Skip to ContentGo to accessibility pageKeyboard shortcuts menu
OpenStax Logo

5.1 Unicellular Eukaryotic Parasites

  • 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.
  • The protists include important pathogens and parasites.

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
  • Members of Zygomycota, Ascomycota, and Basidiomycota produce deadly toxins
  • Important differences in fungal cells, such as ergosterols in fungal membranes, can be targets for antifungal medications, but similarities between human and fungal cells make it difficult to find targets for medications and these medications often have toxic adverse effects

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 and aquatic wildlife and contaminate seafood with toxins that cause paralysis
  • Algae are important for producing agar, which is used as a solidifying agent in microbiological media, and carrageenan, which is used as a solidifying agent

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
Order a print copy

As an Amazon Associate we earn from qualifying purchases.


This book may not be used in the training of large language models or otherwise be ingested into large language models or generative AI offerings without OpenStax's permission.

Want to cite, share, or modify this book? This book uses the Creative Commons Attribution License and you must attribute OpenStax.

Attribution information
  • If you are redistributing all or part of this book in a print format, then you must include on every physical page the following attribution:
    Access for free at
  • If you are redistributing all or part of this book in a digital format, then you must include on every digital page view the following attribution:
    Access for free at
Citation information

© Jan 10, 2024 OpenStax. Textbook content produced by OpenStax is licensed under a Creative Commons Attribution License . The OpenStax name, OpenStax logo, OpenStax book covers, OpenStax CNX name, and OpenStax CNX logo are not subject to the Creative Commons license and may not be reproduced without the prior and express written consent of Rice University.