08-Mycology

Fundamentals of Mycology and Fungi Characteristics

  • Definitions and Disciplines

    • Mycology: The scientific discipline dedicated to examining fungi.

    • Mycologists: Scientists specialized in the study of fungi.

    • Mycoses: Diseases and infections caused by fungi.

  • General Characteristics of Fungi

    • Cellular Nature: Eukaryotic microorganisms that are larger than bacteria.

    • Morphological Forms:

      • Yeasts: Unicellular cells.

      • Molds / Filamentous Fungi: Colonial organisms characterized by hyphae.

    • Structure:

      • Hyphae: Filamentous structures where colonies contain many cells not fully divided by cell walls.

      • Pores: Located in the cell divisions to facilitate material transport between hyphal cells.

      • Mycelium: A mass consisting of branched threads of hyphae.

    • Cell Wall Composition: Contains chitin, glucan, and mannan. Crucially, it does not contain cellulose.

    • Nutrition: Heterotrophs that utilize absorptive nutrition, meaning they absorb nutrients in dissolved form across their entire surface.

    • Biodiversity:

      • Estimated number of species is close to 1×1061 \times 10^{6}.

      • Number of described species is approximately 75,00075,000 (this count excludes lichens).

  • Taxonomic Comparisons: Fungi vs. Plants vs. Animals

    • Multicellularity: Fungi can be multicellular (molds) or unicellular (yeasts); plants and animals are typically multicellular.

    • Cell Walls: Present in Fungi (chitin) and Plants (cellulose); absent in Animals.

    • Chloroplasts: Present in Plants; absent in Fungi and Animals.

    • Mobility: Fungi and Plants are generally non-mobile; Animals are mobile.

    • Organelles with Nucleus: Present in all three (Eukaryotes).

    • Method of Nutrient Retrieval:

      • Plants: Autotrophs (Photosynthesis).

      • Animals: Heterotrophs (Ingestion).

      • Fungi: Heterotrophs (Absorption).

Biology and Metabolism of Fungi

  • Respiration and Lifestyle

    • Oxygen Requirements: Primarily aerobic organisms, though some are occasionally facultative anaerobes.

    • Lifestyles:

      • Saprotrophic: Decomposers that degrade organic material (significant ecological importance).

      • Symbiont: Form associations with plants, such as mycorrhiza.

      • Parasite: Cause infections in humans and animals; also of high phytopathological (plant disease) importance.

  • Reproduction

    • Yeast: Reproduce via budding or fission.

    • Molds: Reproduce by sexually producing spores.

  • Scientific and Industrial Importance

    • Toxicology: Study of fungal toxins.

    • Medicine: Use of mushrooms for medicinal purposes.

    • Pathology: Human and veterinary pathology.

    • Agriculture: Plant protection mycology and forestry mycology.

    • Research: Mycorrhiza research and biotechnology (model organisms).

    • Industry: Used in alcohol production and the food industry.

Anatomy of Fungal Cells

  • Yeasts (Unicellular)

    • Shape and Size: Spherical shape with a diameter of approximately 3−5×10−6 m3-5 \times 10^{-6}\,m (micrometers).

    • Pseudohypha Formation: Emergence of fully septate hyphae with separate cells.

    • Key Species:

      • Saccharomyces cerevisiae

      • Schizosaccharomyces pombe

      • Candida albicans

    • Dimorphic Pathogenic Fungi: These exhibit an alternation of shapes; they grow as yeast within the host body but as molds (filamentous shape) outside the body.

  • Molds (Multicellular)

    • Hyphae: Multicellular units forming a mycelium.

    • Septum: Cross-walls between cells that are not fully closed (non-septate hyphae also exists).

    • Hyphae Types:

      • Vegetative hyphae: Grow within the substrate to absorb nutrients.

      • Aerial hyphae: Grow above the surface of the media and are responsible for producing spores.

  • Subcellular Components

    • Cell Wall: Solid yet flexible; determines cell shape and protects from environmental stress. Composed of polysaccharide fibrils including chitin, glucan, and mannan.

    • Periplasmic Space: The gap between the cell wall and plasma membrane. It contains enzymes for cell wall synthesis and extracellular enzymes like invertase and acid phosphatase.

