14: Pathogenic Eukaryotes

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.

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, however, for the purpose of this lab, we will focus on the five groups of clinically relevant pathogenic fungi.

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.

Procedures

A. The Fungi

Recall that most fungi grow slowly and are too dangerous to work with in the lab because of the possibility of inhalation of spores. Perform a simple stain using methylene blue and observe the non-pathogenic fungi, Saccharomyces cervisiae, also known as brewers yeast. Working with S. cervisiae is a safe alternative to view budding yeast! Describe your observations. Additionally, view the Candida albicans demonstration slide and describe your observations. 

S. cervisiae Simple Stain

• Name of slide:
• Magnification used:
• Observations:

C. albicans Demonstration Slide

• Name of slide: 
• Magnification used:
• Observations:

B. The Protozoa

Observe the slides listed below. Describe your observations. Then use reference material from lecture, lab, or the internet to fill in the information below for each organism in Table 14.2.1.

Slides to be observed:

• Plasmodium falciparum
  • Name of slide:
  • Magnification used:
  • Observations:
• Plasmodium vivax
  • Name of slide:
  • Magnification used:
  • Observations:
• Trypanosoma cruzi
  • Name of slide:
  • Magnification used:
  • Observations:
• Trichomonas vaginalis
  • Name of slide:
  • Magnification used:
  • Observations:

Table 14.2.1: Protozoa

C. The Helminthes

Observe the following slides and preserved specimens. Use the spaces provided to record your observations. Then use the sources of information provided to fill in Table 14.2.2 (your instructor may assign you to research a particular species or group of species)

Slides to be observed:

• Schistosoma mansoni 
  • Name of slide:
  • Magnification used:
  • Observations:
• Dipylidium caninum
  • Name of slide:
  • Magnification used:
  • Observations:
• Enterobius vermicularis
  • Name of slide:
  • Magnification used:
  • Observations:

Table 14.2.2: Helminthes

LAB ASSIGNMENT

1. Summarize the major differences between prokaryotes and eukaryotes.

2. What type of organism alternates between a trophozoite and a cyst form? What is the difference between a trophozoite and a cyst?

3. List some differences between fungi, protists and helminthes.

4. Why is it important to learn about parasitic infections that don’t normally occur in the United States?

5. Would you expect an antibiotic like penicillin to be an effective treatment for a fungal or protist infection? Why or why not?