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

Fungi: General Characteristics

Definition and Overview

Fungi are a diverse group of eukaryotic organisms studied in the field of mycology. They are primarily chemoheterotrophs, meaning they obtain energy by decomposing organic matter. Fungi can be aerobic or facultative anaerobic and play essential roles as decomposers and pathogens.

• Mycology: The study of fungi.

• Chemoheterotrophs: Require organic compounds for energy and carbon.

• Most fungi are multicellular; yeasts are unicellular.

• Fungi do not carry out photosynthesis.

• They absorb nutrients from their environment.

• Include pathogens and saprobes (decomposers).

Comparison: Fungi vs. Bacteria

Key Features

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

Nutritional Adaptations of Fungi

Environmental Tolerance

Fungi are adapted to grow in environments that may be challenging for other organisms.

• Optimal growth at pH 5.

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

• Capable of growth in low moisture content.

• Can metabolize complex carbohydrates.

Vegetative Structures of Fungi

Molds and Fleshy Fungi

The main body of a mold or fleshy fungus is called the thallus, which 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 two forms—yeastlike at 37°C and moldlike at 25°C. Many pathogenic fungi are dimorphic.

Hyphal Structure

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

Classification and Types

Fungi reproduce by forming spores, which are important for classification. Spores may be asexual or sexual.

• Asexual spores: Arise from mitosis (no genetic variation).

• Sexual spores: Arise from meiosis (genetic variation).

Asexual Spores

• Conidiospore: Not enclosed in a sac.

• Sporangiospore: Enclosed in a sac.

• Arthroconidia: Fragmentation of septate hyphae.

• Blastoconidia: Buds of the parent cell.

• Chlamydoconidium: Spore within a hyphal segment.

Sexual Spores

• Zygospore: Fusion of nuclei from two similar cells.

• Ascospore: Nuclei fuse in a saclike ascus.

• Basidiospore: 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

Major Phyla

• Mucoromycota (Zygomycota): Conjugation fungi; coenocytic hyphae; asexual sporangiospores; sexual zygospores. Example: Rhizopus stolonifer (black bread mold).

• Microsporidia: No mitochondria; obligate intracellular parasites; cause chronic diarrhea and keratoconjunctivitis.

• Ascomycota: Sac fungi; septate hyphae; produce sexual (ascospores) and asexual (conidiospores) spores. Some are anamorphic (asexual only).

• Basidiomycota: Club fungi; septate hyphae; produce sexual (basidiospores) and asexual (conidiospores) spores. Includes mushrooms.

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 fuse at the tips of hyphae	Phylum Zygomycota; Rhizopus
Sexual Fungal Spores	Ascospores	Haploid gametes form within a sac (ascus)	Phylum Ascomycota; truffles, morels, cup fungi
Sexual Fungal Spores	Basidiospores	Buds off a pedestal structure called the basidium	Phylum Basidiomycota; mushrooms

Fungal Diseases (Mycoses)

Types of Mycoses

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

• Subcutaneous mycoses: Beneath the skin.

• Cutaneous mycoses: Affect hair, skin, nails; also called dermatomycoses.

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

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

Examples and Pathogens

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

• Disruption of normal microbiota can lead to yeast infections (e.g., Candida).

• Some fungi are true pathogens infecting healthy hosts (e.g., Histoplasmosis, Coccidioidomycosis).

• Dermatophytes: Infect skin, hair, nails; cause "tinea" (e.g., athlete's foot, ringworm).

• Some fungi produce mycotoxins (e.g., Claviceps purpurea produces ergot toxin, a potent neurotoxin).

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, other animals	Diarrhea, keratoconjunctivitis
Ascomycota	Conidia, dimorphic, septate hyphae	Conidia, arthroconidia	Aspergillus, Claviceps, Blastomyces, Histoplasma, Microsporum	Ubiquitous, grasses, soil, animals	Systemic, cutaneous, toxin ingestion
Anamorphs	Yeastlike, pseudohyphae, unicellular	Chlamydoconidia, none	Candida, Pneumocystis	Human lungs, mucous membranes	Systemic, cutaneous, mucocutaneous
Basidiomycota	Septate hyphae	Conidia	Cryptococcus, Malassezia, Amanita	Soil, bird feces, human skin	Systemic, cutaneous, toxin ingestion

Economic Effects of Fungi

Applications and Impacts

• Aspergillus niger: Production of citric acid.

• Aspergillus terreus: Source of statins (cholesterol inhibitors).

• 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: Biocontrol agent for crop fungi.

• Paecilomyces: Used to control termites.

Protozoa and Helminths (Overview)

Protozoa

• Unicellular eukaryotes; require water for survival.

• Motility by cilia, flagella, or pseudopodia.

• Complex life cycles; some cause human diseases (e.g., malaria by Plasmodium).

Helminths

• Parasitic worms: Platyhelminthes (flatworms) and Nematoda (roundworms).

• Complex life cycles; may lack digestive system and have reduced nervous system.

• Spread via microscopic forms (eggs, larvae).

Additional info: For more details on protozoa and helminths, refer to the full chapter or lecture notes.