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5: The Eukaryotes of Microbiology

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    Although bacteria and viruses account for a large number of the infectious diseases that afflict humans, many serious illnesses are caused by eukaryotic organisms. One example is malaria, which is caused by Plasmodium, a eukaryotic organism transmitted through mosquito bites. Malaria is a major cause of morbidity (illness) and mortality (death) that threatens 3.4 billion people worldwide.1 In severe cases, organ failure and blood or metabolic abnormalities contribute to medical emergencies and sometimes death. Even after initial recovery, relapses may occur years later. In countries where malaria is endemic, the disease represents a major public health challenge that can place a tremendous strain on developing economies.

    Worldwide, major efforts are underway to reduce malaria infections. Efforts include the distribution of insecticide-treated bed nets and the spraying of pesticides. Researchers are also making progress in their efforts to develop effective vaccines.2 The President’s Malaria Initiative, started in 2005, supports prevention and treatment. The Bill and Melinda Gates Foundation has a large initiative to eliminate malaria. Despite these efforts, malaria continues to cause long-term morbidity (such as intellectual disabilities in children) and mortality (especially in children younger than 5 years), so we still have far to go.

    Micrograph of a long cell. Micrograph of blood cells. Some have a red ring labeled “ring form”, others have dark dots and a larger ring labeled “young trophozoite”, and others have many dark dots and an amorphous structure labeled “schizont”. A photograph of mosquito netting over a bed.
    Figure \(\PageIndex{1}\): Malaria is a disease caused by a eukaryotic parasite transmitted to humans by mosquitos. Micrographs (left and center) show a sporozoite life stage, trophozoites, and a schizont in a blood smear. On the right is depicted a primary defense against mosquito-borne illnesses like malaria—mosquito netting. (credit left: modification of work by Ute Frevert; credit middle: modification of work by Centers for Disease Control and Prevention; credit right: modification of work by Tjeerd Wiersma)

    • 5.1: Unicellular Eukaryotic Microorganisms
      Protists are a diverse, polyphyletic group of eukaryotic organisms. Protists may be unicellular or multicellular. They vary in how they get their nutrition, morphology, method of locomotion, and mode of reproduction. Important structures of protists include contractile vacuoles, cilia, flagella, pellicles, and pseudopodia; some lack organelles such as mitochondria. Taxonomy of protists is changing rapidly as relationships are reassessed using newer techniques.
    • 5.2: Parasitic Helminths
      Helminth parasites are included within the study of microbiology because they are often identified by looking for microscopic eggs and larvae. The two major groups of helminth parasites are the roundworms (Nematoda) and the flatworms (Platyhelminthes). Nematodes are common intestinal parasites often transmitted through undercooked foods, although they are also found in other environments. Platyhelminths include tapeworms and flukes, which are often transmitted through undercooked meat.
    • 5.3: Fungi
      The fungi include diverse saprotrophic eukaryotic organisms with chitin cell walls. Fungi can be unicellular or multicellular; some (like yeast) and fungal spores are microscopic, whereas some are large and conspicuous. Reproductive types are important in distinguishing fungal groups. Medically important species exist in the four fungal groups Zygomycota, Ascomycota, Basidiomycota, and Microsporidia.
    • 5.4: Algae
      Algae are a diverse group of photosynthetic eukaryotic protists. Algae may be unicellular or multicellular. Large, multicellular algae are called seaweeds but are not plants and lack plant-like tissues and organs. Although algae have little pathogenicity, they may be associated with toxic algal blooms that can harm aquatic wildlife and contaminate seafood with toxins that cause paralysis.
    • 5.5: Lichens
      Lichens are a symbiotic association between a fungus and an algae or a cyanobacterium. The symbiotic association found in lichens is currently considered to be a controlled parasitism, in which the fungus benefits and the algae or cyanobacterium is harmed. Lichens are slow growing and can live for centuries in a variety of habitats. Lichens are environmentally important, helping to create soil, providing food, and acting as indicators of air pollution.
    • 5.E: The Eukaryotes of Microbiology (Exercises)


    1. 1 Centers for Disease Control and Prevention. “Impact of Malaria.” September 22, 2015. Accessed January 18, 2016.
    2. 2 RTS, S Clinical Trials Partnership. “Efficacy and safety of RTS,S/AS01 malaria vaccine with or without a booster dose in infants and children in Africa: final results of a phase 3, individually randomised, controlled trial.” The Lancet 23 April 2015. DOI:

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