Fungi, Protists, and Parasitic Animals: Structure, Classification, and Medical Importance

Fungi

Introduction to Fungi

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. Most fungi are aerobic or facultative anaerobic.

• Mycology: The scientific study of fungi.

• Chemoheterotrophs: Organisms that obtain both energy and carbon from organic compounds.

• Fungi decompose organic matter, playing a crucial role in nutrient cycling.

• Most are aerobic, but some can survive in anaerobic conditions.

Comparison of Fungi and Bacteria

Fungi and bacteria differ in several structural and functional aspects, as summarized below:

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 different species.

• Most are multicellular; yeasts are unicellular.

• Do not carry out photosynthesis.

• Absorb nutrients from their environment.

• Include both pathogens and saprobes (decomposers).

Nutritional Adaptations

Fungi have unique nutritional adaptations that allow them to thrive in diverse environments.

• Grow better at pH of 5.

• Resistant to osmotic pressure; can grow in high sugar and salt concentrations.

• Can grow in low moisture content.

• Can metabolize complex carbohydrates.

Vegetative Structures

Molds and Fleshy Fungi

• The fungal thallus (body) consists of hyphae filaments; a mass of hyphae is called a mycelium.

• Vegetative hyphae obtain nutrients; aerial hyphae are involved in reproduction.

Yeasts

• Nonfilamentous and unicellular.

• Budding yeasts divide unevenly; fission yeasts divide evenly.

Dimorphic Fungi

• Yeastlike at 37°C and moldlike at 25°C.

• Cycle between hyphal and yeastlike forms.

• Many pathogenic fungi are dimorphic.

Hyphal Structure

• Most fungi grow as tubular structures called hyphae.

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

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

Fungal Spores and Reproduction

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

• Spores may be asexual (from mitosis, no genetic variation) or sexual (from meiosis, genetic variation).

• Reproduction can be sexual or asexual, with spores detaching and germinating into new molds.

Asexual Spores

• Produced via mitosis and cell division by hyphae of one organism.

• 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 Spores

• Arise from meiosis, resulting in genetic variation.

• Include zygospores, ascospores, and basidiospores.

• Sexual reproduction involves three phases:

  1. Plasmogamy: Haploid donor cell nucleus (+) penetrates cytoplasm of recipient cell (−).

  2. Karyogamy: + and − nuclei fuse to form diploid zygote.

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

Medically Important Fungi

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

• Microsporidia: No mitochondria, obligate intracellular parasites, cause diseases like chronic diarrhea and keratoconjunctivitis.

• Ascomycota: Sac fungi, septate hyphae, produce conidiospores (asexual) and ascospores (sexual). Example: Claviceps purpurea.

• Basidiomycota: Club fungi, septate hyphae, produce conidiospores (asexual) and basidiospores (sexual). Example: mushrooms.

Fungal Spores Table

Type	Name	Form	Examples
Asexual Fungal Spores	Conidiospores	Chains of spores; not enclosed in a sac	Penicillium (source of penicillin), Aspergillus species
Asexual Fungal Spores	Sporangiospores	Spores formed within a sac called a sporangium	Absidia species (cause mucormycosis in humans)
Sexual Fungal Spores	Zygospores	Haploid gametes found at the tips of hyphae	Phylum Zygomycota; includes black bread molds (Rhizopus species)
Sexual Fungal Spores	Ascospores	Haploid gametes found within a sac called an ascus	Phylum Ascomycota; includes truffles, morels, many yeasts, and cup fungi
Sexual Fungal Spores	Basidiospores	Buds off of a pedestal structure called the basidium	Phylum Basidiomycota; includes mushrooms

Fungal Diseases (Mycoses)

Fungal infections are called mycoses. They are classified based on the site and severity of infection.

• Systemic mycoses: Deep within the body, affecting multiple tissues and organs.

• Subcutaneous mycoses: Beneath the skin.

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

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

• Opportunistic mycoses: Harmless fungi become 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 yeast infections). Some fungi are true pathogens and can infect healthy hosts (e.g., histoplasmosis).

• Dermatophytes: True pathogens causing skin, hair, and nail infections ("tinea" or ringworm).

• Some fungi produce mycotoxins or stimulate allergies. Example: Claviceps purpurea produces ergot toxin, a potent neurotoxin (LSD) causing seizures, psychosis, nausea, vomiting, and death.

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	None	Encephalitozoon, Nosema	Humans, other animals	Diarrhea, keratoconjunctivitis
Ascomycota	Dimorphic or septate hyphae	Conidia	Aspergillus, Claviceps purpurea	Ubiquitous, grasses	Systemic, toxin ingestion
Basidiomycota	Septate hyphae; includes rusts and smuts, plant pathogens, and mushroom-producing cells	Conidia	Cryptococcus, Malassezia	Soil, human skin	Systemic, cutaneous, toxin ingestion

Economic Effects of Fungi

• Aspergillus niger: Production of citric acid.

