Fungal Infections- Classified into: superficial (invade stratum corneum, hair, and nails), subcutaneous (usually due to implantation) and deep (systemic) infection
- Further subdivided into true and opportunistic pathogens
DEFINITIONS- Yeast: unicellular fungus, round to ovoid organisms with asexual reproduction (budding or binary fission), pseudohyphae (long chain of yeast cells with constrictions rather than true septae), form moist colonies
- Mold: multicellular filamentous fungus with hyphae (tubular branching cells, regular septae), reproduction via spore development and dispersal; can be geophilic (growth primarily in soil), zoophilic (predominantly infects animals), or anthrophilic (infects humans), cell membrane with unique sterol (ergosterol)
- Dimorphic fungi: grow as either yeast or mold, depending on environmental conditions (yeast form in tissue at 37°C, but mycelial form in environment at 25°C)
- Dematiaceous fungi: fungi with pigmented hyphae (green, brown, or black); appearance of brown-black coloration on artificial culture media
- Mycelium: large intertwined mass of hyphae; different types (see below)
| | | | | | | | Types of Mycelia | | | Spiral hyphae | | Regular hyphae with occasional spiral coils (T. mentagrophytes) | | | Pectinate bodies | | Hyphal ends with protuberances resembling comb (M. audouinii) | | | Nodular bodies | | Knot of twisted hyphae | | | Racquet hyphae | | Hyphae with club-shaped cells (C. immitis) | | | Favic chandeliers | | Hyphae terminate in broad branches resembling antlers
(T. schoenleinii) |
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- Spores: reproducing bodies of fungi; asexual or sexual
- Asexual spores: often characteristic for particular species, thus used as basis for identification; two types: sporangiospores and conidia
- Conidia: free spores produced directly from hyphae or supporting conidiophores, different types
| | | | | | | Types of Asexual Conidia | | | Arthroconidia | | Barrel-shaped spores released by fragmentation of hyphae (C. immitis) | | | Chlamydoconidia | | Spherical, thickened, resistant hyphal cell (C. albicans, T. tonsurans) | | | Microconidia | | Small asexual spores | | | Macroconidia | | Large leaf or club-shaped asexual spores | | | Blastoconidia | | Conidia formed by budding | | |
| | | | | | | | | Of note, macro- and microconidia may be found on branches of same mycelium filament | | | | | | | | |
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Direct Stains- Potassium hydroxide (KOH): dissolves keratin but leaves behind the hyphae (faster if dimethyl sulfoxide [DMSO] added)
- Chlorazol black E: chitin-specific blue-black stain
- Calcofluor: colorless dye, binds cellulose and chitin in fungal cell walls, seen under fluorescent microscope (apple-green fluorescence)
Histology- Gomori methenamine silver (GMS): outlines fungal elements black
- Periodic acid-Schiff (PAS): outlines fungal elements magenta with green background
- Fontana-Masson: stains dematiaceous fungi
- Mucicarmine: stains capsule of Cryptococcus neoformans pink
Media (Figure 4.20A–C) - Sabouraud Dextrose Agar (SDA): gold standard (peptone, glucose, water, agar)
- Modified SDA (Mycosel or Mycobiotic): SDA + cycloheximide + chloramphenicol
- Cycloheximide inhibits saprophytic fungi (Prototheca, H. werneckii, Scytalidium, Candida other than albicans, C. neoformans)
- Chloramphenicol inhibits bacteria
- Dermatophyte Test Medium (DTM): peptones, dextrose, cycloheximide, phenol red, chlortetracycline, and gentamicin
- Dermatophytes turn media from amber to red color due to alkaline by-products
- Non-dermatophytes cause media to turn yellow (or stay amber-colored)
| | Figure 4.20
A: DTM, non-dermatophyte*
B: DTM, dermatophyte *
* Courtesy of Hardy
Diagnostics,
www.HardyDiagnostics.com
C: DTM, non-dermatophyte
(left) and dermatophyte (right)
(Courtesy of Sandra Arduin,
Michigan Department of
Community Health) |
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