Chapter 12: The Eukaryotes—Fungi, Algae, Protozoa, and Helminths (Microbiology Study Notes)

Fungi

Introduction to Fungi

Fungi are a diverse group of eukaryotic organisms studied in the field of mycology. They play essential roles in ecosystems as decomposers and can be pathogenic to plants, animals, and humans.

• Mycology: The study of fungi.

• Chemoheterotrophs: Obtain energy and carbon from organic compounds; decompose organic matter.

• Most fungi are aerobic or facultative anaerobic.

Comparison: Fungi vs. Bacteria

Fungi and bacteria differ in cell structure, metabolism, and reproduction.

Feature	Fungi	Bacteria
Cell Type	Eukaryotic	Prokaryotic
Cell Membrane	Sterols present	Sterols absent (except in Mycoplasma)
Cell Wall	Glucans, mannans, chitin (no peptidoglycan)	Peptidoglycan
Spores	Sexual and asexual reproductive spores	Endospores (not for reproduction); some asexual reproductive spores
Metabolism	Limited to heterotrophic; aerobic, facultatively anaerobic	Heterotrophic, autotrophic; aerobic, facultatively anaerobic, anaerobic

General Characteristics of Fungi

• Over 600,000 species identified.

• Most are multicellular; yeasts are unicellular.

• Do not perform photosynthesis.

• Absorb nutrients from their environment.

• Include pathogens and saprobes (decomposers).

Nutritional Adaptations

Fungi are adapted to survive in diverse and often extreme environments.

• Optimal growth at pH 5.

• Can grow in high sugar and salt concentrations; resistant to osmotic pressure.

• Can tolerate low moisture content.

• Capable of metabolizing complex carbohydrates.

Vegetative Structures

The body of a fungus, called the thallus, consists of long filaments known as hyphae. A mass of hyphae forms a mycelium.

• Vegetative hyphae: Obtain nutrients.

• Aerial hyphae: Involved in reproduction.

Yeasts and Dimorphic Fungi

• Yeasts: Nonfilamentous, unicellular, facultative anaerobes.

• Budding yeasts: Divide unevenly.

• Fission yeasts: Divide evenly.

• Dimorphic fungi: Exhibit both yeastlike (at 37°C) and moldlike (at 25°C) forms; many pathogenic fungi are dimorphic.

Types of Hyphae

• Septate hyphae: Have cross-walls (septa) dividing cells; appear as a string of individual cells.

• Coenocytic (Aseptate) hyphae: Lack septa; form a long continuous chain with many nuclei.

Fungal Spores and Reproduction

Fungi reproduce by forming spores, which can be asexual or sexual. Spores help classify fungi and allow them to disperse and survive adverse conditions.

• Asexual spores: Produced by mitosis (no genetic variation).

• Sexual spores: Produced by meiosis (genetic variation).

Asexual Spores

• Conidiospores: Not enclosed in a sac.

• Sporangiospores: Enclosed in a sac called a sporangium.

• Arthroconidia: Fragmentation of septate hyphae.

• Blastoconidia: Buds of the parent cell.

• Chlamydoconidium: Spore within a hyphal segment.

Sexual Spores

• Zygospores: Fusion of nuclei from two similar cells.

• Ascospores: Formed in a saclike ascus after fusion of nuclei.

• Basidiospores: Formed externally on a pedestal called a basidium.

Phases of Sexual Reproduction

• Plasmogamy: Haploid donor cell nucleus (+) penetrates cytoplasm of recipient cell (-).

• Karyogamy: + and - nuclei fuse to form diploid zygote.

• Meiosis: Diploid nucleus produces haploid nuclei (sexual spores).

Medically Important Fungi

Fungi are classified into four major phyla, some of which include important human pathogens.

• Mucoromycota (Zygomycota)

• Microsporidia

• Ascomycota

• Basidiomycota

Mucoromycota (Zygomycota)

• Conjugation fungi; coenocytic hyphae.

• Asexual reproduction: sporangiospore.

• Sexual reproduction: zygospore (fusion of nuclei).

• Example: Rhizopus stolonifer (common black bread mold).

