Fungal Identification and Classification Quiz

Clinically Important Fungi

• Approximately 80,000 species of fungi exist, but less than 400 are medically important. Fewer than 50 fungi species cause over 90% of human and animal fungal infections.
• Mycoses (mycosis) are fungal infections. Fungi are either exogenous (from the environment) or endogenous (part of the normal flora).
• Most pathogenic fungi are exogenous.
  • Environments include water, soil, and organic debris (e.g. bird droppings).
• Endogenous mycoses (e.g., candidiasis, dermatophytosis) are caused by fungi that are part of the normal body flora.

General Properties of Fungi

• Fungi grow in two basic forms: molds and yeasts.

Molds

• Molds grow by producing multicellular, filamentous colonies.
• Colonies consist of hyphae, which are branching, tubular structures.
• Hyphae are ~2-10 µm in diameter and divided into cell-like units by cross-walls (septa).
• The entire mass of hyphae is called a mycelium.
• Common examples: Aspergillus flavus, Aspergillus niger (images shown).
• Laboratory characteristics include:
  • Pigmentation (e.g., black, green)
  • Growth rate (fast 24-48 hours or slow 4 weeks)
  • Texture (powdery or fuzzy)
• Images of Aspergillus flavus and Aspergillus niger colonies are presented.
• Image of Aspergillus flavus hyphae.

Yeasts

• Yeasts are single-celled organisms, typically spherical to ellipsoid in shape.
• Yeast cells vary in size from 3 to 15 µm.
• Reproduction occurs mainly through budding.
• Some species produce buds that don't detach, elongating into pseudohyphae.
• One example is Candida albicans (images shown).
• Laboratory characteristics include:
  • Soft, opaque colonies
  • 1-3 mm size
  • Cream-colored

Dimorphic Fungi

• Certain fungal species can exist as either yeasts or molds, contingent on environmental factors (e.g., temperature).
• Thermally dimorphic fungi grow as yeast-like organisms at 35-37°C. Culture at 25-30°C transforms these into filamentous fungi (mold).
• Clinically important dimorphic fungi include: Sporothrix schenckii, Blastomyces dermatitidis, Coccidioides immitis, Penicillium marneffei, and Histoplasma capsulatum.
• Images of *Sporothrix schenckii * at 37°C and 25°C are included.

Fungal Cell Wall

• All fungi have a rigid cell wall defining their shape.
• The cell wall composition includes carbohydrate layers, glycoprotein and lipids.
• Cell wall mediates attachment to host cells.
• Activates the complement cascade and elicits an inflammatory response leading to immune response.

Fungi Classification

• Fungi are categorized into four major phyla: Zygomycota, Ascomycota, Basidiomycota, and Chytridiomycota.
• Ascomycota is the most prominent phylum, encompassing ~85% of human fungal infections.
• Species within the Ascomycota phylum include: Histoplasma spp., Blastomyces spp., Penicillium spp., Aspergillus spp., Microsporum spp, Trichophyton spp., Saccharomyces spp, Candida spp, Coccidioides spp, Sporothrix schenckii, and others.
• Conditions caused by Ascomycota include histoplasmosis, blastomycosis, aspergillosis, candidiasis, dermatophytosis.

Zygomycota

• Zygomycota reproduces via sporangia and zygospores, and exhibits sparsely septate hyphae.
• Notable species include Rhizopus spp., Mucor spp., Absidia spp., and Philobolus spp.
• Mucormycosis is a disease associated with this phylum (images of oral and skin mucormycosis are shown).
• Causative agent is Mucor spp.
• Infection might involve sinuses, brain, lungs, oral, gastrointestinal tract, skin, and other organs.

Ascomycota

• Ascomycetes encompasses ~85% of human fungal infections
• Examples include Histoplasma, Blastomyces, Aspergillus, Penicillium,
• Trichophyton*, Candida, and Coccidioides.
• Diseases include histoplasmosis, blastomycosis, candidiasis, tinea capitis/corporis/pedis.

Basidiomycota

• Basidiomycetes includes Cryptococcus neoformans, Malassezia spp. and Trichosporon spp.
• Diseases such as cryptococcosis (eg cryptococcal meningitis and cryptococcosis involving the CNS).
• Cryptococcus neoformans: yeasts with a thick polysaccharide capsule, frequently found in dry pigeon feces. Inhalation causes primary pulmonary infection which can asymptomatically progress to disseminated infection.
  • In immunocompromised hosts, disseminated cryptococcal infection leads to cryptococcal meningoencephalitis or cryptococcal meningitis.
  • Signs and symptoms(presentation) for cryptococcal meningitis include fever, headache, elevated intracranial pressure, nausea, ocular involvement, evidence of dissemination of infection, neck stiffness, seizure, altered mental status, focal neurologic deficits.
  • Labs for diagnosis include: FBC, ESR/C-reactive protein, CXR, sputum cultures, blood cultures, HIV testing, CSF analysis (WBC, protein, glucose), India ink staining and fungal culture.
  • Therapy involves antifungal agents (e.g., Amphotericin B plus flucytosine).

Lab Diagnosis of Fungal Infections

• Methods for diagnosing fungal infections include microscopic examination, culture, identification tests, and sensitivity tests.
• Types of microscopic examinations include saline wet mounts, lactophenol cotton blue (LPCB) preparation, potassium hydroxide (KOH) preparations, Gram stain (e.g., Candida albicans), India ink preparation (Cryptococcus neoformans) and other stains.
• Fungal cultures use primary fungal media: Saboraud Dextrose Agar (SDA), Saboraud Dextrose Agar with cycloheximide and chloramphenicol (SDA-C&C), and Brain Heart Infusion Agar with Blood (BHIAB).
• Incubation times vary, ranging from 3-4 days to 3-4 weeks.
• Growth rate depends on temperature, media, and inhibitors in patient specimens. Culture is kept for a month before deemed negative.
• Yeast identification methods include microscopic morphology, germ tube production, assimilation reactions, urease production, pigment production, API, and PCR.

Treatment

• Antifungal therapy frequently involves a combination of Amphotericin B and flucytosine.

Test your knowledge on various aspects of fungi, including their media, reproduction, and pathogenicity. This quiz covers the identification and characteristics of different fungal species, as well as their role in mycosis. Perfect for students and enthusiasts alike!