    • Plasma Membrane: A liquid mosaic phospholipid bilayer.

      • Ergosterol: A sterol incorporated into the membrane to provide rigidity.

      • Phospholipids: Provide fluidity.

      • Glycolipids/Glycoproteins: Act as signal molecules on the outer surface.

    • Cytoplasm and Cytoskeleton: The cytosol is a matrix of high water content containing proteins, ions, and organic compounds. It is the site for glycolysis and amino acid biosynthesis.

    • Organelles:

      • Microbodies: Peroxisomes and glyoxisomes.

      • Ribosomes: Eukaryotic ribosomes are 80S80S, consisting of a 40S40S and a 60S60S subunit. They are composed of rRNA and proteins, found free-floating (synthesizing cytoplasmic proteins) or attached to the RER.

      • Endoplasmic Reticulum (ER):

        • Rough ER (RER): Covered in ribosomes; site of protein synthesis.

        • Smooth ER (SER): Site of lipid and sterol synthesis.

      • Golgi System: The "cis" surfaces follow the ER; lacks ribosomes; responsible for processing.

      • Lysosomes: Contain hydrolase enzymes (proteases, glucanases, nucleases, lipases, acid phosphatases) for macromolecule breakdown.

      • Vacuoles: Used for storage.

      • Nucleus: DNA is organized into multiple chromosomes; surrounded by a double membrane with pores for molecular transport.

        • S. cerevisiae has 1616 chromosomes.

        • S. pombe has 33 chromosomes.

  • Extracellular Structures

    • Capsule: A well-defined layer of polysaccharides and glycoproteins outside the cell wall. It protects against physical, chemical, and biological stress. In Cryptococcus neoformans, the capsule is a major virulence factor protecting against phagocytes.

    • Fimbriae: Protein extensions of the cell wall involved in cell coupling (sexual agglutination), aggregation, and adhesion to surfaces/tissues.

Fungal Ecology and Life Cycles

  • Environmental Factors

    • Water: Essential as a solvent, transport medium, and reaction partner. High humidity/moisture is necessary for the movement of zoospores, germination, sporulation, and the formation of fruiting bodies.

    • Temperature:

      • Thermophiles: Grow above 45∘C45^{\circ}C. The maximum for eukaryotes is 60∘C60^{\circ}C. Primarily found in Ascomycota (compost, volcanic ash).

      • Mesophiles: Grow between 15−40∘C15-40^{\circ}C (e.g., Aspergillus).

      • Psychrophiles: Optimum growth temperature is below 15∘C15^{\circ}C.

    • pH: Fungi tolerate a wide range but prefer acidic media (pH 3−53-5). Bacteria prefer alkaline media (pH 6−86-8).

    • Oxygen (O2O_2): Primarily aerobes. Some perform fermentation (alcohol production) in high glucose environments.

  • Life Cycles

    • Vegetative: Apical growth accompanied by branching.

    • Reproductive: Can be sexual, asexual, or parasexual.

Medical Mycology

  • Four Types of Mycotic Diseases

    • Hypersensitivity: Allergic reactions to molds and spores.

    • Mycotoxicosis: Poisoning resulting from food/feed contaminated by fungal toxins.

    • Mycetismus: Mushroom poisoning caused by the ingestion of pre-formed toxins.

    • Mycosis: Actual fungal infections of tissues.

  • Sources of Infection

    • Environment (Geophile): Soil, water, air. Aspergillus can cause nosocomial (hospital-acquired) infections via air conditioners. Cryptococcus neoformans is linked to pigeon feces; Histoplasma capsulatum to soil.

    • Animals (Zoophile): Keratinophilic dermatophytes from fur/feathers. Species-specific examples:

      • Trichophyton verrucosum: Cattle.

      • Trichophyton equinum: Horses.

      • Microsporum gallinae: Domestic fowl.

      • Microsporum canis: Primarily cats and dogs, but also horses, rabbits, and guinea pigs.

    • Human (Anthropophilic): Fungi associated with the human body spreading via direct contact or skin scales. Examples: Microsporum audouinii, Trichophyton schoenleinii, and Trichophyton rubrum (found in swimming pools and hotel mats).

