Eukaryotes: Fungi and Protozoa – Structure, Function, and Medical Importance

Eukaryotes: Fungi and Protozoa

Overview

This section explores the defining characteristics, anatomy, reproduction, and medical significance of fungi and protozoa, two major groups of eukaryotic microorganisms. Understanding their biology is essential for microbiology students, especially in the context of infectious diseases and environmental roles.

Comparison of Fungi, Bacteria, and Protozoa

Defining Characteristics

• Cell Type: Fungi and protozoa are eukaryotic, possessing membrane-bound organelles; bacteria are prokaryotic.

• Cell Wall: Fungi have cell walls containing chitin; bacteria have peptidoglycan; protozoa generally lack cell walls.

• Oxygen Requirements: Most molds are obligate aerobes; most yeasts are facultative anaerobes; bacteria vary widely.

• Environmental Tolerance: Fungi tolerate lower pH and higher osmotic pressures than bacteria, requiring less moisture and nitrogen for growth.

• Metabolism: Fungi can metabolize complex carbohydrates (e.g., lignin) that most bacteria cannot.

Feature	Fungi	Bacteria
Cell Type	Eukaryotic	Prokaryotic
Cell Wall	Chitin	Peptidoglycan
Metabolism	Chemoheterotroph	Varied
Oxygen Requirement	Obligate aerobes (molds), facultative anaerobes (yeasts)	Varied
Reproduction	Sexual and asexual spores	Binary fission, spores (endospores)

Mycology: The Study of Fungi

Harmful Effects

• Medical: Fungal infections (mycoses) have increased, especially in immunocompromised individuals. Only about 300 of >150,000 species are pathogenic to humans, but they cause significant disease burden.

• Economic: Plant pathogenic fungi cause $100–200 billion in crop losses annually. Over 10,000 species are plant pathogens.

Beneficial Effects

• Ecological: Fungi decompose dead plant material, replenishing soil nutrients.

• Symbiosis: Most plants rely on mycorrhizal fungi for water and nutrient absorption; fungi receive carbohydrates from plants.

• Human Use: Fungi are used as food (mushrooms), in food production (bread, citric acid), and in pharmaceuticals (alcohol, penicillin).

Characteristics of Fungi

Summary Table

Kingdom	Fungi
Nutritional Type	Chemoheterotroph
Multicellularity	All, except yeasts (unicellular)
Cellular Arrangement	Unicellular, filamentous, fleshy
Food Acquisition	Absorptive
Characteristic Features	Sexual and asexual spores

Identification of Fungi

Methods

• Yeasts: Identified by biochemical tests.

• Multicellular Fungi: Identified by physical appearance, colony characteristics, and reproductive spores.

• Molecular Methods: PCR and serological tests are increasingly used for both yeast and multicellular fungi.

Anatomy of Filamentous Fungi

Main Structures

• Thallus: Vegetative body composed of hyphae.

• Hyphae: Filamentous structures; a network forms the mycelium.

• Coenocytic Hyphae: Multinucleated, long, continuous cells.

• Septate Hyphae: Divided by septa with pores for cytoplasmic flow.

• Vegetative Hyphae: Anchor and absorb nutrients.

• Reproductive/Aerial Hyphae: Bear reproductive spores above the medium.

Yeasts

Characteristics and Reproduction

• Non-filamentous, unicellular fungi; typically spherical or oval.

• Asexual Division:

  • Fission Yeasts: Divide symmetrically (e.g., Schizosaccharomyces pombe).

  • Budding Yeasts: Divide asymmetrically (e.g., Saccharomyces cerevisiae).

  • Pseudohyphae: Buds that fail to detach form chains.

• Replicative Life-Span: One yeast cell can produce ~24 daughter cells before senescence.

• Chronological Life-Span: Time a mother cell survives in a non-dividing state.

Energy Production in Yeast

Metabolic Pathways

• Yeast can use O2 (aerobic respiration) or organic compounds (fermentation) as final electron acceptors.

• Most yeast prefer aerobic respiration, but can ferment carbohydrates to yield ethanol and CO2 in absence of oxygen.

• Crabtree Effect: Some yeast (e.g., S. cerevisiae) produce ethanol aerobically when exposed to high glucose concentrations.

Applications: Alcoholic beverage production, bread leavening.

Dimorphic Fungi

Definition and Significance

• Dimorphic fungi can grow as mold (hyphae) or yeast (budding), depending on environmental conditions.

• Pathogenic dimorphism is often temperature-dependent: yeast-like at 25°C, mold-like at 37°C.

• Candida albicans is sometimes called polymorphic due to its ability to form yeast, pseudohyphae, and hyphae.

Filamentous Fungi Reproduction

Asexual and Sexual Spores

• Fragmentation: Hyphae break apart and form new colonies.

• Spore Formation: True reproductive spores, not endospores.

• Asexual Spores: Genetically identical to parent; arise from mitosis.

• Sexual Spores: Fusion of nuclei from opposite mating strains.

• Telomorphs: Fungi that reproduce both sexually and asexually.

• Anamorphs: Fungi that reproduce only asexually.

Types of Asexual Spores

• Conidiospores (Conidia): Not enclosed in a sac; produced in chains at the end of a conidiophore.

• Sporangiospores: Formed within a sac (sporangium) at the end of an aerial hypha; one sporangium can contain hundreds of spores.

Types of Conidia

• Arthroconidia: Barrel-shaped spores formed by septate fragmentation.

• Chlamydoconidia: Thick-walled spores formed when hyphal cells enlarge.

• Blastoconidia: Formed by budding from parent cell (yeast-like forms).

Sexual Reproduction Stages

• Plasmogamy: Union of two haploid cells with cytoplasmic mixing.

• Karyogamy: Fusion of + and – nuclei.

• Meiosis: Diploid nucleus produces haploid nuclei (sexual spores), some of which are genetic recombinants.

Fungal Classification and Medically Important Phyla

Phyla and Sexual Spores

• Zygomycota: Conjugation fungi; coenocytic hyphae; asexual sporangiospores; sexual zygospores.

• Ascomycota: Sac fungi; septate hyphae; asexual conidiospores; sexual ascospores.

• Basidiomycota: Club fungi; septate hyphae; sexual basidiospores.

• Other phyla: Chytridiomycota, Blastocladiomycota, Entomophthoromycota, Glomeromycota, Neocallimastigomycota, Microsporidia (some do not produce sexual spores).

Sexual Spores

• Zygospore: Large diploid spore in thick wall, formed by fusion of haploid nuclei.

• Ascospore: Produced in saclike ascus, formed by fusion of nuclei and meiosis.

• Basidiospore: Haploid spores formed externally on a pedestal (basidium).

Fungal Diseases (Mycoses)

Definition and Classification

• Mycosis: Any fungal disease; generally chronic due to slow fungal growth.

• Treatment Difficulty: Fungi and animals are both eukaryotic, so antifungal drugs may affect animal cells.

• Classification by Site of Infection:

  • Superficial: Outermost layers of skin and hair.

  • Cutaneous (Dermatomycosis): Deeper skin, hair, nails; do not invade living tissue.

  • Subcutaneous: Second skin layer, subcutaneous tissues, muscle, fascia; often via puncture wounds.

  • Systemic: Affect several organs/tissues; usually spread from lungs after inhalation of spores.

  • Opportunistic: Normally harmless, but pathogenic in immunocompromised hosts.

Additional info:

• Further details on protozoa, lichens, and medically important protozoan groups are covered in subsequent slides and notes, including life cycles, nutrition, and disease mechanisms.