Medically Important Fungi

Medically Important Fungi

1. Stains

   • Gomori methenamine silver (GMS)
     • Routine stain used for tissue specimens and smeared material
     • Alternatives include Periodic Acid-Schiff (PAS) and Gridley fungus stain
     • Fungi are gray-black with green background
   • Calcofluor white with KOH
     • Fluorescent stain used for wet preparations or smeared specimens that have been dried. Stains blue
   • Mucicarmine stain
     • Used to detect polysaccharide capsule. Ex: Cryptococcus
   • Fontana-Mason
     • Used for detecting melanin
   • Giemsa
     • Stains fungi and bacteria blue-purple. Useful in bone marrow and blood smears.
   • Hematoxylin and eosin
     • Stains some fungal elements violet to bluish purple. Allows observation of natural pigments of fungi
   • Modified acid-fast
     • Stains filaments of Nocardia pink. Actinomycetes are negative
   • PAS
     • Stains fungi pinkish-red. Demonstrates the double-contoured refractile walls of Blastomyces dermatitidis.
   • Gridley fungus
     • Fungi stain purple-red. Filaments of actinomycetes are not stained
   • Brown and Brenn
     • Gram stain for bacteria. Demonstrates the bacterial filaments of the actinomycetes.

2. Yeast and yeastlike organisms

   • Unicellular organisms that generally reproduce by budding. If buds elongate and remain attached to parent cell, they form pseudohyphae.
   • Few can produce ascospores (sexual form)
   • Colonies are smooth and glabrous and may be moist or dry
   • Opportunistic pathogens
   • Candida species
     • C. albicans
       • Cream colored, smooth colonies often with "feet."
       • Positive germ tube test.
       • Cornmeal-Tween 80 agar
         • Pseudohyphae with clusters of round blastoconidia at the septa. Large, thick-walled terminal clamydospores form.
     • C. glabrata
       • Grows a bit more slowly than other species of Candida
       • Some isolates depend of cholesterol for growth
       • Does not form pseudohyphae; only forms cells with budding.
     • C. krusei
       • Pseudohyphae with elongate blastoconidia
     • C. dubliensis
       • Positive germ tube test.
       • Similar morphology to C. albicans but less common as an isolate.
     • C. tropicalis
       • Forms blastoconidia singly or in small groups along long pseudohyphae on cornmeal-Tween 80 agar.
   • Cryptococcus
     • C. neoformans
       • Environmental source is bird and bat droppings.
       • Produces a capsule that is visible using india ink.
     • C. gatti
       • Morphologically similar to C. neoformans. Differentiation is not necessary due to identical treatment strategies.
       • Infection more associated with lesions in the lungs and brain.
     • Infection frequently involves the central nervous system.
   • Geotrichum
     • True hyphae that segment into rectangular arthroconidia. Cells characteristically germinate from one corner.

3. Trichosporon spp.

   • Causes white piedra
     • Superficial infection of the hair that causes white nodules to appear on the hair shafts.
   • Present with hyphae, pseudohyphae, blastoconidia, and arthroconidia.

4. Thermally Dimorphic Fungi

   • Grow hyphae at 25-30C and yeast-like cells at 37C.
     • Histoplasma capsulatum
       • Commonly found in bird and bat droppings. Endemic to the Ohio and Mississippi River valleys.
       • "Mariner's wheel" formation of macroconidia.
       • Causes respiratory and systemic infections.
       • Microconidia grow directly off hyphae.
     • Blastomyces dermatitidis
       • Causes blastomycosis. Characterized by pulmonary infection and occasionally granulomatous lesions.
       • "Lollipop" conidia at the apex of conidiophores or directly on the hyphae.
     • Paracoccidioides brasiliensis
       • South American blastomycosis
         • Chronic, granulomatous disease that begins in the lungs and spreads to the mucous membranes of the nose, mouth, and occasionally gastrointestinal tract.
       • Identified through the yeast form. Thick-walled cells with single or multiple buddings.
     • Penicillium marneffei
       • Causes deep-seated infection that can be focal or disseminated, mostly in immunocompromised patients.
       • Rapid growth compared to other dimorphic fungi.
       • Conidiophore with 4-5 terminal metulae. Metual bear 4-6 phialids. Phialids produce conidia in chains.
       • "Fingers on a hand" morphology.
       • Produce red pigment at edge of mold colonies.
     • Sporothrix schenckii
       • Causes sporotrichosis, a chronic infection that begins as a lesion of the skin and progresses to the lymphatic system.
       • "Rose gardener's thumb"

5. Zygomycetes

   • Moulds :)
     • Etiological agents of zygomycosis
       • Rhizopus
         • Little to no septae. Hypahe form at the base of sporangiophores.
       • Mucor
         • Nonseptate, does not formrhizoids.
       • Rhizomucor
         • Branched sporangiophores and rhizoids at points in-between the sporangiophores.
     • Cunninghamella bertholletiae
       • Uncommon cause of cutaneous infections following traumatic inoculation.
       • Can grow at 40-45C
       • Almost no septae

6. SBA (Sabourad's dextrose agar)

   • Additives
     • Chloramphenicol -> inhibits bacteria
     • Cycloheximide -> inhibits rapidly growing molds such as saprophytes and focuses on dermatophytes

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