Candidemia, hepatosplenic candidiasis (Candida, C. neoformans, A. fumigatus, P. marneffei)

Question 5

Diagnosis of fungal infections

Answer 5

1. Direct visualisation
   - Yeast: Gram smear (Candida), Indian ink smear (C. neoformans)
   - Mould: KOH smear
   - Biopsy and histopathology
2. Culture
   - agar plates
   - broth culture
3. Antigen detection
   - Beta-D-glucan
   - C. neoformans: polysaccharide capsule
   - Aspergillus: galactomannan
4. Nucleic acid detection
   - Aspergillus
5. Antibody response of patient
   - P. marneffei

Question 6

Anti-fungal agents (3 drugs)

Answer 6

1. Cell membrane:
   - Polyene: bind to ergosterol and disrupt cell membrane e.g. Nystatin (topical), Amphotericin B (systemic)
   - Azole: inhibit ergosterol synthesis: Fluconazole (systemic), Itraconazole
2. Nucleoside analogue: 5-flucytosine
3. Cell wall synthesis: Echinocandins e.g. Caspofungin

Question 7

Characteristics of Candida albicans

Answer 7

• Normal commensals in skin, GIT, female genital tract —> endogenous infection
• Risk factors: broad spectrum antibiotics, immunocompromised (steroid, HIV, neutropenia)

Question 8

Prevention of fungal infection

Answer 8

1. Avoid overuse of antibiotic
2. Fluconazole prophylaxis for transplant patient
3. NO vaccine