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Biology LibreTexts

10.1: Introduction

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  • Skills to Develop

    • Learn about the characteristics of fungi, protozoa and helminthes, including pathogenic species.
    • Observe prepared slides and agar plates with several types of fungi including yeasts and molds.
    • Observe prepared slides of several protozoan pathogens, and live protozoans in a lake water sample.
    • Observe prepared slides and preserved specimens of parasitic worms.
    • Prepare a collective forehead swab for microbial identification in Lab 12

    So far our laboratory exercises have been primarily focused on prokaryotic organisms—those that lack a nucleus and other membrane-bound organelles. However, there are other types of eukaryotic microorganisms that exist in nature, some of which cause human disease. In this exercise we will examine representative types of eukaryotic microorganisms: fungi, protozoa, and parasitic worms (helminthes), and learn about the diseases that they cause.

    The Fungi

    Fungi are heterotrophs (organisms that require organic carbon). In nature they are important saprotrophs--organisms that decompose dead organic matter. Many fungi produce enzymes that decompose woody plant material—thus making them of critical importance for nutrient recycling in forests. Some types of fungi live in a symbiotic relationship with a photosynthetic algae or bacterium (lichens)—others live in symbiosis with plant roots (mycorrhizae). Fungi are also an important food source for humans and other organisms, and are used in food production as well. Fungi have a cell wall composed primarily of chitin (a polymer of glucose). Members of the Kingdom Fungi exist as multinucleate filaments (molds) or unicellular yeasts. Molds have long branching cellular structures called hyphae that grow continuously without complete division of cytoplasm. Several hyphae may form a visible mat called mycelium. Most hyphae grow along the substrate (vegetative hyphae) but those that produce spores extend upwards to disperse them (aerial or reproductive hyphae). Hyphae may or may not have septa that partially separate the cytoplasm (Figure 10.1.1).


    Figure 10.1.1: Fungal Hyphae

    Yeasts are unicellular fungi with an oval or spherical shape that replicate either by uneven or even cell division (uneven cell division is called budding). Some fungi exhibit thermal dimorphism: they grow as filamentous molds at room temperature, but grow as yeasts at 37° C.

    Among the human pathogens, many can cause opportunistic infections by taking advantage of a weakened or immunocompromised host. Other species of fungi produce toxins that can affect humans when consumed: some affect humans indirectly causing disease in crop plants and animals that humans rely on for food.

    Fungi are often grouped based on the types of spores produced during sexual reproduction.

    • Zygomycetes form zygospores in sexual reproduction and sporangiospores (encased in a sac known as a sporangium) in asexual reproduction.
      • Rhizopus stolonifer (black bread mold)
    • Ascomycetes form ascospores (encased in a sac known as an ascus) in sexual reproduction and unprotected conidiospores in asexual reproduction.
      • Penicillium notatum: produces penicillin
      • Aspergillis—includes A. fumigatus (causes aspergillosis) and A. flavus (produces carcinogenic aflatoxins)
      • Saccharomyces (Baker’s yeast): used in baking and alcohol production
      • Candida albicans: causes yeast infections and thrush
      • Pneumocystis jiroveci: leading cause of pneumonia among AIDS patients
    • Basidiomycetes form basidiospores in sexual reproduction but do not have a well-defined asexual mode of reproduction. The mushroom is a macroscopic fruiting body of this type of fungus--basidiospores are formed on the underside of the mushroom cap (basidiocarp). Although mushrooms are clearly visible without a microscope, most of the living biomass of these fungi exists as microscopic hyphae.
      • Agaricus (edible)
      • Amanita (poisonous)
      • Cryptococcus neoformans: found in pigeon droppings; can cause severe infections in immunocompromised patients

    The Protozoa

    Protozoa are a very diverse group of unicellular, heterotrophic eukaryotes that are members of the kingdom Protista (photosynthetic algae are also in this kingdom). They are found in all types of habitats, including soil, freshwater and saltwater. Some live in symbiosis with other organisms, others are free-living consumers, and a few are parasitic to humans. Protozoans do not have cell walls, but many are surrounded by a proteinaceous outer covering called a pellicle. Both sexual and asexual modes of reproduction occur in this group.

    Protozoan pathogens vary in their mode of transmission, or method of gaining access to a new host. Some are transmitted by vector (an insect or arthropod that transmits a microbial pathogen), by ingestion of contaminated food or water, or even by sexual contact.

    Many species form a dormant stage called a cyst that is resistant to adverse environmental conditions. This allows them to exist outside of a host cell for some time and is often the stage that is transmitted to a new host. The feeding (metabolically active) form of these organisms is known as a trophozoite.

    Protozoa are often grouped based on the type of structures they use for locomotion (motility).

    • Amoebozoans use cytoplasmic projections called pseudopodia
      • ex: Entamoeba histolytica
    • Flagellates use flagella
      • Trypanosoma gambiense
      • Trypanosoma cruzi
      • Giardia lamblia (intestinalis)
      • Trichomonas vaginalis
    • Ciliates use cilia.
      • Balantidium coli
      • Paramecium spp (non-pathogenic)
    • Apicomplexans have no means of locomotion in their mature form
      • Plasmodium vivax
      • Toxoplasma gondii

    The Helminthes (parasitic worms)

    Helminthes are worms that live off of other living organisms. They are multicellular heterotrophs in the kingdom Animalia. There are two major phyla (a taxonomic grouping below the kingdom) that contain parasitic worms that are important to humans.

    1. Phylum Platyhelminthes: these worms are commonly called flatworms because of their flat body structure. They do not have respiratory or circulatory structures, or a digestive tract, and thus rely on diffusion of nutrients and other chemicals. Two distinct types of Platyhelminthes exist: (a) trematodes, or flukes, and (b) cestodes, or tapeworms.

    2. Phylum Nematoda (roundworms): these worms have more complex organ systems and are found in many types of habitats on Earth; a few species are parasitic to humans and other organisms.

    Helminthes vary in their mode of transmission and the part of the body infected. Some are transmitted by cysts, eggs, or larval stages of development. Many parasitic worms infect the digestive tract but may be disseminated to other body parts as well. The severity of a parasitic worm infection varies with type of worm, number of individual organisms present, and whether or not they spread to other organs.

    Key Terms

    Fungi, heterotroph, saprotroph, mold, hyphae, mycelium, aerial (reproductive) hyphae, vegetative hyphae, thermal dimorphism, yeast, budding, opportunistic infections, Zygomycete, Ascomycete, Basidiomycete, ascospore, conidiospore, sporangiospore, zygospore, basidiospore, protozoa, vector, Amoebozoans, ciliates, flagellates, apicomplexans, cyst, trophozoite, helminthes, Platyhelminthes, cestodes, trematodes, nematodes, mode of transmission.