• Aspergillus terreus: Statins that inhibit cholesterol synthesis.

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

• Trichoderma: Produces cellulase.

• Taxomyces: Produces taxol (anticancer drug).

• Tolypocladium inflatum: Produces cyclosporine (immunosuppressant).

• Coniothyrium minitans: Kills fungi on crops.

• Paecilomyces: Kills termites.

Protists

Introduction to Protists

Protists are a diverse group of eukaryotes that may be unicellular, multicellular, or multinucleated. They can be autotrophs or heterotrophs and reproduce both sexually and asexually. Some have cell walls, while others do not.

• Examples include algae, slime molds, and protozoans.

• Protists are thought to represent an evolutionary link to plants, fungi, and animals.

Characteristics of Protozoa

Protozoa are unicellular eukaryotes that require a large supply of water and inhabit water and soil. They have specialized structures for feeding and movement.

• Many have a protective pellicle; ciliates use cilia to direct food into a mouthlike cytosome.

• Amebae phagocytize food; digestion occurs in vacuoles, and wastes are eliminated through an anal pore.

• Animal-like nutrition.

Protozoan Diversity and Pathogenicity

• Over 50,000 species; some are normal microbiota, others cause disease.

• Plasmodium species cause malaria.

• Other pathogens: Toxoplasma gondii, Trichomonas vaginalis, Giardia spp.

• Complex life cycles.

Protozoan Life Cycle

• Feeding and growing form: trophozoite.

• Asexual reproduction: fission, budding, or schizogony (multiple fission).

• Sexual reproduction: conjugation.

• Some produce a cyst to survive adverse conditions.

Classification by Motility

• Amoeboid (pseudopods)

• Flagellated

• Ciliated

• Spore-forming

Medically Important Protozoa

• Feeding grooves (Excavata): Move by flagella and undulating membrane.

  • Diplomonads (Giardia intestinalis): No mitochondria, multiple flagella.

  • Parabasalids (Trichomonas vaginalis): Undulating membrane, no cyst stage.

  • Euglenozoa (Trypanosoma spp.): Photoautotrophs or facultative chemotrophs; hemoflagellates transmitted by blood-feeding insects.

• Amoebozoa (Ameba): Move by pseudopods. Pathogens include Entamoeba histolytica (amebic dysentery), Acanthamoeba (corneal infections), Balamuthia (encephalitis).

• Apicomplexa: Nonmotile, obligate intracellular parasites. Includes Plasmodium (malaria), Toxoplasma gondii, Cryptosporidium, Microsporidia.

• Ciliates: Move by cilia. Balantidium coli causes dysentery.

Spore-Forming Protozoans (Apicomplexa)

• Largest phyla of protozoans; move by gliding.

• Complex life cycles with sexual and asexual stages:

  1. Merogony: Asexual stage; merozoites produced by repeated cell division.

  2. Gamogony: Sexual stage; merozoites produce male and female gametes by meiosis.

  3. Sporogony: Zygote divides to make sporozoites (infective stage).

• Notorious human pathogens: Toxoplasma gondii, Plasmodium vivax (malaria), Cryptosporidium, Microsporidia.

• Some require arthropod vectors (e.g., malaria via mosquitoes).

• Toxoplasma gondii: Transmitted by cats; causes fetal infections.

• Cryptosporidium: Transmitted via feces; causes waterborne illness.

Life Cycle of Plasmodium (Malaria)

• Sexual reproduction occurs in the Anopheles mosquito.

• Mosquito injects a sporozoite into its bite; sporozoite undergoes schizogony in the liver, producing merozoites.

• Merozoites infect red blood cells, forming a ring stage inside the cell.

• Red blood cells rupture, releasing merozoites to infect new cells.

Animals

Introduction to Animals

Animals are multicellular organisms that do not carry out photosynthesis and obtain organic carbon from nutrients. The animal kingdom includes parasitic worms (helminths) and arthropods.

• Estimated ~7.5 million animal species.

• Helminths and arthropods are of medical importance.

Helminths (Parasitic Worms)

• Include roundworms (Nematoda) and flatworms (Platyhelminthes).

• Complex life cycles; usually spread in microscopic form.

• WHO estimates half the world's population is infected with some type of helminth.

*Additional info: The notes cover topics relevant to Ch. 12 (Fungi, Protozoa, and Helminths) in a Microbiology course, including structure, classification, reproduction, diseases, and economic impact. The tables have been reconstructed for clarity and completeness.*