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

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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 include both beneficial and pathogenic species.

Chemoheterotrophs: Obtain energy by decomposing organic matter.
Aerobic or facultative anaerobic: Most fungi require oxygen, but some can survive in low-oxygen environments.
Species diversity: Over 600,000 species; most are multicellular, but yeasts are unicellular.
No photosynthesis: Fungi absorb nutrients from their environment.
Pathogens and saprobes: Some cause disease, others decompose dead matter.

Comparison of Fungi and Bacteria

Fungi and bacteria differ in several key structural and metabolic 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

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

Optimal growth at pH 5.
Resistant to osmotic pressure; can grow in high sugar and salt concentrations.
Can grow in low moisture content.
Capable of metabolizing complex carbohydrates.

Vegetative Structures

Fungi exhibit a variety of vegetative structures that aid in nutrient acquisition and reproduction.

Molds and fleshy fungi: The thallus (body) consists of hyphae filaments; a mass of hyphae is called a mycelium.
Vegetative hyphae: Obtain nutrients.
Aerial hyphae: Involved in reproduction.

Yeasts and Dimorphic Fungi

Yeasts: Nonfilamentous, unicellular. Budding yeasts divide unevenly; fission yeasts divide evenly.
Dimorphic fungi: Exhibit yeast-like growth at 37°C and mold-like growth at 25°C. Many pathogenic fungi are dimorphic.

Types of Hyphae

Septate hyphae: Have cross-walls (septa) dividing cells; appear as strings of individual cells.
Coenocytic (Aseptate) hyphae: Lack septa; form long, continuous chains with many nuclei.

Fungal Spores and Reproduction

Fungi reproduce via spores, which are important for classification and dispersal.

Asexual spores: Produced by mitosis; no genetic variation. Types include conidiospores and sporangiospores.
Sexual spores: Produced by meiosis; result in genetic variation. Types include zygospores, ascospores, and basidiospores.

Asexual Spore Types

Conidiospore: Not enclosed in a sac.
Arthroconidia: Fragmentation of septate hyphae.
Blastoconidia: Buds of the parent cell.
Chlamydoconidium: Spore within a hyphal segment.
Sporangiospore: Enclosed in a sac.

Sexual Spore Formation

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 several phyla, some of which include important human pathogens.

Mucoromycota (Zygomycota): Conjugation fungi; coenocytic hyphae. Example: Rhizopus stolonifer (black bread mold).
Microsporidia: Obligate intracellular parasites; lack mitochondria. Cause diseases such as chronic diarrhea and keratoconjunctivitis.
Ascomycota: Sac fungi; septate hyphae. Produce ascospores in a saclike ascus. Example: Penicillium.
Basidiomycota: Club fungi; septate hyphae. Produce basidiospores externally on a basidium. 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 found at the tips of hyphae	Phylum Zygomycota; Rhizopus
Sexual Fungal Spores	Ascospores	Haploid gametes form within a sac called an ascus	Phylum Ascomycota; Penicillium, yeasts, cup fungi
Sexual Fungal Spores	Basidiospores	Buds off a pedestal structure called the basidium	Phylum Basidiomycota; mushrooms

Fungal Diseases (Mycoses)

Fungal infections are called mycoses and are classified by their location and severity.

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 in normal habitat but pathogenic in compromised hosts.

Most mycoses occur in immunocompromised individuals (e.g., Pneumocystis pneumonia in AIDS patients) or those with disrupted microbiota (e.g., Candida infections). Some fungi are true pathogens (e.g., histoplasmosis, coccidioidomycosis).

Dermatophytes

True pathogens infecting skin, hair, and nails.
Dermatophytic infections are called "tinea" (e.g., ringworm).

Mycotoxins

Some fungi produce toxins, such as ergot toxin from Claviceps purpurea, which can cause severe neurological symptoms.

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	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: Source of statins (cholesterol-lowering drugs).
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 against crop fungi.
Paecilomyces: Used to kill termites.

Protists

Introduction to Protists

Protists are a diverse group of eukaryotic organisms that may be unicellular, multicellular, or multinucleated. They can be autotrophic or heterotrophic and reproduce sexually or asexually.

May have a cell wall or lack one.
Include algae, slime molds, and protozoans.
Represent evolutionary links to plants, fungi, and animals.

Characteristics of Protozoa

Protozoa are unicellular eukaryotes that require a large supply of water and inhabit aquatic and soil environments.

Many have a protective pellicle and specialized feeding structures.
Ciliates: Use cilia to direct food into a mouth-like cytosome.
Amebae: Phagocytize food.
Food is digested in vacuoles; wastes eliminated through an anal pore.
Nutrition is animal-like.

Protozoan Life Cycle

Trophozoite: Feeding and growing stage.
Asexual reproduction: By fission, budding, or schizogony (multiple fission).
Sexual reproduction: By conjugation.
Some produce a cyst to survive adverse conditions.

Classification of Protozoa by Motility

Amoeboid: Move by pseudopodia.
Flagellated: Move by flagella.
Ciliated: Move by cilia.
Spore-forming (Apicomplexa): Non-motile; obligate intracellular parasites.

Medically Important Protozoa

Excavata: Includes Giardia intestinalis, Trichomonas vaginalis, Trypanosoma spp.
Amoebozoa: Includes Entamoeba histolytica (causes amebic dysentery), Acanthamoeba (causes blindness).
Ciliates: Includes Balantidium coli (causes dysentery).
Apicomplexa: Includes Plasmodium (malaria), Toxoplasma gondii (toxoplasmosis), Cryptosporidium (cryptosporidiosis).

Apicomplexa Life Cycle

Merogony: Asexual reproduction; produces merozoites.
Gamogony: Sexual reproduction; produces male and female gametes.
Sporogony: Zygote divides to produce sporozoites (infective stage).

Example: Plasmodium (malaria) undergoes sexual reproduction in mosquitoes and asexual reproduction in humans.

Helminths and Arthropods

Helminths (Parasitic Worms)

Helminths are multicellular parasitic animals, including flatworms and roundworms, with complex life cycles.

Platyhelminthes (flatworms): Includes trematodes (flukes) and cestodes (tapeworms).
Nematoda (roundworms): Cylindrical, complete digestive system; includes Ascaris lumbricoides, Enterobius vermicularis (pinworm), Necator americanus (hookworm).
Specialized for parasitism: reduced digestive and nervous systems, complex reproductive systems.
Life cycles may be dioecious (separate sexes) or monoecious (hermaphroditic).

Arthropods as Vectors

Arthropods are animals with segmented bodies and jointed legs. Some act as vectors, transmitting pathogens to humans.

Classes: Arachnida (eight legs), Crustacea (four antennae), Insecta (six legs).
Mechanical transmission: Pathogen is carried on the body surface.
Biological transmission: Pathogen multiplies within the vector.
Definitive host: Microbe's sexual reproduction occurs in the vector.

Table: Important Arthropod Vectors of Human Diseases

Vector	Pathogen Transmitted	Disease
Mosquito	Plasmodium	Malaria
Tick	Borrelia	Lyme disease
Flea	Yersinia pestis	Plague

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