Microsporidia

• No mitochondria; obligate intracellular parasites.

• No sexual reproduction observed (may occur in host).

• Diseases: chronic diarrhea, keratoconjunctivitis.

Ascomycota

• Sac fungi; septate hyphae.

• Teleomorphic fungi: Produce sexual and asexual spores.

• Anamorphic fungi: Lost ability to sexually reproduce (asexual only).

• Asexual: conidiospore; Sexual: ascospore (in ascus).

Basidiomycota

• Club fungi; septate hyphae (includes mushrooms).

• Asexual: conidiospores; Sexual: basidiospores (on basidium).

Table: Fungal Spores

Type	Name	Form	Examples
Asexual Fungal Spores	Conidiospores	Chains of spores; not enclosed in a sac	Penicillium, Aspergillus
Asexual Fungal Spores	Sporangiospores	Spores formed within a sac called a sporangium	Absidia species
Sexual Fungal Spores	Zygospores	Haploid gametes fused at the tips of hyphae	Rhizopus (black bread mold)
Sexual Fungal Spores	Ascospores	Haploid gametes form within a sac (ascus)	Truffles, morels, cup fungi
Sexual Fungal Spores	Basidiospores	Buds off a pedestal structure called the basidium	Mushrooms

Fungal Diseases (Mycoses)

Fungal infections are called mycoses and are classified by the site of infection.

• Systemic mycoses: Deep within the body; affect multiple tissues/organs.

• Subcutaneous mycoses: Beneath the skin.

• Cutaneous mycoses: Affect hair, skin, nails (dermatomycoses).

• Superficial mycoses: Localized, e.g., hair shafts.

• Opportunistic mycoses: Harmless fungi become pathogenic in compromised hosts.

Examples of Fungal Diseases

• Immunocompromised individuals (e.g., Pneumocystis pneumonia in AIDS patients).

• Disruption of normal microbiota (e.g., yeast infections by Candida).

• True pathogens infect healthy hosts (e.g., Histoplasmosis, Coccidioidomycosis).

Dermatophytes

• True pathogens infecting skin, hair, nails.

• Dermatophytic infections are called "tinea" (e.g., athlete's foot, ringworm).

Mycotoxins

• Some fungi produce toxins (mycotoxins) or cause allergies.

• Claviceps purpurea produces ergot toxin (potent neurotoxin, precursor to LSD).

• Exposure can cause seizures, psychosis, nausea, vomiting, and death.

Table: Characteristics of Some Pathogenic Fungi

Phylum	Growth Characteristics	Asexual Spore Types	Human Pathogens	Habitat	Type of Mycosis
Zygomycota	Nonseptate hyphae	Sporangiospores	Rhizopus, Mucor	Ubiquitous	Systemic
Microsporidia	No hyphae	Nonmotile spores	Encephalitozoon, Nosema	Humans, animals	Diarrhea, keratoconjunctivitis
Ascomycota	Conidia, dimorphic	Conidia	Aspergillus, Claviceps, Blastomyces, Histoplasma	Ubiquitous, grasses, soil	Systemic, toxin ingestion
Basidiomycota	Septate hyphae	Conidia	Cryptococcus, Malassezia, Amanita	Soil, bird feces, human skin	Systemic, cutaneous, toxin ingestion

Economic Effects of Fungi

Fungi have significant economic impacts, both beneficial and harmful.

• Aspergillus niger: Production of citric acid.

• Aspergillus terreus: Produces statins (inhibit cholesterol synthesis).

• Saccharomyces cerevisiae: Used in bread, wine, and hepatitis B vaccine production.

• Trichoderma: Produces cellulase (enzyme for breaking down cellulose).

• Taxomyces: Source of taxol (anticancer drug).

• Tolypocladium inflatum: Produces cyclosporine (immunosuppressant).

• Coniothyrium minitans: Kills fungi on crops.

• Paecilomyces: Kills termites.

Additional info: These notes cover the major features, classification, reproduction, diseases, and economic importance of fungi, as outlined in a college-level microbiology course. For more details on life cycles and pathogenic mechanisms, refer to textbook figures and appendices.