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.

• Mycology: The scientific study of fungi.

• Chemoheterotrophs: Obtain energy and carbon by decomposing organic matter.

• Most fungi are aerobic or facultative anaerobic.

Comparison of Fungi and Bacteria

Fungi and bacteria differ in cell structure, metabolism, and reproductive strategies.

General Characteristics of Fungi

Fungi exhibit a wide range of forms and life strategies.

• Over 600,000 different species.

• Most are multicellular; yeasts are unicellular.

• Do not carry out photosynthesis.

• Absorb nutrients from their environment.

• Include pathogens and saprobes (decomposers).

Nutritional Adaptations

Fungi are adapted to survive in diverse and often extreme 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

Fungi have specialized structures for growth and reproduction.

• Molds and fleshy fungi: The fungal thallus (body) consists of hyphae filaments; a mass of hyphae is 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; many pathogenic fungi are dimorphic.

Hyphal Types

Hyphae are the main structural units of fungi.

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

• Coenocytic (Aseptate) hyphae: Lack septa; appear as long continuous chains with many nuclei.

Fungal Spores and Reproduction

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

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

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

• Fungi reproduce sexually and asexually via spores that detach from the parent and germinate into new molds.

Asexual Spores

• 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

• Zygospores, ascospores, and basidiospores.

• Formed by fusion of nuclei from two opposite mating strains.

• Three 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

Several fungal phyla contain species of medical significance.

• 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 chronic diarrhea and keratoconjunctivitis.

• Ascomycota: Sac fungi; septate hyphae; produce conidiospores (asexual) and ascospores (sexual).

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

Table: Fungal Spores

Fungal Diseases (Mycoses)

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

• Systemic mycoses: Deep within the body, affecting multiple tissues/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 (e.g., histoplasmosis).

• Dermatophytes: True pathogens infecting skin, hair, nails; cause "tinea" (ringworm).

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

Table: Characteristics of Some Pathogenic Fungi

Economic Effects of Fungi

Fungi have significant economic impacts, both beneficial and harmful.

• Aspergillus niger: Production of citric acid.

• Aspergillus terreus: Statins for cholesterol synthesis.

• Saccharomyces cerevisiae: Bread, wine, hepatitis B vaccine.

• Trichoderma: Cellulase production.

• Taxomyces: Taxol (anticancer drug).

• Tolypocladium inflatum: Cyclosporine (immunosuppressant).

• Coniothyrium minitans: Kills crop fungi.

• Paecilomyces: Kills termites.

Protists

Introduction to Protists

Protists are a diverse group of eukaryotes, including unicellular, multicellular, and multinucleated forms. They can be autotrophic or heterotrophic and reproduce sexually or asexually.

• May have a cell wall or lack one.

• Examples: Algae, Slime molds, Protozoans.

Characteristics of Protozoa

Protozoa are unicellular eukaryotes that require water and often inhabit soil or aquatic environments.

• Many have a protective pellicle and specialized feeding structures.

• Ciliates use cilia to direct food to a mouthlike cytosome.

• Amebae phagocytize food.

• Food is digested in vacuoles; wastes eliminated via an anal pore.

Protozoan Diversity and Pathogenicity

There are over 50,000 species of protozoa, some of which are normal microbiota, while others cause disease (e.g., Plasmodium species cause malaria).

• Pathogens include Toxoplasma gondii, Trichomonas vaginalis, Giardia spp.

• Complex life cycles.

Protozoan Life Cycle

• Feeding/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 (flagella)

• Ciliated (cilia)

• Spore-forming (nonmotile)

Medically Important Protozoa

• Feeding grooves (Excavata): Move by flagella/undulating membrane. Includes Giardia intestinalis, Trichomonas vaginalis, Trypanosoma spp.

• Amoebozoa: Move by pseudopods. Includes Amoeba proteus, Naegleria fowleri, Acanthamoeba, Entamoeba histolytica (causes amebic dysentery).

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

• Ciliates: Move by cilia. Includes Paramecium (free-living), Balantidium coli (causes dysentery).

Spore-Forming Protozoans (Apicomplexa)

Apicomplexa are the largest phyla of protozoans, characterized by complex life cycles and obligate intracellular parasitism.

• Move by gliding; lack flagella, cilia, or pseudopodia.

• Life cycle includes sexual and asexual stages: merogony (asexual), gamogony (sexual), sporogony (spore formation).

• Merozoites produced by repeated asexual cell division.

• Gamogony: Merozoites produce male/female gametes by meiosis.

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

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

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

• Cryptosporidium: Transmitted via feces; causes waterborne illness.

• Plasmodium: Causes malaria; sexually reproduces in Anopheles mosquito. Sporozoite injected into host, undergoes schizogony in liver, merozoites infect red blood cells, forming ring stage; RBCs rupture, releasing more merozoites.

Animals

Introduction to Animals

Animals are multicellular organisms that do not carry out photosynthesis and obtain organic carbon from nutrients. They include parasitic worms (helminths) and arthropods.

• Estimated 7.5 million animal species.

Helminths (Parasitic Worms)

Helminths are parasitic worms with complex life cycles, often spread in microscopic forms. WHO estimates half the world's population is infected with some type of helminth.

• Two phyla: Platyhelminthes (flatworms) and Nematoda (roundworms).

• Specialized for host life: may lack digestive system, reduced nervous system, limited locomotion, complex reproductive system.

• Life cycle: Dioecious (separate sexes) or Monoecious (hermaphroditic); egg → larvae → adult.

Platyhelminths

• Trematodes (flukes): Flat, leaf-shaped; ventral/oral suckers; absorb food through cuticle. Examples: Paragonimus (lung fluke), Schistosoma (blood fluke).

• Cestodes (tapeworms): Scolex (head with suckers), absorb food through cuticle, proglottids (body segments with reproductive organs). Examples: Taenia solium (pork tapeworm), Echinococcus granulosus (hydatid cysts).

Nematodes

• Roundworms; cylindrical; complete digestive system; dioecious (males have spicules).

• Eggs infective: Ascaris lumbricoides, Baylisascaris procyonis, Trichuris trichiura, Enterobius vermicularis.

• Larvae infective: Strongyloides, Necator americanus, Ancylostoma duodenale (hookworms), Dirofilaria immitis (heartworm).

Arthropods as Vectors

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

• Classes: Arachnida (eight legs), Crustacea (four antennae), Insecta (six legs).

• Mechanical transmission: Pathogen carried on body parts.

• Biological transmission: Pathogen multiplies in vector.

• Definitive host: Microbe's sexual reproduction occurs in vector.