Introduction to Mycology: Study of Fungi

Mycology: The Study of Fungi

• Mycology: Study of fungi, genetics, biochemistry, and disease.
• Mycoses: Fungal infections in humans/animals.
• Fungal Characteristics: Eukaryotic, chitin cell wall; unicellular (yeasts) or multicellular (molds); saprobes (decomposers) or parasites.
• Ergosterol: Fungal cell membrane sterol; analogous to cholesterol in animals; inhibited by fungistatic medications (e.g., azoles). Directly targeted by fungicidal medications (e.g., polyenes).

Morphological Forms

• Hyphae: Thread-like structures forming fungal mycelium.
• Septate Hyphae: Divided by cross walls.
• Nonseptate Hyphae: Continuous, no cross walls.
• Yeast: Single-celled, oval/round; reproduce by budding.
• Pseudohyphae: Chains of elongated yeast cells remaining attached.
• Dimorphic Fungi: Two forms depending on environment: Yeast (pathogenic, 37°C) and mold (environmental, 25°C).

Reproductive Spores

• Asexual Spores:
  • Conidia: Spores borne on specialized structures.
  • Sporangiospores: Formed inside a sporangium.
• Sexual Spores:
  • Zygospores: Fusion of two hyphae.
  • Ascospores: Contained in an ascus.
  • Basidiospores: Produced on a basidium.

Fungal Culture

• Dimorph Characteristics:
  • Oxygen Tolerance: Yeast phase – facultative anaerobes; mold phase – aerobic.
  • Temperature: Yeast phase – grows at 37°C; mold phase – grows at 25°C.
• Sabouraud's Agar (Sab agar): Slightly acidic (pH ~5.6), inhibits bacteria, promotes fungal growth.

Contagiousness of Fungal Infections

• Most fungal infections are not contagious.
• Exceptions: Dermatophytes (e.g., ringworm) are transmitted via direct or indirect contact.

Modes of Fungal Disease

• Infections: Invasive fungal growth (e.g., Candida, Aspergillus).
• Toxins: Mycotoxins from fungi (e.g., aflatoxins from Aspergillus).
• Allergies: Hypersensitivity to fungal spores.

Diagnosis

• KOH Test: Dissolves keratin, visualizing fungal elements microscopically.
• GMS Stain: Highlights fungal cells in tissue samples.
• Colony Morphology: Observed on Sabouraud's agar.

Antifungal Medications

• Fungistatic: Inhibit fungal growth (e.g., Azoles – Fluconazole, Itraconazole; side effects: liver toxicity, fewer side effects than polyenes);
• Fungicidal: Kill fungi (e.g., Polyenes – Amphotericin B, Nystatin; side effect: kidney toxicity from membrane interactions in host cells)
• Other Antifungals: Flucytosine (used with Amphotericin B for severe infections), Griseofulvin (treats skin, hair, and nail dermatophyte infections).
• Prophylactic Antifungals: e.g., Posaconazole, for immunocompromised individuals (e.g., chemotherapy/transplant patients).

Pathogenic Fungi

• Histoplasma capsulatum: Dimorphic fungus causing histoplasmosis, linked to bird/bat droppings.
• Coccidioides immitis: Produces spherules (large structures containing endospores) in infected tissues.

Opportunistic Fungi

• Candida albicans: Thrush (oral/throat fungal infection—white patches/plaques).
• Cryptococcus neoformans: Reservoir: soil contaminated with pigeon droppings.
• Penicillium chrysogenum: Produces penicillin; generally not pathogenic.

Superficial Fungal Infections (Tinea)

• Tinea versicolor: "Spaghetti and meatballs" under microscope due to hyphae/spores.
• Types: Capitis (head), corporis (body/ringworm), pedis (athlete's foot), cruris (jock itch), barbae (beard), manuum (hand), unguium (onychomycosis; nail infection).

Subcutaneous Fungi

• Sporothrix schenckii: "Rose gardener's disease;" contracted from thorn pricks or handling contaminated plant matter.

Protozoa: General Characteristics

• Protozoa: Unicellular, eukaryotic organisms in moist environments.
• Motility:
  • Flagella: (e.g., Giardia, Trypanosoma).
  • Cilia: (e.g., Balantidium coli).
  • Pseudopodia: (e.g., Entamoeba histolytica).
  • Non-motile: (e.g., Plasmodium – rely on vectors).

Trophozoite vs. Cyst

• Trophozoite: Active, motile feeding, replicating stage.
• Cyst: Dormant, resilient stage; survives outside host; forms under unfavorable conditions.
• Encystation: Protozoa form cysts during harsh conditions.
• Excystation: Cysts turn into trophozoites in favorable environments.

Definitive & Intermediate Hosts

• Definitive Host: Host for sexual reproduction (e.g., Anopheles mosquito for Plasmodium).
• Intermediate Host: Host for asexual reproduction/development (e.g., humans for Plasmodium).

