Fungi: Disease and Beneficial Roles in Microbiology

Fungi and Disease

Definitions and Types of Fungal Disease

Fungi can cause disease in humans through infection or toxin production. Understanding the terminology and mechanisms is essential for microbiology students.

• Mycosis: Infection with fungus. Some fungi are opportunistic pathogens (cause disease mainly in immunocompromised hosts), while a few are true pathogens (can infect healthy individuals).

• Mycotoxicosis: Poisoning, usually by ingestion of fungal toxins. Some fungi produce toxins without causing infection; a few, such as Aspergillus flavus, can cause both infection and toxicosis.

Types of Mycoses

Fungal infections are classified based on the site and depth of infection.

• Superficial Mycoses: Affect skin, hair, or nails. May be caused by true or opportunistic pathogens.

• Subcutaneous Mycoses: Affect deeper layers of skin and underlying tissue. Usually caused by opportunistic pathogens, but some true pathogens exist.

• Systemic Mycoses: Affect internal organs and systems. Both true and opportunistic pathogens can be responsible.

Dimorphic Fungi and Systemic Mycoses

True pathogens causing systemic mycoses are often dimorphic, meaning they exist in two forms depending on the environment.

• Dimorphic Fungi:

  • Multicellular saprobe in the environment (25°C)

  • Parasitic yeast in the body (37°C)

  • The saprobic form produces infectious spores.

• Dimorphic fungus infections are not communicable (do not spread person-to-person).

• Infection typically starts in the lungs after inhalation of spores. It may be asymptomatic, mild, or progress to severe respiratory disease.

• Dissemination from the lungs can occur with heavy exposure or in immunocompromised individuals.

Monomorphic Fungi

Monomorphic fungi maintain a single form regardless of environmental conditions.

• Can be single-celled or multicellular in all conditions.

• Systemic mycoses involving monomorphic fungi are usually opportunistic infections (e.g., Aspergillus causing aspergillosis).

Trends in Fungal Infections

There is an increase in fungal infections due to a growing population of immunocompromised individuals (e.g., HIV/AIDS, organ transplant recipients, cancer patients).

Beneficial Fungi

Fungi in Food

Many fungi are important in food production and as edible products.

• Mushrooms: Edible species include button, chanterelle, morel, shiitake, and truffles.

• Corn Smut (Ustilago maydis): A fungus disease of corn considered a delicacy in some cultures (known as huitlacoche).

• Cheese Production: Penicillium species (Division: Ascomycota) are used in blue cheese, Brie, Camembert, and Gorgonzola.

• Meat Products: Fungi are involved in the fermentation of dry-cured ham and old world salami.

• Soy Sauce and Fermented Sauces: Aspergillus oryzae and related species (Ascomycota) are used in fermentation.

• Bread and Alcoholic Beverages: Saccharomyces cerevisiae (monomorphic yeast, Ascomycota) produces carbon dioxide and ethanol, essential for leavened bread, beer, wine, and sake.

Fungi in Medicine

Fungi are sources of important pharmaceuticals.

• Antibiotics: Penicillium chrysogenum produces penicillin.

• Immunosuppressants: Some Ascomycota produce cyclosporin, used to prevent organ transplant rejection.

Other Industrial Products

• Enzymes: Aspergillus species produce enzymes used in products like Beano and stain removers.

• Genetically Engineered Products: Saccharomyces cerevisiae is used to produce recombinant proteins such as Humulin (insulin), Hepatitis B vaccine, and human growth hormone (HGH).

Bioremediation

Fungi can metabolize toxic compounds, making them useful in environmental cleanup.

• Some fungi (e.g., turkey tail and oyster mushrooms) can break down dioxin, a toxic pollutant.

• Fungi may be used to clean up contaminated soil.