Exercise 9: Eukaryotic Microorganisms

Introduction

Microorganisms may be divided into two major groups based on their cell structure – prokaryotes and eukaryotes. Eukaryotic cells are typically much larger and contain membrane-bound organelles for compartmentalizing cellular functions. The largest and most recognizable organelle is the nucleus containing the cell’s chromosomes enclosed by a nuclear membrane. Other organelles are small, membrane-bound intracellular structures that perform specific functions like mitochondria or lysosomes. Eukaryotic microbes include protozoa, algae, fungi, and helminths (parasitic worms). Familiarity with eukaryotic cells is important for differentiation from bacterial cells when examining environmental or clinical specimens. The classification of eukaryotes is complex and still under debate. Therefore, we will focus on grouping by morphological description.

The protozoa include single-celled “animal-like” microorganisms. Many of the protozoa are capable of independent movement (motility) and this characteristic can be used in their identification. Some protozoa also have complex life cycles including a trophozoite (feeding stage) and a cyst (dormant stage) and sometimes more. Amoebas are motile by pseudopods or “false feet” that are a result of cytoplasmic extensions of the cell membrane. Flagellates such as Giardia, Trichomonas and Leishmania are motile by one or more flagella on the cell. Trypanosoma have an undulating membrane in addition to the flagella. Balantidium and Paramecium are covered with many cilia across their surface. Cilia are used to sweep food toward an oral groove as well as for movement.

The fungi include multicellular molds and unicellular yeasts. The molds consist of filamentous hyphae and have characteristic sexual and asexual fruiting bodies that are helpful in their identification. We will focus on the asexual structures of Penicillium, Rhizopus and Aspergillus. Penicillium produce asexual spores called conidiospores on a branched fruiting body that looks like the tip of a paint brush. Aspergillus have similar looking structures, but their conidiospores radiate outward from a central vesicle. Rhizopus produces asexual spores in an enclosed sporangium. When the sporangium breaks open all the mature sporangiospores are released at once. Yeasts are unicellular fungi that can reproduce by budding – a special type of mitosis in which there is unequal distribution of the cell cytoplasm. This results in what looks like a miniature yeast cell growing off the side of the parent cell.

The helminths are parasitic worms that may grow to be excessively large. Some tapeworms can grow to be 20 feet long! All helminthes have an egg or larval stage that is microscopic and is involved in transmission of the infection to a new host. Tapeworms are flatworms (cestodes) that are composed of a scolex for attachment (with hooklets and suckers) and a segmented body of proglottids that are solely responsible for reproduction. Each proglottid can contain up to 100,000 fertilized eggs. Hookworms like Ancyclostoma are roundworms (nematodes) that have a complete digestive tract and separate sexes. Students should be able to identify the mouth of the worm with one to several teeth. Schistosoma mansoni is a trematode with an oral sucker and an incomplete digestive tract. Schistosomes have separates sexes and produce eggs with a distinctive spine.

Course Intended Outcomes

Properly use a compound microscope, identify its parts and describe concepts of microscopy such as magnification and resolution.

Materials Used

Students will work individually but slides can be shared within your group. Each group will need the following materials for this exercise.

• Microscope
• Lens paper and cleaner
• Prepared slides used will be assigned by your instructor

Procedure

Technique:

1. Refer to the general microscopy techniques covered in exercise 4.
2. Some organisms are best viewed in low or scanning power while others require 400X total magnification to observe detail.
3. It is NOT necessary to use the 100X oil immersion lens to view the eukaryotic organisms.

Sampling/Inoculation:

1. Your instructor will assign the prepared slides to be used by your group.
2. Collect the required materials for the lab.
3. Work as a group to view ALL required slides.

Figure 9.1: Representative species of Fungi (L. to R.):  Penicillium notatum, Aspergillis niger, Rhizopus stolonifera.

Figure 9.2: Representative species of Protista (L. to R.):  Entamoeba coli, trophozoite; Entamoeba coli, cyst; Balantidium coli, trophozoite.

Figure 9.3: Representative species of Protista, continued (L. to R.): Leshmania donovani; Plasmodium species, trophozoite; Trypanosoma brucei.

Figure 9.4: Representative species of Helminthe adults (L. to R.): Taenia soleum, adult; S. japonicum, adult; Necator americanus, adult.

Figure 9.5: Representative species of Helminthe eggs (L. to R.): Taenia soleum, egg ; S. japonicum, egg ; Necator americanus; egg. Hookworm Egg Undergoing Cellular Division by the Centers for Disease Control and Prevention, available through the CDC Public Health Image Library, Image ID 5220. Public domain.

Lab Clean-up

Clean your slides and return them to the proper slide trays.

Clean your microscope and return it to the lab prep room.

Results and Interpretation

Create a table as below in your lab notebook. Record the names of the specimens, the best total magnification for viewing and a brief description of the organism.

Draw illustrations of the organisms viewed in your lab notebook.

Critical Thinking Questions

Based on your optimum total magnification results, which eukaryotic specimens were the largest and which were smallest?

Which specimens viewed are capable of movement (motility) and by what means/mode?