notes - fungi

Objectives

• Describe properties of fungi contributing to disease in humans.

• List the most important fungal diseases.

• Explain how parasites cause disease.

• Identify factors increasing susceptibility to parasitic disease.

• Describe the role of insects as disease vectors.

Eucaryotic Microorganisms

• includes animals, plants, fungi, algae, and protozoa.

• can be microscopic or have microscopic stages.

• Generally cause minor diseases in healthy individuals but serious effects in immunocompromised individuals (opportunistic pathogens).

Opportunistic Infections

• Infections from microorganisms that do not normally cause disease in individuals with intact immune systems but can affect those with altered defense mechanisms.

• Major cause of morbidity and mortality in AIDS patients.

Characteristics of Fungi

• Widespread and diverse group of eucaryotic organisms.

• Two main types of fungi affecting humans:

  • Yeasts (single-celled, do not have hyphae)

  • Moulds (filamentous, produce airborne spores).

• Growth environments:

  • High sugar/salt concentrations, low moisture, low temperatures, acid pH.

• Nutritional aspect:

  • Heterotrophic (require carbon for growth).

  • Saprophytes: organisms feeding on dead, woody plant materials.

Benefits of Fungi

• Source of antibiotics (e.g. penicillin, cephalosporin) and immunosuppressive drugs (e.g. cyclosporin).

• Yeasts are important in food production (bread, wine, beer, soy sauce).

• Moulds contribute flavor to cheeses and are a source of mushrooms.

Yeasts: Characteristics

• Found on surfaces of plants and fruits, ferment sugars to produce alcohol.

• Example: Saccharomyces cerevisiae used for bread, wine, and beer; also genetically engineered for vaccine production.

• Candida albicans: normal flora in humans; can become pathogenic under certain conditions.

  • Has a dimorphic nature, existing as a filamentous form at room temperature and as yeast at body temperature.

Fungal Diseases

• Serious diseases include:

  • Candidiasis, aspergillosis, cryptococcosis, histoplasmosis.

• Minor diseases primarily involve dermatophytic fungi causing infections like ringworm and athlete's foot.

• Fungal spores can also cause allergic reactions.

Candida albicans

• Can exist as a single-celled budding yeast or in filamentous form (pseudohyphae).

• Causes infections like thrush in immunocompromised patients through its invasive characteristics.

Moulds

• Filamentous fungi with long filaments (hyphae), form a dense mat called mycelium.

• Grow in various habitats (e.g. stale bread, cheese).

Fungal Infections

• Approximately 100 species of fungi can cause human disease, primarily through:

  • Superficial mycoses (e.g. tinea versicolor).

  • Cutaneous mycoses (e.g. tinea pedis, ringworm).

  • Subcutaneous mycoses (e.g. sporotrichosis).

  • Systemic mycoses affecting immunocompromised patients (e.g. histoplasmosis, pneumocystis pneumonia).

Factors Leading to Opportunistic Infections

• Weakened immune system (HIV, cancer, diabetes) allows pathogens like candidiasis to thrive.

• Environmental factors can predispose individuals (e.g. corticosteriod use can alter flora).

Specific Fungal Infections

• Pneumocystis pneumonia: major cause of illness in individuals with impaired immunity.

• Cryptococcal infections: often linked to pigeon excreta, leading to potentially severe pneumonia in immunocompromised individuals.

Transmission

• Fungal infections can be transmitted through soil, contact with infected materials, or inhalation of spores.

• Example: Candida is a commensal organism but can lead to infections when defenses are compromised (e.g. in the case of a disrupted normal flora).

Treatment Options

• Topical antifungals for cutaneous infections; systemic infections require careful management due to side effects.

• Specific treatments include fluconazole for candidiasis in AIDS patients, amphotericin for neutropenic patients, and oral azoles for onychomycosis.

Health Hazards of Fungi

• Mould in damp environments can lead to respiratory problems and allergic reactions.

• Toxic effects include food contamination, such as aflatoxins leading to liver cancer and hallucinogenic effects from ergotamine in rye.