The Eukaryotes: Fungi, Algae, Protozoa, and Helminths – Study Notes

Fungi

Introduction to Fungi

Fungi are a diverse group of eukaryotic organisms that play essential roles in ecosystems as decomposers, pathogens, and symbionts. The study of fungi is known as mycology. Fungi are chemoheterotrophs, meaning they obtain energy by consuming organic molecules. They can be aerobic or facultative anaerobes and are found in a variety of environments.

• Decompose organic matter and recycle nutrients in ecosystems.

• Most are multicellular (molds), but yeasts are unicellular.

• Do not carry out photosynthesis; absorb nutrients from their environment.

• Include both pathogens and saprobes (decomposers).

Nutritional Adaptations

Fungi have unique adaptations that allow them to thrive in diverse and sometimes extreme environments.

• Grow best at a pH of 5 (more acidic than most bacteria prefer).

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

• Capable of growing in low moisture environments.

• Can metabolize complex carbohydrates that many other organisms cannot.

Vegetative Structures

The main body of a fungus is called the thallus, which consists of long filaments known as hyphae. A mass of hyphae is called a mycelium. Hyphae can be classified as:

• Vegetative hyphae: Obtain nutrients.

• Aerial hyphae: Involved in reproduction.

Yeasts are nonfilamentous, unicellular fungi. They reproduce by budding (uneven division) or fission (even division). Some fungi are dimorphic, meaning they can exist as both hyphae (mold-like) and yeast-like forms, often depending on environmental conditions.

Types of Hyphae

• Septate hyphae: Have cross-walls (septa) dividing the cells.

• Aseptate (coenocytic) hyphae: Lack septa, forming a continuous cytoplasmic mass with many nuclei.

Fungal Reproduction and Spores

Fungi reproduce by forming spores, which can be classified as asexual or sexual. Spores 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.

Life Cycle of Fungi

The fungal life cycle includes both asexual and sexual reproduction. Sexual reproduction involves three main phases:

• Plasmogamy: Fusion of the cytoplasm from two parent mycelia.

• Karyogamy: Fusion of nuclei to form a diploid zygote.

• Meiosis: Production of haploid spores.

Medically Important Fungal Phyla

• Mucoromycota (Zygomycota): Conjugation fungi with coenocytic hyphae. Example: Rhizopus stolonifer (black bread mold).

• Microsporidia: Obligate intracellular parasites, lack mitochondria, cause diseases like chronic diarrhea and keratoconjunctivitis.

• Ascomycota: Sac fungi with septate hyphae, produce ascospores in sacs (ascus). Example: Penicillium.

• Basidiomycota: Club fungi with septate hyphae, produce basidiospores on a basidium. Example: mushrooms.

Fungal Diseases (Mycoses)

Mycoses are fungal infections classified by the site of infection:

• Systemic mycoses: Deep within the body.

• Subcutaneous mycoses: Beneath the skin.

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

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

• Opportunistic mycoses: Pathogenic in immunocompromised hosts.

Some fungi are true pathogens (e.g., Histoplasma, Coccidioides), while others are opportunistic (e.g., Candida).

Economic Effects of Fungi

• Aspergillus niger: Production of citric acid.

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

• Trichoderma: Produces cellulase for biofuel production.

• Taxomyces: Source of taxol (anticancer drug).

• Tolypocladium inflatum: Produces cyclosporine (immunosuppressant).

Protists

Introduction to Protists

Protists are a diverse group of eukaryotic microorganisms that include algae, protozoa, and slime molds. They can be unicellular, multicellular, or multinucleated masses, and may be autotrophic or heterotrophic.

• Reproduce sexually and asexually.

• May have a cell wall or lack one.

Protozoa

Protozoa are unicellular eukaryotes that require a moist environment. They exhibit animal-like nutrition and are classified based on their means of locomotion.

• Some are normal microbiota; others are pathogenic (e.g., Plasmodium, Giardia).

• Complex life cycles, often involving multiple hosts.

• Feeding and growing stage is called a trophozoite.

• Can reproduce asexually (fission, budding, schizogony) or sexually (conjugation).

• Some form cysts to survive harsh conditions.

Classification of Protozoa by Motility

• Amoeboid: Move by pseudopods (e.g., Entamoeba histolytica).

• Flagellated: Move by flagella (e.g., Giardia lamblia, Trypanosoma).

• Ciliated: Move by cilia (e.g., Balantidium coli).

• Spore-forming (Apicomplexa): Non-motile, obligate intracellular parasites (e.g., Plasmodium).

Medically Important Protozoa

• Excavata: Flagellated protozoa, some lack mitochondria (e.g., Giardia intestinalis, Trichomonas vaginalis).

• Amoebozoa: Move by pseudopods (e.g., Entamoeba histolytica).

• Apicomplexa: Spore-forming, complex life cycles (e.g., Plasmodium, Toxoplasma gondii).

• Ciliates: Move by cilia (e.g., Balantidium coli).

Spore-Forming Protozoans (Apicomplexa)

Apicomplexans are obligate intracellular parasites with complex life cycles involving sexual and asexual stages:

• Merogony: Asexual reproduction, producing merozoites.

• Gamogony: Sexual reproduction, producing gametes.

• Sporogony: Formation of sporozoites, the infective stage.

Examples include Plasmodium (malaria), Toxoplasma gondii (toxoplasmosis), and Cryptosporidium (cryptosporidiosis).

Helminths

Introduction to Helminths

Helminths are multicellular parasitic worms that infect humans and animals. They are classified into two main phyla:

• Platyhelminthes: Flatworms (e.g., flukes and tapeworms).

• Nematoda: Roundworms.

Helminths have complex life cycles, often involving multiple hosts. They may be dioecious (separate sexes) or monoecious (hermaphroditic).

Platyhelminths (Flatworms)

• Trematodes (flukes): Leaf-shaped, have suckers for attachment, absorb food through their cuticle. Examples: Paragonimus (lung fluke), Schistosoma (blood fluke).

• Cestodes (tapeworms): Have a scolex (head) with suckers, absorb food through their cuticle, and have body segments called proglottids. Examples: Taenia solium (pork tapeworm), Echinococcus granulosus (hydatid disease).

Nematodes (Roundworms)

• Cylindrical, complete digestive system, usually dioecious.

• Some species' eggs are infective (e.g., Ascaris lumbricoides, Enterobius vermicularis).

• Others have larvae that are infective (e.g., hookworms, Dirofilaria immitis).

Arthropods as Vectors

Introduction to Arthropods

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

• Arachnida: Eight legs (e.g., ticks).

• Crustacea: Four antennae (e.g., crabs).

• Insecta: Six legs (e.g., mosquitoes, fleas).

Transmission can be mechanical (passive) or biological (pathogen multiplies in vector). The definitive host is where the microbe's sexual reproduction occurs.