  • Transmission Methods

    • Exogenous: Inhalation (Aspergillus, Cryptococcus), skin injury, or sexual contact (Candida).

    • Endogenous: Arises from the host's own reservoir (e.g., the gastrointestinal tract is a reservoir for yeasts). Occurs in opportunistic settings or in newborns via the birth canal (candidosis oris).

Classification and Pathogenesis of Mycoses

  • Localizations

    • Skin/Mucous Membrane: Caused by keratin-utilizing fungi.

    • Cutaneous and Subcutaneous: Often require traumatic inoculation; caused by yeasts/molds.

    • Systemic and Internal: Occur in immunologically impaired patients.

  • Dermatomycoses

    • Definition: Infections of the skin, hair, and nails.

    • Predisposing Factors: Tight clothes, synthetic materials, high sugar diet, antibiotic/immunosuppressive therapy, surgery, and immunodeficiency.

    • Pathogenesis: Damage is mechanical and enzymatic. Key enzyme is keratinase (specific, chymotrypsin-like), alongside neutral and acidic proteinases. These fungi generally cannot survive in serum and do not penetrate deep tissues.

    • Immune Response: Primarily cellular immunity (late-type hypersensitivity reaction). Previous infections do not provide protection.

  • Clinical Forms of Dermatomycosis

    • Mycosis capitis (Scalp): Forms plaques (0.5−5 cm0.5-5\,cm diameter). Caused by M. canis, T. mentagrophytes, T. rubrum, and M. gypseum.

    • Mycosis barbae (Beard): Ring-shaped rashes and kerions (inflamed areas) leading to scarring.

    • Mycosis corporis (Body): Itchy, rounded lesions with vesicular edges. Anthropophilic types may form map-like plaques.

    • Mycosis intertriginosa (Skin Folds): Common in axilla, sub-mammary, and abdominal folds. Seen in overweight/diabetic individuals. Caused by Trichophyton sp., Epidermophyton floccosum, or yeasts.

    • Mycosis manus (Hand): Peeling, cracking, and erosion between fingers. Usually affects only one palm.

    • Mycosis pedis (Athlete's Foot): Affects both feet. Symptoms include erosion, peeling, and itching. Associated with communal areas and rubber boots.

    • Onychomycosis (Nail Fungus): Endemic in 4−20%4-20\% of adults. T. rubrum is common for toenails; yeasts are more common for fingernails.

  • Specific Yeast and Other Infections

    • Candidosis oris (Oral Thrush): White, removable deposits on oral mucosa; common in infants and denture wearers.

    • Vaginal Yeast Infection: Caused by Candida species (especially albicans, glabrata, krusei, tropicalis). Triggered by pH imbalance, antibiotics, hormonal changes, or weak immunity.

    • Cryptococcosis: Skin nodules caused by Cryptococcus neoformans via contaminated pigeon feces.

    • Pityriasis versicolor: Caused by Malassezia furfur. Results in reddish/brown or depigmented scaly spots on the trunk.

Prevention and Treatment

  • Prevention Strategies

    • Low sugar diet; avoiding yeast and alcohol.

    • Drying body folds thoroughly; wearing slippers in showers.

    • Using well-ventilated, non-synthetic clothing.

    • Proper hygiene after animal contact.

  • Therapeutic Agents

    • Topical Disinfectants: Iodine, organic acids (salicylic, benzoic), sulfur, and dyes (fuchsin). Borax glycerin is used for infant thrush.

    • Special Agents:

      • Ketoconazole: Shampoo for dandruff/dermatitis (note: do not use with steroids).

      • Clotrimazole: Broad-spectrum; used for 1−21-2 weeks.

      • Econazole: Skin fungi.

      • Nystatin: Candidiasis of skin, mucous membranes, and GI tract.

    • Triazoles (Systemic):

      • Itraconazole: Broad-spectrum; effective against Aspergillus.

      • Fluconazole: Bioavailability of 90%90\%. Not affected by gastric pH. Penetrates the central nervous system and is excreted in active form in urine.