Prevention of Insect-transmitted/Food/Water-Borne Diseases

• Insect-Transmitted: Repellents, treated bed nets, eliminate standing water, prevent contact.
• Water/Food-Borne: Consume safe water, thoroughly cook food, wash produce.

Antiprotozoal Medications

• Metronidazole: (Flagyl) Treats Entamoeba, Giardia, Trichomonas
• Chloroquine: Treats Plasmodium (malaria)
• Artemisinin-based Combination Therapy (ACT): Treats multi-drug-resistant malaria.
• Nitazoxanide: Treats Cryptosporidium & Giardia.
• Pentamidine: Treats Trypanosoma brucei.
• Suramin & Melarsoprol: Treats African trypanosomiasis.
• Paromomycin: Treats Amoebiasis and Cryptosporidiosis.

How Antibiotics Kill Protozoa

• Some antibiotics (e.g., metronidazole) disrupt DNA synthesis in protozoa. Others (e.g. paromomycin) inhibit protein synthesis.

Specific Protozoa

• Plasmodium (malaria): Symptoms – cyclic fever; vector – Anopheles mosquito; asexual/sexual stages in humans and mosquitoes.
• Toxoplasma gondii: Definitive host – cats; intermediate host – humans & other warm-blooded animals.
• Balantidium coli: Pathogenic ciliate; transmission – fecal-oral; zoonotic source – pigs.
• Giardia lamblia: "Beaver fever;" zoonotic; transmission - fecal-oral; damages intestinal villi.
• Trichomonas vaginalis: STD; no cyst stage.
• Trypanosoma cruzi (Chagas disease): Vector – Triatomine bug; complications – heart failure, megacolon, megaesophagus.
• Leishmania donovani (Kala-azar): Vector – sandfly; forms – visceral (fever/enlarged organs) and cutaneous (skin ulcers).
• Entamoeba histolytica: Transmission – fecal-oral; symptoms – dysentery, blood diarrhea; complications – liver abscess.
• Acanthamoeba keratitis: Transmission - contact with contaminated water; Symptoms - eye pain; Route to brain via blood or nerve pathways.
• Naegleria fowleri: Transmission – nasal entry of contaminated water; Symptoms – headache, fever; route to brain - olfactory nerves.

General Characteristics of Helminths

• Multicellular eukaryotes with specialized systems.
• Life cycles with intermediate/definitive hosts.
• Produce eggs/larvae for propagation.
• Classified as Platyhelminthes (flatworms) and Nematodes (roundworms).

Dioecious vs. Monoecious

• Dioecious: Separate male/female organisms.
• Monoecious: Both male/female reproductive organs in one organism.

Specific Nematodes

• Ascaris lumbricoides (large intestinal roundworm): Transmission – fecal-oral, migration to lungs and intestines.
• Enterobius vermicularis (pinworm): Transmission –fecal-oral; symptoms – perianal itching.
• Necator americanus (hookworm): Transmission – skin penetration in contaminated soil. Symptoms – anemia from feeding on blood.
• Trichinella spiralis (pork roundworm): Transmission – undercooked pork; symptoms – muscle pain, fever.
• Wuchereria bancrofti (lymphatic filarial worm): Transmission – infected mosquitoes; symptoms – elephantiasis.
• Dracunculus medinensis (Guinea worm): Transmission – drinking water; painful worm emergence.

Specific Platyhelminthes

• Schistosoma mansoni (blood fluke): Transmission – skin penetration of cercariae; symptoms – rash/fever/liver damage.
• Fasciola hepatica (sheep liver fluke): Transmission – ingesting contaminated plants; symptoms – abdominal pain/liver damage.
• Taenia solium (pork tapeworm) and Taenia saginata (beef tapeworm): Transmission - undercooked meat; symptoms – intestinal infection; T. solium – neurocysticercosis.

Antihelminthic Drugs

• Albendazole/Mebendazole: Broad-spectrum for nematodes and some cestodes; targets microtubule synthesis.
• Praziquantel: Treats blood flukes, liver flukes and tapeworms; causes paralysis.
• Ivermectin: Treats nematodes like Strongyloides and filarial worms; targets the nervous system.
• Pyrantel pamoate: Treats intestinal nematodes; causes paralysis at neuromuscular junctions.

Heartworm Prevention

• Ivermectin/Milbemycin oxime: Kills larval stages of Dirofilaria immitis; monthly dosing to prevent mature worms.

How Antibiotics Kill Helminths Indirectly

• Some antibiotics target symbiotic bacteria within parasitic worms, disrupting the worm's lifecycle.

This quiz covers the fundamental concepts of mycology, including the characteristics of fungi, types of mycoses, and fungal morphology. Learn about the structures such as hyphae and yeasts, along with their reproductive mechanisms. Test your knowledge on the crucial role fungi play in ecosystems and their significance in human health.