Pathogenic Fungi Overview

General Properties, Virulence, Pathogenesis, and Classification of Pathogenic Fungi

• Fungi are eukaryotes, with higher complexity than bacteria.
• Fungi reproduce both sexually and asexually.
• Fungi can be unicellular or have long, branching filaments (hyphae) to form multicellular structures.

Fungal Structure, Metabolism, and Reproduction

• Fungi have a nucleus, nuclear membrane, linear chromosomes, actin microfilaments, tubulin-containing microtubules, ribosomes, and organelles (like mitochondria and endoplasmic reticulum).
• The Golgi apparatus is also present.
• The fungal cell wall is primarily composed of chitin.
• Chitin is a polysaccharide made of N-acetylglucosamine.
• The cell wall also contains other polysaccharides, such as β-glucan, which is a polymer of glucose. β-glucan is the site of action of some antifungal drugs.
• Fungal cell membranes have ergosterol, differing from cholesterol in human cell membranes.
• This difference is exploited by some antifungal drugs (e.g., amphotericin B, azoles).

Types of Fungi

• Fungi can be yeasts or molds.
• Yeasts are single-celled organisms that reproduce asexually by budding.
• Molds grow as long filaments called hyphae to form a mat-like structure (mycelium).
• Hyphae can be divided by cross-walls (septate) or not (coenocytic); nonseptate hyphae are multinucleated.
• Some fungi are thermally dimorphic.
• They change their structures (e.g., mold to yeast) depending on the temperature.

Fungal Growth and Reproduction

• Some fungi reproduce sexually (zygospores, ascospores, basidiospores) and asexually by producing conidia.
• Fungi, in general, require a preformed organic carbon source.
• Most fungi are obligate aerobes—some are facultative anaerobes.

Virulence, Pathogenesis, and Classification

• Approximately 80,000 fungi species have been described; however, only about 400 are medically important; less than 50 cause most infections.
• A majority are beneficial to humans, enhancing quality of life and being essential for recycling organic matter (e.g., for food and bio-active molecules such as penicillin)
• Fungal infections (mycoses) often have chronic or subacute relapses, unlike other infections.
• Most pathogenic fungi originate from the environment.
• Candida albicans and dermatophytes are frequently present in humans, are part of the normal microbiota, and are therefore opportunistic pathogens that cause infection in immunocompromised people.
• Fungi are opportunists usually infecting immunocompromised individuals, with altered floral communities and excessive antibiotic intake.

Fungal Adherence

• Several, especially yeasts, colonize mucosal surfaces.
• Adherence is key for colonization and virulence.
• Adherence depends on adhesins on the fungus binding to receptors on host cells (very often a surface mannoprotein).

Fungal Invasion

• Invasion, passing through barriers (skin, membranes) is crucial to fungal infections.
• Some fungi change morphology (e.g. from mold to yeast) to better invade host tissues.
• Fungi may produce extracellular enzymes, like proteases and elastases, contributing to their ability to invade host tissues.

Fungal Injury

• Fungi can produce mycotoxins in the environment.
• The injury or damage from fungal infections is primarily due to the destructive effects of inflammation and immunologic responses, especially in those with immunocompromised systems (e.g. neutropenic patients suffering from invasive mold infections).

Host Defense and Fungal Resistance

• Innate immunity for healthy individuals includes phagocytic cells (neutrophils, macrophages), the complement system, and Pattern Recognition Receptors (PRRs).
• Immunity to fungal infections often depends on the cellular immune response (TH1-mediated immunity).
• Impaired cellular immunity, often found in immunocompromised individuals, leads to more severe fungal infections

Humoral and Cellular Immunity

• Antifungal antibodies can be detected in some fungal infections, but may not signify resistance.
• Cellular immunity plays a significant role, while decreased cellular immunity contributes to more severe fungal infections.

Laboratory Diagnosis of Fungal Infections

• Specimen collection and transport are crucial for successful fungal diagnosis.
• Clinical specimens are screened for fungi using microscopy and culture.
• KOH is often used in microscopy.

Laboratory Diagnosis and Management

• Laboratory safety precautions are important for mycology laboratories.
• Mold cultures and specimens must be handled in appropriate biosafety cabinets (class II).

Types of Molecular Assays

• DNA probes provide rapid and earlier diagnosis of fungal infections than other techniques.

Types of Immunoassays

• Antibody detection or titer measurement can assist in diagnosis of immunocompromised individuals.
• Tests like complement fixation and latex agglutination are often used.

Molecular and Immunoassays

• These testings provide details about detection and confirmation of proper diagnoses for fungal infections