Cultivation of fungi, and microbiology diagnosis of fungal infections Flashcards
What is the standard media for fungi?
Sabourad’s agar.
Beef broth-dextrose mixture.
Glucose - Carbon source
Peptone - Nitrogen source
Peptone provides nitrogen and creates a pH ~5.0.
The low pH inhibits most bacterial growth.
Growth can be slow, and 4 weeks is needed to prove a negative reseult.
What are the other possible agars.
Chrome agar = differentiates species of candida by color.
Normal agar, fungi grow well on most agars
Blood agar 5-10% is good for dimorphic fungi.
Czapek’s solution
Used for Aspergillus and Penicillinum
Rice tween agar: for chlamydospores (candida albicans)
TTC: Tetrazolium chloride.
Penicillin, streptomycin are often added to inhibit bacterial growth.
Cycloheximide can be added to slow the growth rate of rapidly growing yeasts.
Microbiological diagnosis of fungal infections
Scraping or or clinical samples stained with a wet mount in 10%-30% KOH.
KOH dissolves the skin cells but not the fungus with its cell wall.
PCR for fungal DNA in blood or sample is very good and sensitive.
Serology tests - generally insensitive, except for histoplasmosis and coccidioidomycoces.
Skin tests, for allergic bronchopulmonary apergillosis, Arthus test
What stains can be used for fungi?
Gomori/Grocott silver stain is the best. Stains all fungi.
KOH and India ink.
India ink is a negative stain, staining the background black and the capsulated yeasts will still be light. -cryptococcus
PAS, stains yeasts and hyphi.
Mucicarmine red stains capsule - cryptococcus neoformans
Calcofluor white stain. Stains P carnii cell wall.
Giemsa stain can show intracellular
histoplasma capsulatum or intracystic P. carniii.
Gram stains
H&E stains also both work
